GOING BACK TO BASICS
FAULTY ELECTRICAL WIRINGS – A CLOSER LOOK
(Last Part of a Series of Six)
By Doods A. Amora, PEE 1821
(February, 2008)
10) SIZING THE FEEDERS, LV/LV TRANSFORMERS & POWER CENTERS
In reality, whenever there’s a major renovation done on a building electrical system, it usually becomes a condition leading to faulty electrical wiring. As feeders & sub-feeders already exist in a building, they could not just be torn-down as so much money had already been spent on these parts of the system. Hence, as a result of the surprises in new load requirements; new feeders or sub-feeders had to be inserted into terminals of existing circuit breakers, its subsequent conduits (if there are any) squeezed in whatever available space in a crowded room, or just plain naked cables tied to existing conduits runs. Some cases may have resorted to the usual “skin, splice and tape” method. These “ragtag, make-do” additions to the system always make the otherwise neat & workmanlike original installations into messy and faulty conditions.
For example:
Somewhere in the country, a new office building has just been inaugurated for business. A few months thereafter, the building management became frantic to install an additional power center and subsequently the new distribution feeders to satisfy clients’ needs who happen to lease half of the building. In the end, it was the client who financed and installed its own power center in an already cramped & messy room allocated supposedly for electrical services. And this is not even an Internet Hotel, the building is just an edifice built for corporate office clientele.
The building owner or its representative architects did not recognize that today, there are new electrical footprints that have to be dealt with in designing power systems for buildings. Computers, IT peripherals & office electronic equipments and the harmonics they bring – they are not loads in the past, but now are eating more power than that of the traditional plug-in appliance loads. This is not an isolated case. It’s happening anywhere else in the world.
How are the “one-time components” in the electrical system of a commercial building designed? Should the electrical design engineer wait for the architects & mechanical engineers to complete their respective designs as inputs? And from there, the electrical engineer usually counts the loads, make circuit arrangements and determine the panelboards, MCC’s, etc. From these arrangements will subsequently shape the sub-feeders, the feeders and eventually, the transformers. Such is the usual practice, of course…
If such is the case, the electrical designer would have to base his system from the loads given by the architects (lighting, GPR’s, etc) & mechanical engineers (air-conditioning). With some allocation for future loads, then presto – all’s well and the building is energized. Fine…
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(The "One-Time Events" in the electrical system of a building: the Power Center, the Feeders, the Busways, the LV/lv Transformers and the Distributors)
But then, what if in reality, the lessee’s business has a different hunger for loads? Does it mean that the electrical system (the feeders/sub-feeders, the LV/lv transformers and even the main service substation transformers) now embedded in the building has to be overhauled to fit the load?
Certainly not! By plain logic, the building electrical system must be all time ready to provide clients what they need; whoever and whatever they are – without overhauling the system.
But how are these done?
11) THE LOAD DENSITY METHODOLOGY
In the USA and other parts of the world, the sizes of feeders/sub-feeders and the transformers are mandated to be based on the Load Density Methodology.
“But Sir, following this Code methodology will cost a fortune. It will result to large feeders and transformers than what are actually needed!”
NEC Table 220-3(b) lists certain occupancies (types of buildings) for which load densities (lighting & general-purpose outlets) specified in volt-amperes per square foot. The PEC also lists down the lighting load densities in terms of volt-amperes per square meter. Both Codes have a purpose.
In each type of occupancy, there must be adequate feeder circuit capacity to handle the total load that is represented by the product of volt-amperes per unit area times the area of the building. The KVA load derived from these computations shall then be multiplied with a demand factor in order to approximate the maximum demand load ever possible that soon to be served by feeders or sub-feeders. The maximum demand plus a load growth factor of 20% to 30% is recommended imbedded in the design until the ultimate service transformer. In design practice, a 65%-70% loaded feeder or transformer in a brand new office building is usually acceptable.
Aside from lighting & general purpose receptacles in office occupancy, a substantial load of 55% - 65% of the whole building load happens to be the ventilating and air-conditioning loads. The same is true to manufacturing concerns whose production lines are air-conditioned. Load densities for air-conditioning systems are also provided by the Code and IEEE publications. Thus the engineer can approximate safely the air-conditioning loads even if the mechanical group has not yet completed their designs.
