Re: Relocating Residential Service
If there are any nonlinear loads, the grounded conductor (neutral) is counted as a current carrying conductor.
Where does the NEC say that?
310.15(B)(4) Neutral Conductor.
(a)A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(2)(a).
(b)In a 3-wire circuit consisting of two phase wires and the neutral 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 and shall be counted when applying the provisions of 310.15(B)(2)(a).
(c)On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
My question. In a dwelling where one may have 1 or 2 computers and maybe a few flourescents, is that enough of a nonlinear load to qualify the neutral of a three wire multiwire branch circuit to be included as a current carrying conductor?
I only see references to treating the neutral as a current carrying conductor for 4-wire, 3-phase wye circuits where "the major portion of the load consists of nonlinear loads".
IMO one or two computers and some fluorescent lighting is not a major portion of the load.
I guess you could say 310.15(B)(4)(a) requires this but if they meant (a) to cover non-linear loads what would be the need for (c)?
Anyone with the 2002 NEC Handbook should take a look at the commentary following 310.15(B)(4). They present a lot of info on Non-Linear loads and the effect on equipment.
Here are a couple of quotes.
The data were obtained from consultants, equipment manufacturers, and testing laboratories, and included hundreds of feeder and branch circuits involving 3-phase, 4-wire, wye-connected systems with nonlinear loads. The data revealed that many circuits had neutral conductor current greater than the phase conductor current, and approximately 5 percent of all circuits reported had neutral conductor current exceeding 125 percent of the highest phase conductor current.
125% sounds extremely damaging until they go on to point out.
.....the total continuous load on any overcurrent device located in a panelboard should not exceed 80 percent of its rating (the exception being assemblies listed for continuous operation at 100 percent of its rating). Because the neutral conductor is usually not connected to an overcurrent device, derating for continuous operation is not necessary. Therefore, neutral conductor ampacity is usually 125 percent of the maximum continuous current allowed by the overcurrent device.
There is much more in the handbook it is worth a look.
[ February 21, 2004, 07:44 AM: Message edited by: iwire ]
