GOING BACK TO BASICS
FAULTY ELECTRICAL WIRINGS – A CLOSER LOOK
(Fifth Part of a Series of Six)
By Doods A. Amora, PEE 1821
(February, 2008)
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9.0) THE FEEDERS & SUB-FEEDERS
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As one looks into the upstream of the system, there are lots of feeders & sub-feeders that have to be dealt with along the way. Several articles in the Electrical Code are devoted in sizing feeder conductors necessary for the safety and operability of electrical systems.
“Feeders” are conductors which carry electric currents from the service switchgear (or generator switchboard) to the distributors, or groups of distributors, or groups of MCC’s, or load centers supplying bulk loads. “Sub-feeders” originate from distribution centers and supply one or more other sub-distribution boards, motor control centers or panelboards. Code rules on feeders also apply to sub-feeders.
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As one looks into the upstream of the system, there are lots of feeders & sub-feeders that have to be dealt with along the way. Several articles in the Electrical Code are devoted in sizing feeder conductors necessary for the safety and operability of electrical systems.
“Feeders” are conductors which carry electric currents from the service switchgear (or generator switchboard) to the distributors, or groups of distributors, or groups of MCC’s, or load centers supplying bulk loads. “Sub-feeders” originate from distribution centers and supply one or more other sub-distribution boards, motor control centers or panelboards. Code rules on feeders also apply to sub-feeders.
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(Examples of Good & Workmanlike Distribution Feeder Installations)
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At the onset, feeders and sub-feeders must be capable to carry the amount of current required by the present load, plus any current that may be required in the future. Selection of the sizes of a feeders depends on the magnitude and nature of the known loads as computed from the branch circuits, the unknown but anticipated loads, the voltage drop as well, and more importantly, the surprises in future loads.
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- LV Feeders in commercial buildings usually originate from the main LV Switchgear of the Power Center to Distributors or group of Distributors (First Frame).
- MCC’s as in the above picture usually are powered by Sub-Feeders coming from Distributors or Sub-Distributors (Second Frame).
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As feeder circuits are usually not within visual sight (beneath walls or ceilings most of the time), conductor overheating that results in insulation breakdown or meltdown always go unnoticed. Feeder circuits are usually protected by over-current devices (as fuses or circuit breakers), but then in many cases, these same over-current devices had given user-occupants the wrong security that the circuits are adequately protected.
Now note that the situation in the industrial plant is much better than that of a commercial building. The operation & maintenance in industrial plants usually are supervised by electricians and engineers in shifts. Any unusual manifestations on the system are most likely detected and fixed promptly. That explains why seldom that we hear an industrial plant burned out because of electrically-started fires.
On the other hand, most commercial buildings in the country are not supervised by qualified personnel – in fact, a majority of them (except for the large shopping malls & high-rise buildings) don’t have electrical operation & maintenance personnel at all. Electrical problems like blown-off fuses or tripped circuit breaker are generally attended to by an ‘all-around utility guy’ who may have dangled with electricity before.
The Threat of Feeder Burn-Outs
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Feeders and Sub-Feeders in commercial buildings or industrial plants are considered ‘one-time events’, as such these circuits are usually forgotten over the years - thus the danger of unnoticed feeder overloading to occur.
The fact that feeders/sub-feeders are inherently carrying heavy or high ampere loads, most electrical fires in buildings originate from these circuits. As we say, electrical fires usually happen after several years of operation when loads are added indiscriminately and capacities of feeder cables went overloaded.
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1) A Picture of an overloaded Sub-Feeder2) Note the imminent insulation breakdown of the overloaded Sub-Feeder Conductor
3) A burned out MV/LV Transformer due to overloading
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Now remember that commercial buildings are open to lessees or locators. The lease could be the entire building or may be a floor or several floors in the building. Note that loads proportionately cater to the appetite of the lessee’s business. A commercial bank of course significantly differs from a hardware store, or a fine-dining restaurant from an ordinary office.
For instance, a change of client from a motel business to a high-end heavily air-conditioned & exotically lighted disco joint may mean tripling of continuous loads. Because capacities of feeders & sub-feeders are not fully consumed in the beginning, then there seem to be no problem. ‘Business as usual’ has now been on-stream and nobody cares - after all, there was no tripping off circuits.
