george t. everett
Senior Member
- Location
- New York
When adjusting conductor ampacity is the neutral of a 3-wire or 4-wire circuit counted as a current carrying conductor?
Wire derating
- Thread starter george t. everett
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wirebender
Senior Member
- Location
- North Central Texas
Re: Wire derating
If the 3-wire circuit is single phase 120/240 the grounded conductor (neutral) is not counted.310.15(B)(4)(a).
If the 3-wire circuit is on a 4-wire,3-phase,wye-connected system the neutral is counted.310.15(B)(4)(b)
On a 4-wire,3-phase wye circuit, it is not counted unless the major portion of the load is nonlinear loads(ballasts,etc.).310.15(B)(4)(c).
[ July 10, 2004, 08:51 AM: Message edited by: wirebender ]
If the 3-wire circuit is single phase 120/240 the grounded conductor (neutral) is not counted.310.15(B)(4)(a).
If the 3-wire circuit is on a 4-wire,3-phase,wye-connected system the neutral is counted.310.15(B)(4)(b)
On a 4-wire,3-phase wye circuit, it is not counted unless the major portion of the load is nonlinear loads(ballasts,etc.).310.15(B)(4)(c).
[ July 10, 2004, 08:51 AM: Message edited by: wirebender ]
paulternstrom
Member
Re: Wire derating
Also on a similar note, we have a plan checker on one of our projects saying that if our panel load is more than 10% out of balance phase-to-phase, then we have to count the neutral as "current carrying". I can't find anything in the code to support this.
I have always tried to balance the phase loads on panels to within 10% of each other as a good design criteria, but I can't support it by a code reference. Anybody have any input on this?
Who defines what is a "major portion"? 50.5%? Is there anything specific in the NEC?
Also on a similar note, we have a plan checker on one of our projects saying that if our panel load is more than 10% out of balance phase-to-phase, then we have to count the neutral as "current carrying". I can't find anything in the code to support this.
I have always tried to balance the phase loads on panels to within 10% of each other as a good design criteria, but I can't support it by a code reference. Anybody have any input on this?
Re: Wire derating
As far as out phase-phase ballance goes: any increase in neutral current is offset by a corresponding decrease in the current in a phase conductor. (If all phases are maxed out, there is no unbalance. Then an unbalance occurs as the current on one phase decreases.)
I can't quote an article that says you don't have to worry about phase ballance, but I don't think that was the intent of the code.
Steve
I think the handbook says 50% or more. The odd thing is there is no real definition for non-linear. That also comes in varying degrees... 5% THD, 10% THD, ect. So who decides if the lighting ballasts with less than 10% thd is non-linear?
As far as out phase-phase ballance goes: any increase in neutral current is offset by a corresponding decrease in the current in a phase conductor. (If all phases are maxed out, there is no unbalance. Then an unbalance occurs as the current on one phase decreases.)
I can't quote an article that says you don't have to worry about phase ballance, but I don't think that was the intent of the code.
Steve
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Wire derating
I once worked out a proof that the I*2R losses (that create the heat within a raceway) in the four wires (A, B, C, and N) can never be higher than the heat losses from a balanced load on A, B, and C, no matter how the loads are actually balanced. I don?t have it handy, so I can?t post it. But I recall that it was a simple matter of trigonometric additions.
Very true. Consider the extreme case. If you load a 3-phase panelboard with all the loads on phases A and B, then C will have no current, and the neutral will carry the same current as A and B. You will see three conductors carrying current.
I once worked out a proof that the I*2R losses (that create the heat within a raceway) in the four wires (A, B, C, and N) can never be higher than the heat losses from a balanced load on A, B, and C, no matter how the loads are actually balanced. I don?t have it handy, so I can?t post it. But I recall that it was a simple matter of trigonometric additions.
george t. everett
Senior Member
- Location
- New York
Re: Wire derating
Does the nonlinear load only effect the neutral current on Wye systems or does it also effect delta systems too?
Does the nonlinear load only effect the neutral current on Wye systems or does it also effect delta systems too?
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: Wire derating
It affects both. It adds current to the normal 60 cycle load. The current flows in the transformer windings and in the conductors to and from the loads. It all adds up.
It affects both. It adds current to the normal 60 cycle load. The current flows in the transformer windings and in the conductors to and from the loads. It all adds up.
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