Let us now look closer at modern office buildings. The new computer age has also brought along the much needed transformation in the power system landscape that the design of office building wants to achieve. Computer loads are special as they require special treatment, too. They also generate harmonics that makes power systems not only dirty but also cause to reflect more loads into the system. As they themselves are vulnerable to system disturbances they help create, they therefore need to be isolated. These special pieces of office equipment even have special plugs thus needing special outlets. But then in the past and even in the present time, these special loads are not getting the attention they deserve from electrical designs.
Traditionally, computers & peripherals are not recognized as loads with “new identity”. They are just considered as part of the general purpose receptacles usually provided by traditional designs. But then, recent experiences show that computers and electronics equipment in offices now eat up more power than the traditional “plug-in” & appliance loads.
Using load densities intended for GPR’s (general purpose receptacles) with the thinking that computer and IT loads are part of it; is ‘highly arguable’ for reasons that the load densities recommended by the Electrical Code for GPR’s were established long before the advent of computers. Moreover, IEEE recommends that these loads shall have dedicated 3-wire single phase circuits, home-running to dedicated 3-phase, 5-wire panelboards, to be fed by the so-called PDU’s (Power Distribution Units) and to be served by dedicated delta-wye isolating transformer with a sufficient factor in sizing system components to address harmonics.
Modern corporate offices today are expected to be replete with office computers and peripherals that would fill up 85% to 90% of the entire office desks of the building. But alas, the most recent PEC or NEC hasn’t established yet load densities for office computers & peripherals. It has been learned that today, authorities & experts in the USA are still in the process of surveying or making census on usages of computers in offices. But then, even though load densities for computer loads in offices are not yet available in the present Electrical Code, power allocation for office IT equipments has to be inputted in any office building design. Otherwise then, will be the frantic calls for the ragtag cure to overload conditions.
The Seemingly Large Feeders
According to the IEEE Gray Book, “In many modern buildings, the actual maximum demand loads will be substantially less than that calculated under the NEC methodology; but where the NEC or equivalent Code is in effect, the Code calculations using the load density method must be used in sizing service, feeders, switchboards and panelboards”.
Admittedly, the resulting feeders calculated or derived through Load Density Method would seem to appear significantly larger than the actual count of demand loads based on the actual circuits drawn on plans. However, this NEC Methodology has a purpose. Again, in the USA, this method is imposed over & above any other method for feeders and the subsequent transformers.
But why the Load Density Method results to larger feeders than what appears needed? Why these feeders have to terminate in Distributors? Why do sub-feeders have to terminate in Sub-Distributors, and so on and so forth?
Let us remember that feeders are supposed to be ‘one-time-event’ installations. The feeders derived from the Load Density Method are usually larger than the actual count of demand loads based on the actual layouts on a plan. This is so because the commercial building must be ready enough to accommodate surprises in load demands brought by assorted clients’ appetite for power. Whatever happens, the feeders must be there available and ready.
The Distributors & Sub-Distributors along with panelboards designed under the Load Density method usually provide ready-to-use circuit breakers that can be utilized anytime when the event comes, without disturbing the embedded feeders and sub-feeders. In effect, messy additions into the system would be avoided and the original well-installed system would be preserved, no matter who and what businesses the clients are.
Feeder Oversizing
Again, in real-life system designing, feeders and sub-feeders are usually oversized to accommodate present load conditions, the anticipated load growth but as well, the surprises in client’s peculiarity in demand. Load Growth Factors imbedded in cabled feeders usually ranges from 20% - 30%. If the feeder is a busway system; the more that it must be oversized (normally at 25% to 35%) - because should it happen that the bus ducts would be short of capacity in the future, can you imagine replacing them?
Certainly not! By plain logic, the building electrical system must be all time ready to provide clients what they need; whoever and whatever they are – without overhauling the system.
But how are these done?
11) THE LOAD DENSITY METHODOLOGY
In the USA and other parts of the world, the sizes of feeders/sub-feeders and the transformers are mandated to be based on the Load Density Methodology.
“But Sir, following this Code methodology will cost a fortune. It will result to large feeders and transformers than what are actually needed!”
NEC Table 220-3(b) lists certain occupancies (types of buildings) for which load densities (lighting & general-purpose outlets) specified in volt-amperes per square foot. The PEC also lists down the lighting load densities in terms of volt-amperes per square meter. Both Codes have a purpose.
