When are you allowed to downsize the neutral?
- Thread starter zappy
- Start date
- Status
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Are you asking about services, feeders or branch circuits?
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
See 220.61, but also 215.2(A)(2).
Probably trivia for you: 705.95(B).
Probably trivia for you: 705.95(B).
MAC702
Senior Member
- Location
- Clark County, NV
Yes.
kwired
Electron manager
- Location
- NE Nebraska
For most part the general rule is it only needs ampacity to carry the connected load, but for services can never be smaller then SSBJ and for feeders can never be smaller than the required EGC.
- Location
- Bremerton, Washington
The minimum size of the neutral is determined by if its a white wire or white wire with an an imaginary green stripe.
For services, the neutral is sized to carry the maximum unbalanced current (white) or the minimum size for a fault (white with green stripe)
White size is determined by a load calculation and the white with imaginary green stripe is table 250.66.
We often don't use the minimum size allowed and for dwelling units its mostly 4/0-4/0-2/0 Al where the minimum size would be 2 awg Al
Questions
Why 2/0 when the minimum is 2 AWG?
What would the inspector do if he/she saw a 2 AWG AL neutral?
The rules for feeders are a bit different and are based on the OCPD size.
For services, the neutral is sized to carry the maximum unbalanced current (white) or the minimum size for a fault (white with green stripe)
White size is determined by a load calculation and the white with imaginary green stripe is table 250.66.
We often don't use the minimum size allowed and for dwelling units its mostly 4/0-4/0-2/0 Al where the minimum size would be 2 awg Al
Questions
Why 2/0 when the minimum is 2 AWG?
What would the inspector do if he/she saw a 2 AWG AL neutral?
The rules for feeders are a bit different and are based on the OCPD size.
kwired
Electron manager
- Location
- NE Nebraska
Where is this white with imaginary green stripe mentioned in NEC?:huh:
You messing with us, auto correct change something on you, or have I been missing something for a very long time?
As far as 4/0 AL with 2/0 neutral, I think many inspectors will accept that combination without you having to prove the maximum neutral load, especially on 120/240 single phase and similar for many other combinations, usually one or two sizes smaller on the neutral and you don't need to prove neutral load.
I recently ran 3-4/0 and #2 service conductors for something that doesn't have any neutral load. That is a case where no calculation is needed but you must still have a minimum grounded conductor size.
All of the above. Mainly for commercial or industrial applications.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
Gentle correction... the correct table would be 250.102(C)(1), not 250.66.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
The service neutral need not have extend beyond the service disconnect in that case. The neutral ends, the electrodes are bonded, and the EGC system begins all in the same place, and all can be bare.
kwired
Electron manager
- Location
- NE Nebraska
Correct, except in my case underground aluminum can't be bare.
- Location
- Bremerton, Washington
White with a green imaginary stripe is a concept I picked up from mike holt. I used it in my classes to explain sizing the neutral.
We use 2/0 as it comes with 4/0 in triplex. NEC used to specify neutral was two trade sizes smaller, but now its either sized on unbalance current or min fault current size.
If you use anything smaller than 2/0 (for dwelling unit) the AHJ will want load calcs.
There is a difference between the code answer and the trade answer.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
A neutral for a service can be downsized as small as the grounding electrode conductor if the load calculates that low, however in most commercial jobs you will not see a spec with a downsized neutral. In fact, you are more likely to see an oversized neutral.
A feeder also may be downsized based on load calculation and can be as small as an equipment grounding conductor. However, not all feeders even need a neutral but if you use one it cannot be smaller than the equipment grounding conductor.
Most branch circuits do not allow a downsized neutral. Ranges and dryer are 2 exceptions in the NEC. I assume if it is allowed in some cases then the smallest it can be sized would also be the size of the equipment grounding conductor.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
There are repeated references in this thread indicating/implying that it can be sized to the GEC as a minimum. This is incorrect and always has been. The minimum would be 250.102.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
You are correct except that in 99% of the time it is the same... They changed it in the last cycle or so and the bonding conductor is the limit-- which is often the same as the grounding electrode conductor until you get to really large services of over 1750 kcm auminum or it's equivalent in cm. withparallel conductors
Did you count the neutral as a CCC ??
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
Yes, I would agree, in most cases it will be the same as 250.66. As I'm sure you know, the issue is that GECs max out at 3/0 but grounded conductors, bonding conductors and jumpers never max out in size. Making this difference clear was one of the driving forces of creating Table 250.102(C)(1) starting in the 2014 edition.
Even prior this this change a grounded conductor did not max out out 3/0 in table 250.66, it was just harder to understand the notes that required it.
kwired
Electron manager
- Location
- NE Nebraska
Didn't carry any current for one thing, was neutral for wye system with non majority of load being non-linear load, so worst case it only carries imbalance current from the phase conductors anyway. The load was a 75 HP motor and a center pivot irrigation machine - no neutral load for either, type of installation that is about all that is ever there for load and no neutral load is normally expected.
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