Normally, the new lessee would bring along his contractor to renovate the system to suit to the new business. However, note that the contractor doing a renovation on a building space previously occupied by someone else is only concerned on his scope - which is only confined within the interest leased by his client. New branch circuits and even new panelboards may have been installed for the new client and there seem to be no problem.
Meanwhile the electrical system of the disco joint may have increased the load of the building system significantly but remember that the contractor doing the renovation job has no business on the system of the entire building. Again, the contractor’s interest is only “where to tap”. As feeders & sub-feeders are not readily & physically visible, what if these have been overloaded and have already started to melt-down? Yes, there was no circuit tripping off because the over-current protection may have happened to be oversized. Who really knows?
Worse, how many commercial buildings in the country use industrial fans placed in front of distribution panels to cool off the protective devices in an effort to prevent them from tripping off? If the panelboards are unusually hot, what’s happening then to the non-visible feeder & sub-feeder conductors?
For instance, a change of client from a motel business to a high-end heavily air-conditioned & exotically lighted disco joint may mean tripling of continuous loads. Because capacities of feeders & sub-feeders are not fully consumed in the beginning, then there seem to be no problem. ‘Business as usual’ has now been on-stream and nobody cares - after all, there was no tripping off circuits.
Normally, the new lessee would bring along his contractor to renovate the system to suit to the new business. However, note that the contractor doing a renovation on a building space previously occupied by someone else is only concerned on his scope - which is only confined within the interest leased by his client. New branch circuits and even new panelboards may have been installed for the new client and there seem to be no problem.
Meanwhile the electrical system of the disco joint may have increased the load of the building system significantly but remember that the contractor doing the renovation job has no business on the system of the entire building. Again, the contractor’s interest is only “where to tap”. As feeders & sub-feeders are not readily & physically visible, what if these have been overloaded and have already started to melt-down? Yes, there was no circuit tripping off because the over-current protection may have happened to be oversized. Who really knows?
Worse, how many commercial buildings in the country use industrial fans placed in front of distribution panels to cool off the protective devices in an effort to prevent them from tripping off? If the panelboards are unusually hot, what’s happening then to the non-visible feeder & sub-feeder conductors?
The Threat of LV/LV Transformer Burn-Outs
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Like feeders & sub-feeders, LV/LV transformers when installed & energized are considered permanent fixtures in a system. They are usually oversized above the present load conditions and are expected to carry future additional loads. But then a number of electrical fires are pinpointed unto them.
Relatively large commercial complexes power its buildings with 480v, 3-phase, 60 Hz. The 480v takes care of the centralized air-conditioning system, the ancillary pumps, elevators, escalators & other motor loads. Several sub-systems downstream are the common LV/LV transformers transforming voltage to 240v for general lighting, store lighting and offices’ loads. In the Philippines, sizes ranging from 25 to 300 kVA are common installations. And there are several of them scattered within the building.
But an innocent-looking LV/LV transformer installed in a messy corner of a stock room or in whatever space available in some floors of an old building that is converted into an apartelle, bank or department store rolled into one; could be a candidate culprit to ignite a full-blown fire. As pointed out earlier, LV/LV transformers as “one-time events” are usually oversized above the present load conditions and are expected to carry future additional loads. Why then and in what instance shall an oversized transformer burn out?
Oversized from the beginning, transformers like the feeders are usually forgotten as years go by - even as loads keep on increasing and increasing. If electrical operation of the building is not supervised by qualified electrical practitioners (as in many commercial establishments), chances are, the transformer burns out without even knowing that same transformer had already been overloaded. The question is why, when there are protection devices installed? Here lies the issue! How can a transformer be protected from overloading?
This leads to one of the many reasons why there is a need for protection of transformers. But protecting transformers vary in sophistication as to the voltages involved, the size and the degree of importance. Probably if the transformer is LV/LV, then this could be the reason why less serious attention is given to these types of apparatuses.
LV/LV transformers may not be so expensive pieces of equipment compared to others in the system but its value can be very significant because they might be powering essential loads, computer loads & other IT business tools. Moreover, these transformers usually make the lighting systems and general purpose loads in a business operable. Its value in the electrical system is equally important because of the interruption it brings during failures.
