Sizing of neutral for BALANCED 240V single phase service?

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I?m putting in a geothermal system where the power company allows separate metering of the power usage, and a reduced rate for the ?furnace,? blower, well pump, auxiliary resistance heat, and electric water heater.

I?ll need to run about 200 feet of cable from the service drop to the 125 amp breaker box that only powers the above geothermal equipment. The power supply is 240V, single phase, and at full load the system will draw about 96 amps. I plan to use 2/0 aluminum or #1 copper feeders with a #4 aluminum or #6 copper grounding cable attached to grounding rods. All of the equipment from the breaker box is 240V, thus the load is fully balanced. The neutral carries no current, and is only there for ground fault protection.

My question is: since the load is balanced, do I size the neutral the same size as the grounding cable? Where is this addressed in the NEC?
 

augie47

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Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
Where is the 1st means of disconnect (the service disconnect) ?
(Sizng the neutral ahead of the service disconnect/OCP is different from sizing after the OCP.)
 

Smart $

Esteemed Member
Location
Ohio
Where is the 1st means of disconnect (the service disconnect) ?
(Sizng the neutral ahead of the service disconnect/OCP is different from sizing after the OCP.)
One 125 volt branch circuit would be required for a service outlet per 210.63.
The principle is the same... can't be smaller than the GEC or EGC (i.e. service or feeder respectfully)... which would exceed the minimum size based on the maximum unbalanced neutral current as determined by 220.61.

For services 230.42(C)... which takes you to 250.24(C)... then Table 250.66 [2011]

For feeders 215.2(A)(2) takes you to 250.122.
 
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texie

Senior Member
Location
Fort Collins, Colorado
Occupation
Electrician, Contractor, Inspector
The principle is the same... can't be smaller than the GEC or EGC (i.e. service or feeder respectfully)... which would exceed the minimum size based on the maximum unbalanced neutral current as determined by 220.61.

For services 230.42(C)... which takes you to 250.24(C)... then Table 250.66 [2011]

For feeders 215.2(A)(2) takes you to 250.122.

I might add a word here. Yes, on the supply side it needs to be equal to the GEC, BUT if the phase conductors are larger than 1100 CU, then you have to use the 12.5% rule. Just wanted to clarify that the grounded conductor minimum never max's out like a GEC.
As you know, the was redone in the 2014 edition in an attempt to make this more clear but the end result is the same.
 
Thanks for help. Further clarification.

Thanks for help. Further clarification.

Thanks to everyone for your excellent advice.

For your own reference, here's the general layout:

METER ---5 ft --> 125 AMP DISCONNECT -- 12 ft --> GENERAC TRANSFER SWITCH -- 180 ft --> SUBPANEL W/ 125 AMP MAIN BREAKER -- 20 ft --> GEOTHERMAL FURNACE, WELL PRESSURE SWITCH, HW HEATER

(The system is being installed in Michigan, and with the new geothermal system, a simple portable generator isn't enough to keep the house warm in the winter. The system will have a 20 Kw whole house backup generator.)

I'd welcome any other comments regarding the design.
 
Further questions

Further questions

One 125 volt branch circuit would be required for a service outlet per 210.63.

I have two questions about this:

Is a service outlet required even though a 125 V outlet, powered by the general power supply to the structure, is within 3 feet of the (a) main disconnect, (b) a transfer switch, and (c) the subpanel?

If service outlets are required, are they required at each of the above three locations. If not, where would they be needed?
 

Smart $

Esteemed Member
Location
Ohio
I have two questions about this:

Is a service outlet required even though a 125 V outlet, powered by the general power supply to the structure, is within 3 feet of the (a) main disconnect, (b) a transfer switch, and (c) the subpanel?

If service outlets are required, are they required at each of the above three locations. If not, where would they be needed?
The referenced service outlet is for HVAC equipment. None are required for or located by the items you mention.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
The Code requires a neutral to your 125 amp main disconnect at the meter 250.24(C) and your transfer switch may need one, but if there is no 120v load at the 125 amp panel at the end of your 180ft run there is no neutral required.
Size your equipment ground per 250.122 keeping in mind the need to increase the equip. ground if you upsize your feeder conductors. IF you do need a neutral at the panel, 215,2 requires it be no smaller than the equipment ground,.
 

jrohe

Senior Member
Location
Omaha, NE
Occupation
Professional Engineer
The referenced service outlet is for HVAC equipment. None are required for or located by the items you mention.

If under the 2014 NEC, a 125 volt receptacle is required within 50 feet of the service equipment (the 125 amp disco) per 210.64 unless it is a single or two-family dwelling.
 

Smart $

Esteemed Member
Location
Ohio
If under the 2014 NEC, a 125 volt receptacle is required within 50 feet of the service equipment (the 125 amp disco) per 210.64 unless it is a single or two-family dwelling.
Good catch. That's entirely new for 2014. Still on 2011 here in Ohio. Same for OP'er in Michigan.
 
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