Feeder Neutral Question

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boku0003

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feeder

feeder

Smart $ said:
IMO, the "more info" makes the question even more confusing...

Again making assumptions, an 800A 480/277V service would require at least parallel 500kcm Cu 75?C conductors for 701A to 760A calculated load, or parallel 600's fro 761 up to 800A calculated load. The minimum neutral service conductor by 250.66 GEC sizes respectively would be 2/0 and 3/0 Cu, neither of which are close to 400, the lowest value answer to choose from.

There has got to be some pertinent info missing :confused:
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Perhaps then the answer is 800 amps, which would mean the neutral is sized the same size as the service conductor.
 
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wawireguy

Senior Member
Was wondering where the NEC discusses using the larger of 220.61 or 250.24(C)(1) ?

I'm studying sizing of grounded conductors and am a little confused on that point.



Smart $ said:


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boku0003,

Sizing a service or feeder neutral is a two part qualification. First you must calculate the maximum unbalanced current on the neutral and determine the size necessary to carry such current. Next, you determine the minimum size of a grounding electrode conductor per Table 250.66 based on the service or feeder line conductor size. The neutral must be sized to the larger of the two size determinations. The larger of the two sizes is used to ensure the service or feeder neutral is capable of carrying the full amount of trip current in the event of a short-circuit.

Generally speaking, if all of your total load is line-to-neutral loads and balanced, the maximum unbalanced neutral current will require the neutral to be sized larger than a minimum permitted GEC size, because the neutral conductor will be the same, or nearly the same size as the line conductors (because neutral conductors, without OCP, are permitted to be sized at 100% of the continuous current, whereas line conductors have to be sized using 125% of continuous current).
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Smart $

Smart $

Esteemed Member
Location
Ohio
wawireguy said:
Was wondering where the NEC discusses using the larger of 220.61 or 250.24(C)(1) ?

I'm studying sizing of grounded conductors and am a little confused on that point.
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The NEC does not discuss the matter directly... it is left to logical deduction.

The code says (reference is an excercise left for the user ;)), paraphrasing, your conductors must have an ampacity sufficient to carry maximum loading.

220.61 specifies how the maximum service or feeder neutral load is to be calculated.

250.24(C)(1) simply imposes a minimum size.
 
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boku0003

Member
The neutral conductor (grounded) needs to be sized between a minimum of table 250.66 and the largest ungrounded conductor. It can be somewhere in between. There are a number of ways to determine this and that all depends on what the circuit is. 220.61 and 310.15(B)(2)(4) help determine this. But basically, it is the unbalanced current. You can apply a 70% demand factor given certain conditions (see 220.61).

So if you have 300 amps on one leg and 350 on the other, it needs to be sized to handle 50 amps, 8 AWG. However, the largest ungrounded conductor is rated for 350 amps, 500 kcmil, which limits the neutral to 1/0 according to 250.66.

For some circuits (2-wire), the neutral carries the load current, so it is sized the same size as the ungrounded conductor.
 
Smart $

Smart $

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Location
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boku0003 said:
...

So if you have 300 amps on one leg and 350 on the other, it needs to be sized to handle 50 amps, 8 AWG. ....
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Ummm... not so.

If you have a 3-wire circuit (L1, L2, N) with 300A of L-N loads on L1 and 350A of L-N loads on L2, then the Neutral must be sized to carry the maximum unbalanced current of 350A.

When you determine the maximum unbalanced current, it is as though the line with the most line-to-neutral loading is connected to the neutral as if it were a 2-wire circuit, or all other line-to-neutral loads are turned off.
 
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boku0003

Member
Here is an example from the website:

"When sizing the load for a 3-wire circuit, the grounded neutral conductor must carry the unbalanced load of the two ungrounded phase conductors. This type of installation has an unbalanced load - unless both ungrounded conductors pull the same amount of current on each ungrounded phase conductor.

Example: What is the unbalanced neutral load for a 3-wire circuit carrying 64A and 52A on the ungrounded phase conductors?

Step 1: Find amperage per Sec. 220-22 and Sec. 310-15(b)(4)(a).

Ungrounded phase conductor: Phase A = 64A

Ungrounded phase conductor: Phase B = 52A

Unbalanced load = 12A

Solution: The grounded neutral conductor load is 12A for the unbalanced condition."

According to 310.15(B)(4)(b), "In a 3-wire circuit consisting of 2 phase conductors and the neutral conductor of a 4-wire, 3-phase, wye connected system, a common conductor carries approximately the same current as the line-to-neutral load currents of the other conductors..."