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Sidebar 1: By the way, how do we electrical engineers provide overloading protection to LV/LV transformers? Simple..., huh? But not quite because it could be tricky. Consult the Code, then make a scenario and try placing OCPD’s on the transformer circuit. Then analyze/simulate if the transformer can be protected from overloading. Better still, consult books by Joseph MacPartland where good sample scenarios are presented.
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Like feeders & sub-feeders, LV/LV transformers when installed & energized are considered permanent fixtures in a system. They are usually oversized above the present load conditions and are expected to carry future additional loads. But then a number of electrical fires are pinpointed unto them.
Relatively large commercial complexes power its buildings with 480v, 3-phase, 60 Hz. The 480v takes care of the centralized air-conditioning system, the ancillary pumps, elevators, escalators & other motor loads. Several sub-systems downstream are the common LV/LV transformers transforming voltage to 240v for general lighting, store lighting and offices’ loads. In the Philippines, sizes ranging from 25 to 300 kVA are common installations. And there are several of them scattered within the building.
But an innocent-looking LV/LV transformer installed in a messy corner of a stock room or in whatever space available in some floors of an old building that is converted into an apartelle, bank or department store rolled into one; could be a candidate culprit to ignite a full-blown fire. As pointed out earlier, LV/LV transformers as “one-time events” are usually oversized above the present load conditions and are expected to carry future additional loads. Why then and in what instance shall an oversized transformer burn out?
Oversized from the beginning, transformers like the feeders are usually forgotten as years go by - even as loads keep on increasing and increasing. If electrical operation of the building is not supervised by qualified electrical practitioners (as in many commercial establishments), chances are, the transformer burns out without even knowing that same transformer had already been overloaded. The question is why, when there are protection devices installed? Here lies the issue! How can a transformer be protected from overloading?
This leads to one of the many reasons why there is a need for protection of transformers. But protecting transformers vary in sophistication as to the voltages involved, the size and the degree of importance. Probably if the transformer is LV/LV, then this could be the reason why less serious attention is given to these types of apparatuses.
LV/LV transformers may not be so expensive pieces of equipment compared to others in the system but its value can be very significant because they might be powering essential loads, computer loads & other IT business tools. Moreover, these transformers usually make the lighting systems and general purpose loads in a business operable. Its value in the electrical system is equally important because of the interruption it brings during failures.
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Sidebar 1: By the way, how do we electrical engineers provide overloading protection to LV/LV transformers? Simple..., huh? But not quite because it could be tricky. Consult the Code, then make a scenario and try placing OCPD’s on the transformer circuit. Then analyze/simulate if the transformer can be protected from overloading. Better still, consult books by Joseph MacPartland where good sample scenarios are presented.
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------------------------Sidebar 2: At this point, it is worthwhile to mention one common mistake in electrical systems of buildings - the installation of outdoor type oil-immersed transformers inside the building. Although a mortal sin, bringing inside the building oil-immersed transformers (whether MV/LV or LV/LV) is a practice not so uncommon in the Philippines. To recall, the oil used in this type of transformer is mineral oil - a highly flammable liquid. As the transformer burns out (as seen in the picture at the right), ignition into a full-blown fire is a sure formula.
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Derating of Conductors
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Most buildings were burned not because of short circuits, but by overloads. A short circuit being in the kilo-ampere magnitude will surely trip even an oversized continuous ampere-rated circuit breaker somewhere, that is, assuming that the short circuit current does not outsize the interrupting rating of the circuit breaker. Fact is, a breaker with KAIC rating not outsized by the fault current interrupts the fault without damage to itself.
On the other hand, overloads mean overheating. And overheating if sustained for long will start a fire. Of course circuits do have protection, but what if the circuit breaker continuous ampere-trip rating happens to be oversized viz-a-viz the feeders and the transformer they are protecting? Engineers therefore are supposed to be concerned in overloads as well because an overload coupled with an incorrectly sized thermal ampere-trip rating of a molded case circuit breaker is also a sure formula for melt-downs and eventually the fire.
But how come when the circuit breakers placed in the system matched with the ampacity of the conductors?
That brings us to the subject of derated ampacities of conductors.
