I want to run a 125 amp feeder to an outbuilding. The building with have a on demand water heater with (2) 40 amp 240 volt circuits, (1) 15 amp 240 volt circuit to a ductless mini split system, (1) 20 amp 120 volt circuit for receptacle outlets, and (1) 15 amp 120 volt circuit for lighting. I'm going to use 1/0 aluminum for the hot legs but I'm wondering what the minimum size wire would be required for the neutral since most of the load is going to be 240 volt. Would I be OK just running an #8 copper?
Sizing neutral for feeder
- Thread starter crtemp
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winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
I would think the neutral would need to be at least as large as the required EGC, which would be #6Cu or #4Al. But as you have described the install the max neutral current is 20A, which is all the ampacity required.
I presume you are using single conductors at 75C where 1/0 Al has an ampacity of 120A, protected at 125A for your '125A feeder'.
-Jon
I presume you are using single conductors at 75C where 1/0 Al has an ampacity of 120A, protected at 125A for your '125A feeder'.
-Jon
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I'm looking at Table 250.66 and it is calling for a #8 copper EGC with 1/0 aluminum feeders. Is that not correct?
winnie
Senior Member
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- Springfield, MA, USA
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- Electric motor research
250.66 is for sizing Grounding Electrode Conductors, which connect between the service and the electrodes in the soil.
You want table 250.122 for Equipment Grounding Conductors. There the size of the EGC is based on the rating of the overcurrent device; 100A (or less) circuits are protect by #8 Cu, 200A (or less) by #6 Cu, and so on.
-Jon
You want table 250.122 for Equipment Grounding Conductors. There the size of the EGC is based on the rating of the overcurrent device; 100A (or less) circuits are protect by #8 Cu, 200A (or less) by #6 Cu, and so on.
-Jon
texie
Senior Member
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- Fort Collins, Colorado
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- Electrician, Contractor, Inspector
See 215(A)(2) and 220.61. It can not be smaller than 250.122 but must be large enough to satisfy 220.61.
- Location
- Bremerton, Washington
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- Master Electrician
And when you do a load calc, use KW not amps. See the examples in the back of the NEC. You are looking for the unbalanced load, when you find that convert back to amps
synchro
Senior Member
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- Chicago, IL
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- EE
If the kVA of loads is available then that's preferable to kW because kVA includes any reactive power (kVAR) that will increase the current above that calculated from kW if the power factor is less than 1.0 .
Most of the time it will not make much difference unless there's a significant amount of reactive loads such as motors. I suspect Tom may have meant kVA in the first place.
Do I only need to include the 20 amp 120 volt circuit? Or do I add the 20 and the 15 amp circuit together to get 35A?
(Sorry, I'm not much of a theory guy
)
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
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- Electrical Contractor
The most current the neutral would be subject to is the greatest load placed on one of the two lines.
So, if you put the two 15a loads on one line, and the 20a on the other, then the max is 30a.
If you put the 20a load on one line with one of the 15a loads, the max is 35a.
Unless you expect a lot of plug-in loads, I'd put the AC on one line and the rest on the other.
So, if you put the two 15a loads on one line, and the 20a on the other, then the max is 30a.
If you put the 20a load on one line with one of the 15a loads, the max is 35a.
Unless you expect a lot of plug-in loads, I'd put the AC on one line and the rest on the other.
So if I install (3) 20 amp 120 volt circuits and (1) 15 amp 120 volt circuit, my total max of imbalance would be 40 amps (40 amps on phase a and 35 amps on phase b)?
- Location
- Chapel Hill, NC
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You need a minimum #6 to comply with 250.122 and #6 is more than enough for what you have there.
Perfect. Thanks again guys
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Yes I did I am old school, way back it was WattsMost of the time it will not make much difference unless there's a significant amount of reactive loads such as motors. I suspect Tom may have meant kVA in the first place.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Yes. The greatest neutral current occurs when the more heavily-loaded line is maxed out and the other line has no load. As you load that second line, neutral current drops by the same amount.
However, when you add a panel in the remote building, you have a feeder, not a MWBC, so you need a grounding electrode system. You don't need a main breaker unless you exceed six circuits.
Yes, this is what I installed. I had a total of 7 breakers so I used a 100 amp main breaker panel. Pounded 2 ground rods and ran 4 wires from the main service panel. (2) 1/0 aluminum, (1) #6 copper for neutral and a #8 copper for the ground. Separated grounds and neutrals in the subpanel. Also ended up just installing a 100 amp breaker in the main panel. My original 125 amp was a little overkill. Should be good to go.
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