In each type of occupancy, there must be adequate feeder circuit capacity to handle the total load that is represented by the product of volt-amperes per unit area times the area of the building. The KVA load derived from these computations shall then be multiplied with a demand factor in order to approximate the maximum demand load ever possible that soon to be served by feeders or sub-feeders. The maximum demand plus a load growth factor of 20% to 30% is recommended imbedded in the design until the ultimate service transformer. In design practice, a 65%-70% loaded feeder or transformer in a brand new office building is usually acceptable.
Aside from lighting & general purpose receptacles in office occupancy, a substantial load of 55% - 65% of the whole building load happens to be the ventilating and air-conditioning loads. The same is true to manufacturing concerns whose production lines are air-conditioned. Load densities for air-conditioning systems are also provided by the Code and IEEE publications. Thus the engineer can approximate safely the air-conditioning loads even if the mechanical group has not yet completed their designs.
Let us now look closer at modern office buildings. The new computer age has also brought along the much needed transformation in the power system landscape that the design of office building wants to achieve. Computer loads are special as they require special treatment, too. They also generate harmonics that makes power systems not only dirty but also cause to reflect more loads into the system. As they themselves are vulnerable to system disturbances they help create, they therefore need to be isolated. These special pieces of office equipment even have special plugs thus needing special outlets. But then in the past and even in the present time, these special loads are not getting the attention they deserve from electrical designs.
Traditionally, computers & peripherals are not recognized as loads with “new identity”. They are just considered as part of the general purpose receptacles usually provided by traditional designs. But then, recent experiences show that computers and electronics equipment in offices now eat up more power than the traditional “plug-in” & appliance loads.
Using load densities intended for GPR’s (general purpose receptacles) with the thinking that computer and IT loads are part of it; is ‘highly arguable’ for reasons that the load densities recommended by the Electrical Code for GPR’s were established long before the advent of computers. Moreover, IEEE recommends that these loads shall have dedicated 3-wire single phase circuits, home-running to dedicated 3-phase, 5-wire panelboards, to be fed by the so-called PDU’s (Power Distribution Units) and to be served by dedicated delta-wye isolating transformer with a sufficient factor in sizing system components to address harmonics.
Modern corporate offices today are expected to be replete with office computers and peripherals that would fill up 85% to 90% of the entire office desks of the building. But alas, the most recent PEC or NEC hasn’t established yet load densities for office computers & peripherals. It has been learned that today, authorities & experts in the USA are still in the process of surveying or making census on usages of computers in offices. But then, even though load densities for computer loads in offices are not yet available in the present Electrical Code, power allocation for office IT equipments has to be inputted in any office building design. Otherwise then, will be the frantic calls for the ragtag cure to overload conditions.
The Seemingly Large Feeders
According to the IEEE Gray Book, “In many modern buildings, the actual maximum demand loads will be substantially less than that calculated under the NEC methodology; but where the NEC or equivalent Code is in effect, the Code calculations using the load density method must be used in sizing service, feeders, switchboards and panelboards”.
Admittedly, the resulting feeders calculated or derived through Load Density Method would seem to appear significantly larger than the actual count of demand loads based on the actual circuits drawn on plans. However, this NEC Methodology has a purpose. Again, in the USA, this method is imposed over & above any other method for feeders and the subsequent transformers.
But why the Load Density Method results to larger feeders than what appears needed? Why these feeders have to terminate in Distributors? Why do sub-feeders have to terminate in Sub-Distributors, and so on and so forth?
Let us remember that feeders are supposed to be ‘one-time-event’ installations. The feeders derived from the Load Density Method are usually larger than the actual count of demand loads based on the actual layouts on a plan. This is so because the commercial building must be ready enough to accommodate surprises in load demands brought by assorted clients’ appetite for power. Whatever happens, the feeders must be there available and ready.
The Distributors & Sub-Distributors along with panelboards designed under the Load Density method usually provide ready-to-use circuit breakers that can be utilized anytime when the event comes, without disturbing the embedded feeders and sub-feeders. In effect, messy additions into the system would be avoided and the original well-installed system would be preserved, no matter who and what businesses the clients are.
Feeder Oversizing
Again, in real-life system designing, feeders and sub-feeders are usually oversized to accommodate present load conditions, the anticipated load growth but as well, the surprises in client’s peculiarity in demand. Load Growth Factors imbedded in cabled feeders usually ranges from 20% - 30%. If the feeder is a busway system; the more that it must be oversized (normally at 25% to 35%) - because should it happen that the bus ducts would be short of capacity in the future, can you imagine replacing them?
“But Sir.., is oversizing not a violation of the Code?”
