Neutral Calculation

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Npstewart

Senior Member
I know there have been several forums on neutral calculations however the ones I saw always seem to be for residential applications, or didn't quite answer the question exactly for me.

I have a project where we have to reduce the neutral in order to use the existing conduit. Incoming voltage is 208-3 phase

I was told I need to add up all the loads on phase A,B,&C and divide the largest one by 120V, would this be correct? I wouldn't divide by the incoming to the voltage of 208-3?

Side question:

The smaller conductor would still be protected by the over current protection for the larger wires, what would be protecting this smaller conductor?"
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
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Retired Electrical Contractor
Here is what the NEC states

220.61 Feeder or Service Neutral Load.
(A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.
Exception: For 3-wire, 2-phase or 5-wire, 2-phase systems, the maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor multiplied by 140 percent.
(B) Permitted Reductions. A service or feeder supplying the following loads shall be permitted to have an additional demand factor of 70 percent applied to the amount in 220.61(B)(1) or portion of the amount in 220.61(B)(2) determined by the basic calculation:
(1) A feeder or service supplying household electric ranges, wall-mounted ovens, counter-mounted cooking units, and electric dryers, where the maximum unbalanced load has been determined in accordance with Table 220.55 for ranges and Table 220.54 for dryers
(2) That portion of the unbalanced load in excess of 200 amperes where the feeder or service is supplied from a 3-wire dc or single-phase ac system; or a 4-wire, 3-phase, 3-wire, 2-phase system; or a 5-wire, 2-phase system
(C) Prohibited Reductions. There shall be no reduction of the neutral or grounded conductor capacity applied to the amount in 220.61(C)(1), or portion of the amount in (C)(2), from that determined by the basic calculation:
(1) Any portion of a 3-wire circuit consisting of 2 ungrounded conductors and the neutral conductor of a 4-wire, 3-phase, wye-connected system
(2) That portion consisting of nonlinear loads supplied from a 4-wire, wye-connected, 3-phase system
FPN No. 1: See Examples D1(a), D1(b), D2(b), D4(a), and D5(a) in Annex D.
FPN No. 2: A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral-conductor currents.
Click to expand...
 
V

Volta

Senior Member
Location
Columbus, Ohio
Npstewart said:
I know there have been several forums on neutral calculations however the ones I saw always seem to be for residential applications, or didn't quite answer the question exactly for me.

I have a project where we have to reduce the neutral in order to use the existing conduit. Incoming voltage is 208-3 phase

I was told I need to add up all the loads on phase A,B,&C and divide the largest one by 120V, would this be correct? I wouldn't divide by the incoming to the voltage of 208-3?
Click to expand...
The neutral can't be smaller than the required EGC (assuming not a service) 215.2(A), but only will carry the neutral loads. So three-phase loads (without a neutral) will not add current to the neutral. The phase leg with the largest total load to the neutral divided by the voltage to neutral should work.
Side question:

The smaller conductor would still be protected by the over current protection for the larger wires, what would be protecting this smaller conductor?"
Click to expand...
Just math.
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Here is the basic calculation

A) Basic Calculation. The feeder or service neutral load shall be the maximum unbalance of the load determined by this article. The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.
 
H

haskindm

Senior Member
Location
Maryland
Be sure that you understand that there is no neutral load on three phase equipment. A three phase A/C unit for example will probably not even have a neutral (Grounded) conductor run to the unit as the load on the grounded (neutral) conductor is zero. You may even come up with a total calculated neutral load of zero amps for this panel, if all of the equipment is 3-phase. Then you would size the neutral wire equivalent to the Grounding electrode conductor using table 250.66. There is never (well almost never) an overcurrent device place in the neutral (grounded) conductor.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Dennis Alwon said:
The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor.
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That says it all. The neutral must be either that required above, or the size a reuired EGC would have to be, whichever is larger.
 
N

Npstewart

Senior Member
Ok, and according to a post on page 2 I believe, when doing the neutral calc, if you have a 3 phase load, you don't count that at all? For instance I have a RTU that has 3.5KW on each leg, A,B,C. When im doing the neutral calc, I wouldn't count 3.5KW for each phase? I know there is no neutral wire going to the RTU however it is adding a load to each phase.

So in theory if I had a panel with one RTU on it, my neutral load would be zero, and I could size the neutral going to that panel by immediately using table 250.66 if I really wanted to?
 
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Npstewart said:
Ok, and according to a post on page 2 I believe, when doing the neutral calc, if you have a 3 phase load, you don't count that at all? For instance I have a RTU that has 3.5KW on each leg, A,B,C. When im doing the neutral calc, I wouldn't count 3.5KW for each phase? I know there is no neutral wire going to the RTU however it is adding a load to each phase.

So in theory if I had a panel with one RTU on it, my neutral load would be zero, and I could size the neutral going to that panel by immediately using table 250.66 if I really wanted to?
Click to expand...

That is correct. Three phases loads with no neutral do not count for the neutral calculation.

If you had a 3 phase load with no neutral as you mention above then you don't need to pull a neutral at all. If you had a panel with rtu and other loads then you would use T. 250.122 for the lowest possible conductor for the neutral. It cannot be smaller then the egc.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Npstewart said:
Ok, and according to a post on page 2 I believe, when doing the neutral calc, if you have a 3 phase load, you don't count that at all? For instance I have a RTU that has 3.5KW on each leg, A,B,C. When im doing the neutral calc, I wouldn't count 3.5KW for each phase? I know there is no neutral wire going to the RTU however it is adding a load to each phase.

So in theory if I had a panel with one RTU on it, my neutral load would be zero, and I could size the neutral going to that panel by immediately using table 250.66 if I really wanted to?
Click to expand...
Correct, for a service panel. For a feeder panel, 250.122.

Line-to-line loads (i.e. 3? and 1? L-L) do not contribute to neutral current. The only loads that contribute to the maximum unbalanced neutral current are connected line-to-neutral.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
Npstewart said:
... when doing the neutral calc, if you have a 3 phase load, you don't count that at all?
Click to expand...
Right. If you could open two of the three phases, only line-to-neutral loads on the remaining phase would have any current.

Whichever line has the greatest line-to-neutral load current is the one the neutral needs to be sized for.
 
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