Neutral sizing in Panel Board
- Thread starter ajengin
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bphgravity
Senior Member
- Location
- Florida
Re: Neutral sizing in Panel Board
Assuming you question does not pertain to a 4-wire, wye-connected, 3-phase system, the maximum neutral load must not exceed 85-amperes @ 75? if sized 4 AWG.
If you are referring to a service neutral conductor, I would not suggest a reduction in the size at all. The grounded neutral conductor at the service also serves as the effective fault path and should probably always be sized the same as the ungrounded conductors. In no case should it be sized smaller than that required in Table 250.66.
Assuming you question does not pertain to a 4-wire, wye-connected, 3-phase system, the maximum neutral load must not exceed 85-amperes @ 75? if sized 4 AWG.
If you are referring to a service neutral conductor, I would not suggest a reduction in the size at all. The grounded neutral conductor at the service also serves as the effective fault path and should probably always be sized the same as the ungrounded conductors. In no case should it be sized smaller than that required in Table 250.66.
- Location
- Massachusetts
Re: Neutral sizing in Panel Board
I do not understand, we can reduce the neutral of that system.
??
I do not understand, we can reduce the neutral of that system.
- Location
- Massachusetts
Re: Neutral sizing in Panel Board
Take a look at 220.22.
Take a look at 220.22.
Re: Neutral sizing in Panel Board
220.61 provides guidance for the size of Feeder or Service Neutral Load.
Lets not get into the question of what a neutral is there is a NFPA Task Force working on that as we speak.
The 3 phase 4 wire (I Think) allows a demand factor of 70% for that portion of the load over 200A ???
If you have a 3 wire 2 pahse or 5 wire 2 phase the neutral must be increased by 40%.
But I'm pretty sure you can have a reduced neutral.\!!!!!
But the 2005 requires feeder neutrals to be sized not smaller than the EGC of Table 250.122
Charlie
220.61 provides guidance for the size of Feeder or Service Neutral Load.
Lets not get into the question of what a neutral is there is a NFPA Task Force working on that as we speak.
The 3 phase 4 wire (I Think) allows a demand factor of 70% for that portion of the load over 200A ???
If you have a 3 wire 2 pahse or 5 wire 2 phase the neutral must be increased by 40%.
But I'm pretty sure you can have a reduced neutral.\!!!!!
But the 2005 requires feeder neutrals to be sized not smaller than the EGC of Table 250.122
Charlie
bphgravity
Senior Member
- Location
- Florida
Re: Neutral sizing in Panel Board
Oh, my mistake!
I forgot to add the qualifier of "nonlinear loads" or when 2-phase wires are used with the neutral.
Oh, my mistake!
I forgot to add the qualifier of "nonlinear loads" or when 2-phase wires are used with the neutral.
- Location
- Massachusetts
Re: Neutral sizing in Panel Board
Bryan, thanks, you had me wondering if I was misreading 220.22, it is a long and complicated paragraph, at least to me.
Bryan, thanks, you had me wondering if I was misreading 220.22, it is a long and complicated paragraph, at least to me.
Re: Neutral sizing in Panel Board
Just because the "hots" are 1/0 & the "neutral" is #4 does not necessarily indicate a Code violation. 220.22 is the basic method of calculating "neutral" loading allowance for feeder or service.
Example: A 480Y/277V. panelboard that supplies mostly 3-phase 3-wire motor loads & a relatively
small 277V. lighting load, may well have a feeder "neutral" that is much smaller than the "hots".
Just because the "hots" are 1/0 & the "neutral" is #4 does not necessarily indicate a Code violation. 220.22 is the basic method of calculating "neutral" loading allowance for feeder or service.
Example: A 480Y/277V. panelboard that supplies mostly 3-phase 3-wire motor loads & a relatively
small 277V. lighting load, may well have a feeder "neutral" that is much smaller than the "hots".
izak
Senior Member
- Location
- Springfield, MO
Re: Neutral sizing in Panel Board
the question about 'the definition of a neutral' reminds me of a question...
what is considered a 'current carrying conductor' and what is not and why?
the question about 'the definition of a neutral' reminds me of a question...
what is considered a 'current carrying conductor' and what is not and why?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Neutral sizing in Panel Board
The "why" is simple enough. If the neutral carries a significant amount of current, then the heat that it generates will influence the ability of the other conductors to reject their heat to the surrounding environment.
All phase conductors are counted. As to whether to count the neutral, see 310.15(B)(4). This still leaves open the question of how much harmonic current can the system contain, before it becomes a ?major portion of the load.? The code does not give a value, so it is left up to the designer and the approval authority.
The "why" is simple enough. If the neutral carries a significant amount of current, then the heat that it generates will influence the ability of the other conductors to reject their heat to the surrounding environment.
sceepe
Senior Member
Re: Neutral sizing in Panel Board
Personal opinion: Even if you can down size the neutral don't. There are too many reasons (Harmonics, non linear loads) to name a few. Given the nature of the types of loads you see today the question should be whether to go with full size or oversized neutrals.
NEC is the bare minimum. A code compliant installation is nothing to brag about. It like passing a test. Who wants to get a "D" when you can get an "A". - ok i'll get off the soap box now.
If the owner is insisting on undersized neutrals, have him put it in writing. Save it cause you will be showing it to him in a couple of years when he calls you back to look at the fire damage or some other problem.
Personal opinion: Even if you can down size the neutral don't. There are too many reasons (Harmonics, non linear loads) to name a few. Given the nature of the types of loads you see today the question should be whether to go with full size or oversized neutrals.
NEC is the bare minimum. A code compliant installation is nothing to brag about. It like passing a test. Who wants to get a "D" when you can get an "A". - ok i'll get off the soap box now.
If the owner is insisting on undersized neutrals, have him put it in writing. Save it cause you will be showing it to him in a couple of years when he calls you back to look at the fire damage or some other problem.
Re: Neutral sizing in Panel Board
What should we do when bidding a multiple feeder job? If I bid it with full sized neutrals I will not get the work. How can I pass up the savings inherent in sizing the neutral only for the maximum imbalance? Why would I size the neutral for current it will never carry?
--
Tom Horne
What should we do when bidding a multiple feeder job? If I bid it with full sized neutrals I will not get the work. How can I pass up the savings inherent in sizing the neutral only for the maximum imbalance? Why would I size the neutral for current it will never carry?
--
Tom Horne
Re: Neutral sizing in Panel Board
Unless you have project dwgs./specs. showing certain minimum size, then it is an NEC & design consideration.
Meet NEC minimum, of course. Then it is a design issue from then on. NEC is not a training manual. Your knowledge of, & experience with, electrical systems has a lot to do with design considerations.
Do not waste & overkill. Anybody can have 500 kcmils hanging out a 3" weatherhead for a 1500 sq. ft. dwelling service.
But at same time, do not cut things short.
Unless you have project dwgs./specs. showing certain minimum size, then it is an NEC & design consideration.
Meet NEC minimum, of course. Then it is a design issue from then on. NEC is not a training manual. Your knowledge of, & experience with, electrical systems has a lot to do with design considerations.
Do not waste & overkill. Anybody can have 500 kcmils hanging out a 3" weatherhead for a 1500 sq. ft. dwelling service.
But at same time, do not cut things short.
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