So perhaps the example problem they give is a different situation. It is a 3-wire, but they do not specify that it is of a 4-wire, 3-phase wye connected system.
 
Smart $

Smart $

Esteemed Member
Location
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boku0003 said:
Here is an example...

So perhaps the example problem they give is a different situation. ...
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The difference between your example and the requirement of 220.61 is the word "maximum". For instance, in your example, the "maximum" unbalanced current is 64A... same as if you turned off the breaker connecting Phase B to the load.
 
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boku0003

Member
"You must determine the size of the neutral conductor (based on its use with ungrounded circuit conductors) carefully. For example, the manner in which it shares loads between the other conductors determines if you can reduce its ampacity rating."

What examples can you think of where the neutral will not be sized to the largest ungrounded conductor? I know that you can apply 70% demand factor above 200 amps for linear loads (220.61(B)(2)). Is there ever a situation where it will be sized to the unbalanced current between the phase conductors, or to 250.66? Because 220.61(A) says "The feeder or service neutral load shall be the maximum unbalance of the load determined by this artice." Then it goes on to say that "The maximum unbalanced load shall be the maximum net calculated load between the neutral conductor and any one ungrounded conductor". That one statement seems to suggest that you should just size your neutral the same as your ungrounded conductors, which I know is not the case as there are cables that have neutral conductors that are smaller than the ungrounded conductor.
 
Smart $

Smart $

Esteemed Member
Location
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boku0003 said:
...

What examples can you think of where the neutral will not be sized to the largest ungrounded conductor? ...
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Where the service or feeder supplies 3? and line-to-line 1? loads, as neither of these contribute to the neutral current.

Say you have a perfectly balanced 208/120 3? 4W panel with a total load of 30kVA, and exactly 50% of the load was 3? and 1? L-L loads, then only 15kVA would be L-N loads. If the latter is also perfectly balanced the maximum unbalance would be 5kVA. Divide this number by your L-N voltage (120) and you would have your maximum unbalanced current of the feeder neutral. The size necessary to carry this amount of current would be smaller than the ungrounded conductors.
 
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boku0003

Member
Where did you get the 5kVA? If it's perfectly balanced, shouldn't it be 0 (in which case 250.66 would be used for sizing...)?
 
Smart $

Smart $

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Location
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boku0003 said:
Where did you get the 5kVA? If it's perfectly balanced, shouldn't it be 0 (in which case 250.66 would be used for sizing...)?
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If you have perfectly balanced L-N loads of 15kVA on a 208/120 panelboard, the 15kVA is apportioned to three lines at 5kVA each.

Yes if it is perfectly balanced the neutral current would be 0... IF everything is on and drawing calculated power. But what if only the ?A L-N loads are on and all ?B and ?C L-N loads are off? The feeder neutral would be carrying 5kVA of power. This is the maximum unbalanced current condition for the panelboard.
 
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boku0003

Member
Clear up

Clear up

Ok. Just want to clear this up.

Minimum neutral (grounded) conductor is based on T250.66

However is it based on T250.122 if it is a feeder neutral? See 215.2(A) which says "The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122". I thought T250.122 was only used for the grounding electrode conductor.

Perhaps 250.102 (C) and (D) which says 250.66 is for the supply side and 250.122 is for the load side of service equipment clears this up.
 
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boku0003

Member
One other note I'd like to make is that I looked at the tables and it looks like 250.66 is sized a little bigger than 250.122.

For example if you have 200A (after all calcs), you use a 3/0 ungrounded conductor. This would require a minimum 4 AWG neutral/GEC and a 6 AWG EGC.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
boku0003 said:
Ok. Just want to clear this up.

Minimum neutral (grounded) conductor is based on T250.66

However is it based on T250.122 if it is a feeder neutral? See 215.2(A) which says "The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122". I thought T250.122 was only used for the grounding electrode conductor.

Perhaps 250.102 (C) and (D) which says 250.66 is for the supply side and 250.122 is for the load side of service equipment clears this up.
Click to expand...

boku0003 said:
I meant to say that T250.122 is for the equipment grounding conductor.
Click to expand...

boku0003 said:
One other note I'd like to make is that I looked at the tables and it looks like 250.66 is sized a little bigger than 250.122.

For example if you have 200A (after all calcs), you use a 3/0 ungrounded conductor. This would require a minimum 4 AWG neutral/GEC and a 6 AWG EGC.
Click to expand...
Seems like your still missing the full scope of requirements for sizing a grounded neutral service or feeder conductor.

Minimum size is a two-part requirement: Not smaller than the larger of 220.61 or 250.66.
 
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