SIZING THE NEUTRAL & GROUND CONDUCTORS
The sizing of neutral conductors referred to in this article could mean both the current-carrying or non-current carrying Neutral, N. Subsequently, the equipment bonding ground conductor refers to the Protective Earth, PE. Using IEC terminology, if the system is a four-wire system, the fourth conductor is called PE/N (or just plain N by North American terminology) which can be applied as current-carrying neutrals and/or at the same time, acting as the equipment bonding ground conductor. Note that North America’s 3-phase, four-wire system, the IEC counterpart could either be the TN-C or TT four-wire schemes.
There are three main applications for the three-phase 4-wire system. One application is to serve line-to-line-connected three-phase & single-phase loads only; which in this case, the neutral is not a current-carrying conductor (common in the Philippines). The second application is to supply line-to-neutral connected single phase loads as in 220v loads in a 380vY system (EU) or 277v in 480vY or 120v in 208vY systems (USA). The third application is the combination of both line-to-line and line–to-neutral loads. Whenever there are loads connected to the neutral (L-N), the neutral becomes a current-carrying conductor as in the case of the second & third applications.
Almost similar to North America’s three-phase 4-wire system, in the five-wire systems (TN-S or TT 5-wire, which are European), the fourth wire is called the N and the fifth wire, the PE, meaning, the N is separated from the PE. The fourth wire N is meant to be current-carrying while the PE is not, during normal operating condition. The purpose of the PE is for equipotentiality effect and to act as current return path during ground fault conditions.
Before any sizing can be done and for purposes of common understanding, the grounding system discussed here shall be understood to have the following conditions:
a) Typical to the Philippine scenario is that the 3-phase system supplies three-phase or single phase line-to-line loads and is therefore not supposed to accommodate line-to-neutral loads. Thus for a 480v 3-phase system, three-phase loads can only be the 460v 3Φ motors. (Only a few isolated cases that 277v industrial lights are employed in this country). Common are the 480-240v LV/LV transformer sets-up where 230v lighting voltage is derived. The fourth wire in these installations thus is meant for ‘equipment grounding’ or ‘PE’ and therefore must not be mistaken as a current-carrying neutral.
b) The 3-phase 5-wire system referred to as TN-S or TT five-wire schemes are to serve three-phase line-to-line loads as well as single phase line-to-neutral loads. This is typical to European systems but only a few industries in the Philippines are employing this kind of system. This system however is best recommended for IT environments.
The Neutral Conductor N (this time separated from the Protective Earth) is intended to carry currents during normal conditions and therefore an active phase. The role of the PE is to complete the loop of ground current return paths and would only carry currents during faults. Acting as “equipment grounding”, the PE takes care of the stability and uniformity of the ground potential during faults.
c) The phase conductors of a power source (generators or transformers) or service entrance may be large enough that they are paralleled. In grounding system, it is important to establish the “equivalent” of these conductors. The term “equivalent of paralleled phase conductors” is explained as: “parallel phase conductors are to be converted into a single conductor per phase that has a cross-sectional area of its conductor material at least equal to the sum of the cross-sectional areas of the conductor materials of the two or more parallel conductors per phase.” For instance, two parallel 500 MCM per phase copper THW cables conduits would be equivalent to a single conductor with a cross-sectional area of 500 + 500 or 1,000 MCM.
SIZING THE NEUTRAL & PE CONDUCTORS
IN THREE-PHASE 4-WIRE SYSTEM (TN-C or TT)
RELEVANT GROUNDING PRACTICES:
The size of the PE conductor (if not intended as a current-carrying neutral) shall be for simplicity, NEC Table 250.122 provides ready recommendations.
However, if the PE/N is intended to be current-carrying, the minimum size of the PE/N shall be the same as the size of the main phase conductors (100%), or maybe larger if third and other triplen harmonics are prevalent in the system.
The size of the Electrode Conductor (EC) shall be in accordance to NEC Table 250.66.
DOODS A. AMORA, PEE
September 28, 2007
There are three main applications for the three-phase 4-wire system. One application is to serve line-to-line-connected three-phase & single-phase loads only; which in this case, the neutral is not a current-carrying conductor (common in the Philippines). The second application is to supply line-to-neutral connected single phase loads as in 220v loads in a 380vY system (EU) or 277v in 480vY or 120v in 208vY systems (USA). The third application is the combination of both line-to-line and line–to-neutral loads. Whenever there are loads connected to the neutral (L-N), the neutral becomes a current-carrying conductor as in the case of the second & third applications.
Almost similar to North America’s three-phase 4-wire system, in the five-wire systems (TN-S or TT 5-wire, which are European), the fourth wire is called the N and the fifth wire, the PE, meaning, the N is separated from the PE. The fourth wire N is meant to be current-carrying while the PE is not, during normal operating condition. The purpose of the PE is for equipotentiality effect and to act as current return path during ground fault conditions.
Before any sizing can be done and for purposes of common understanding, the grounding system discussed here shall be understood to have the following conditions:
a) Typical to the Philippine scenario is that the 3-phase system supplies three-phase or single phase line-to-line loads and is therefore not supposed to accommodate line-to-neutral loads. Thus for a 480v 3-phase system, three-phase loads can only be the 460v 3Φ motors. (Only a few isolated cases that 277v industrial lights are employed in this country). Common are the 480-240v LV/LV transformer sets-up where 230v lighting voltage is derived. The fourth wire in these installations thus is meant for ‘equipment grounding’ or ‘PE’ and therefore must not be mistaken as a current-carrying neutral.
b) The 3-phase 5-wire system referred to as TN-S or TT five-wire schemes are to serve three-phase line-to-line loads as well as single phase line-to-neutral loads. This is typical to European systems but only a few industries in the Philippines are employing this kind of system. This system however is best recommended for IT environments.
The Neutral Conductor N (this time separated from the Protective Earth) is intended to carry currents during normal conditions and therefore an active phase. The role of the PE is to complete the loop of ground current return paths and would only carry currents during faults. Acting as “equipment grounding”, the PE takes care of the stability and uniformity of the ground potential during faults.
c) The phase conductors of a power source (generators or transformers) or service entrance may be large enough that they are paralleled. In grounding system, it is important to establish the “equivalent” of these conductors. The term “equivalent of paralleled phase conductors” is explained as: “parallel phase conductors are to be converted into a single conductor per phase that has a cross-sectional area of its conductor material at least equal to the sum of the cross-sectional areas of the conductor materials of the two or more parallel conductors per phase.” For instance, two parallel 500 MCM per phase copper THW cables conduits would be equivalent to a single conductor with a cross-sectional area of 500 + 500 or 1,000 MCM.
SIZING THE NEUTRAL & PE CONDUCTORS
IN THREE-PHASE 4-WIRE SYSTEM (TN-C or TT)
RELEVANT GROUNDING PRACTICES:
The size of the PE conductor (if not intended as a current-carrying neutral) shall be for simplicity, NEC Table 250.122 provides ready recommendations.
However, if the PE/N is intended to be current-carrying, the minimum size of the PE/N shall be the same as the size of the main phase conductors (100%), or maybe larger if third and other triplen harmonics are prevalent in the system.
The size of the Electrode Conductor (EC) shall be in accordance to NEC Table 250.66.
DOODS A. AMORA, PEE
September 28, 2007
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