Let’s take a look at some relevant provisions of the Code.
“The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the non-continuous load plus 125 percent of the continuous load. [NEC 215.2 (A)(1)]”. For general service feeders, the Code further said, “the computed load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, after any applicable demand factors permitted (NEC 220.10)”.
For feeders serving group motors, the Code says: “Motor loads shall be computed in accordance with NEC 430.24, 430.25 (NEC 220.14)”. To continue, “Conductors supplying several motors, or motors and other loads, shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads (NEC 430.24)”.
Hence, designing sizes of circuits or feeders ends up in the ampacities of cables or bus. Protecting these circuits by OCPD’s must match these ampacities because after all, the protective devices are supposed to protect circuit conductors. As NEC 240-3 says: “Conductors shall be protected against over-current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes”.
But, “where feeder conductors have an ampacity greater than required by 430.24, the rating or setting of the feeder overcurrent protective device shall be permitted to be based on the ampacity of the feeder conductors (NEC 430-62(b)”.
Therefore, oversizing a feeder (larger than the minimum requirements) is permitted for as long as it is protected properly by protective devices with sizes or settings that match the ampacity of the conductors used. Note that oversizing the feeders above the minimum requirements is a normal event in designing. These are due to the following factors:
a) Anticipation to future loads or load growth,
b) Compensation to voltage drops, necessitating larger cables,
c) Compensation for derating conditions, like too many conductors in a raceway or cable trays,
d) Compensation to correction factors in ambient temperatures,
e) The usual assumption that the loads are all continuous in anticipation to future change of use.
12) THE CONCLUDING PART
Statistics showed that the month of March tops the number of fire incidents in the Philippines compared to other periods in a year. Thus in an effort to raise consciousness prompted the authorities to declare it as the “Fire Prevention Month”. But why is the month of March laden with most incidents of fires? Is it because it’s summer time and the environment is hot?
Probably yes! Dry combustible materials are easy to ignite when the environment is hot and the air abundant. On the other hand, ventilating and cooling comfort appliances as air-conditioners, electric fans are in full blast during these times and the electrical loads are high. Ripe for overloads…, huh? - as many observers say.
Again back to square one, why the overload when there are OCPD’s installed?
The more troubling question is: “Has it occurred to us that the hotter ambient temperature makes the conductor derating conditions in full effect? What if the OCPD’s are sized without considering the derating factors?” (Remember Part 5 of this series).
As pointed out in the earlier episodes; there really exist faulty wirings. The conditions for electrically-started fires cited in this series are only a tip of the iceberg. There are still mountains of issues wanting to be discussed. But to this author, it is now a matter of lifting people’s consciousness - the level of awareness that these articles hope to help achieve.
Although much had been said about “faulty wirings” by the media, significant number of fires are caused by carelessness on the part of the occupants. Aside from these obvious causes, it’s however very difficult to establish faulty electrical wiring as the cause of a fire.
That’s why in these modern times, like the ‘new medical detectives’ we usually see on TV, we need forensic electrical engineers. In the USA, the forensic engineers are not just ordinary ones. They are a select breed of highly experienced PhD’s, complete with laboratories and all having the credentials to represent in court proceedings. Like any medical detectives or forensic pathologists, the forensic engineers reconstruct and establish the interwoven sequence of events that lead to electrical fires.
In the Philippines, we have yet to hear one of this kind. Wishful thinking as it may, such forensic engineers are highly wanting in the recent Glorietta blast controversy.
In the end, Flawed Design, Unworkmanlike Installations, Substandard Electrical Components & Devices, and Cheap Engineering; all of them contributing to each other, constitute to evolve into what we call as: FAULTY ELECTRICAL WIRING.
The Electrical Engineer: The engineering community now faces the task of selecting the correct power-system topology for new building environments. By evaluating popular power-system topologies, there has to emerge a shift in paradigm as deviations from the traditional way of systems designing has dramatically changed.
It is then necessary that we, Filipino electrical engineers have to investigate our traditional electrical practices from that of global standards. Again, there must be a paradigm shift from what was thought of as “traditional practices” to what are global; otherwise the electrical engineer will become irrelevant in these modern times.
What is globalization for?
Doods A. Amora, PEE
February, 2008
Let’s take a look at some relevant provisions of the Code.