Although conductors do have published current-carrying capacities but they are so rated at the ambient temperature of 30 deg C. Also, these ampacities are true only if the are no more than three current-carrying conductors in a raceway or cable. Conditions other than these specifications reduce the capacities of conductors - such derating pushes the conductors to lower net ampacities. Now the question: “Is placing OCPD’s based on the published ampacities of conductors correct?” Maybe yes, maybe not!
Note that placing OCPD’s based on the published ampacities of conductors are correct only if the conditions on “ambient temperature” and the “no more than three current-carrying conductors in a raceway or cable” are met. If not, then the OCPD’s must be sized based of the net ampacity of the wires & cables after being derated.
Does it make sense? Of course, Yes! Because in this case; the OCPD will provide correct protection to the derated cables.
One thing more: The Electrical Code states that in high capacity circuits of 800 amperes & larger, the use of the “next higher protective device ratings” is not permitted. This is so because the ampere increment of protective devices on these levels are already large. Using the “next higher rating” would place the feeder conductors in danger. Worse, some designers & contractors usually don’t consider derating the conductors because these would mean bigger cables and ergo, higher cost. Now, therefore; using the 'next higher rating' OCPD to protect a derated feeder conductor would go farther away from its intended protection.
The secret therefore in high capacity feeder circuits is that the design engineer must place feeders in bigger sizes while maintaining the size of over-current protection. This strategy will take account the voltage drop requirements and the derating factors that lead to overloading of the conductors, as well. Otherwise, what could have been a quality feeder installation will retrogress into faulty electrical wiring!
Derating of Conductors
----------------------------
Most buildings were burned not because of short circuits, but by overloads. A short circuit being in the kilo-ampere magnitude will surely trip even an oversized continuous ampere-rated circuit breaker somewhere, that is, assuming that the short circuit current does not outsize the interrupting rating of the circuit breaker. Fact is, a breaker with KAIC rating not outsized by the fault current interrupts the fault without damage to itself.
On the other hand, overloads mean overheating. And overheating if sustained for long will start a fire. Of course circuits do have protection, but what if the circuit breaker continuous ampere-trip rating happens to be oversized viz-a-viz the feeders and the transformer they are protecting? Engineers therefore are supposed to be concerned in overloads as well because an overload coupled with an incorrectly sized thermal ampere-trip rating of a molded case circuit breaker is also a sure formula for melt-downs and eventually the fire.
But how come when the circuit breakers placed in the system matched with the ampacity of the conductors?
That brings us to the subject of derated ampacities of conductors.
Although conductors do have published current-carrying capacities but they are so rated at the ambient temperature of 30 deg C. Also, these ampacities are true only if the are no more than three current-carrying conductors in a raceway or cable. Conditions other than these specifications reduce the capacities of conductors - such derating pushes the conductors to lower net ampacities. Now the question: “Is placing OCPD’s based on the published ampacities of conductors correct?” Maybe yes, maybe not!
Note that placing OCPD’s based on the published ampacities of conductors are correct only if the conditions on “ambient temperature” and the “no more than three current-carrying conductors in a raceway or cable” are met. If not, then the OCPD’s must be sized based of the net ampacity of the wires & cables after being derated.
Does it make sense? Of course, Yes! Because in this case; the OCPD will provide correct protection to the derated cables.
One thing more: The Electrical Code states that in high capacity circuits of 800 amperes & larger, the use of the “next higher protective device ratings” is not permitted. This is so because the ampere increment of protective devices on these levels are already large. Using the “next higher rating” would place the feeder conductors in danger. Worse, some designers & contractors usually don’t consider derating the conductors because these would mean bigger cables and ergo, higher cost. Now, therefore; using the 'next higher rating' OCPD to protect a derated feeder conductor would go farther away from its intended protection.
The secret therefore in high capacity feeder circuits is that the design engineer must place feeders in bigger sizes while maintaining the size of over-current protection. This strategy will take account the voltage drop requirements and the derating factors that lead to overloading of the conductors, as well. Otherwise, what could have been a quality feeder installation will retrogress into faulty electrical wiring!
(To be continued…)
Next Episode: HOW ARE FEEDERS & TRANSFORMERS SIZED?
Next Episode: HOW ARE FEEDERS & TRANSFORMERS SIZED?
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