“The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the non-continuous load plus 125 percent of the continuous load. [NEC 215.2 (A)(1)]”. For general service feeders, the Code further said, “the computed load of a feeder or service shall not be less than the sum of the loads on the branch circuits supplied, after any applicable demand factors permitted (NEC 220.10)”.
For feeders serving group motors, the Code says: “Motor loads shall be computed in accordance with NEC 430.24, 430.25 (NEC 220.14)”. To continue, “Conductors supplying several motors, or motors and other loads, shall have an ampacity not less than 125 percent of the full-load current rating of the highest rated motor plus the sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A), plus the ampacity required for the other loads (NEC 430.24)”.
Hence, designing sizes of circuits or feeders ends up in the ampacities of cables or bus. Protecting these circuits by OCPD’s must match these ampacities because after all, the protective devices are supposed to protect circuit conductors. As NEC 240-3 says: “Conductors shall be protected against over-current in accordance to their ampacities, but where the ampacity of the conductor does not correspond with the standard ampere rating of a fuse or a circuit breaker, the next higher rating shall be permitted only if this rating does not exceed 800 amperes”.
But, “where feeder conductors have an ampacity greater than required by 430.24, the rating or setting of the feeder overcurrent protective device shall be permitted to be based on the ampacity of the feeder conductors (NEC 430-62(b)”.
Therefore, oversizing a feeder (larger than the minimum requirements) is permitted for as long as it is protected properly by protective devices with sizes or settings that match the ampacity of the conductors used. Note that oversizing the feeders above the minimum requirements is a normal event in designing. These are due to the following factors:
a) Anticipation to future loads or load growth,
b) Compensation to voltage drops, necessitating larger cables,
c) Compensation for derating conditions, like too many conductors in a raceway or cable trays,
d) Compensation to correction factors in ambient temperatures,
e) The usual assumption that the loads are all continuous in anticipation to future change of use.
12) THE CONCLUDING PART
Statistics showed that the month of March tops the number of fire incidents in the Philippines compared to other periods in a year. Thus in an effort to raise consciousness prompted the authorities to declare it as the “Fire Prevention Month”. But why is the month of March laden with most incidents of fires? Is it because it’s summer time and the environment is hot?
Probably yes! Dry combustible materials are easy to ignite when the environment is hot and the air abundant. On the other hand, ventilating and cooling comfort appliances as air-conditioners, electric fans are in full blast during these times and the electrical loads are high. Ripe for overloads…, huh? - as many observers say.
Again back to square one, why the overload when there are OCPD’s installed?
The more troubling question is: “Has it occurred to us that the hotter ambient temperature makes the conductor derating conditions in full effect? What if the OCPD’s are sized without considering the derating factors?” (Remember Part 5 of this series).
As pointed out in the earlier episodes; there really exist faulty wirings. The conditions for electrically-started fires cited in this series are only a tip of the iceberg. There are still mountains of issues wanting to be discussed. But to this author, it is now a matter of lifting people’s consciousness - the level of awareness that these articles hope to help achieve.
Although much had been said about “faulty wirings” by the media, significant number of fires are caused by carelessness on the part of the occupants. Aside from these obvious causes, it’s however very difficult to establish faulty electrical wiring as the cause of a fire.That’s why in these modern times, like the ‘new medical detectives’ we usually see on TV, we need forensic electrical engineers. In the USA, the forensic engineers are not just ordinary ones. They are a select breed of highly experienced PhD’s, complete with laboratories and all having the credentials to represent in court proceedings. Like any medical detectives or forensic pathologists, the forensic engineers reconstruct and establish the interwoven sequence of events that lead to electrical fires.
In the Philippines, we have yet to hear one of this kind. Wishful thinking as it may, such forensic engineers are highly wanting in the recent Glorietta blast controversy.
In the end, Flawed Design, Unworkmanlike Installations, Substandard Electrical Components & Devices, and Cheap Engineering; all of them contributing to each other, constitute to evolve into what we call as: FAULTY ELECTRICAL WIRING.
The Electrical Engineer: The engineering community now faces the task of selecting the correct power-system topology for new building environments. By evaluating popular power-system topologies, there has to emerge a shift in paradigm as deviations from the traditional way of systems designing has dramatically changed.
It is then necessary that we, Filipino electrical engineers have to investigate our traditional electrical practices from that of global standards. Again, there must be a paradigm shift from what was thought of as “traditional practices” to what are global; otherwise the electrical engineer will become irrelevant in these modern times.
What is globalization for?
Doods A. Amora, PEE
February, 2008
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