conductor amperage adjustment

[walton]

I have a situation where a contractor has installed 6 #12 current carrying conductors in a 3/4" conduit. (2 grounded conductors and 4 phase conductors). The circuits supply nonlinear continuos lighting loads. After adjustments for continuos loading and more than 3 current carrying conductors in a raceway should these conductor be up sized. Should the adjustment start at 30 amp per Table 310.16 (90*C) or 20 amps per 240.4(D). Please clarify.

 

[don_resqcapt19]

Re: conductor amperage adjustment

If the conductors are 90C, then you start the adjustment at the point. See 110.14(C)
Don

 

[walton]

Re: conductor amperage adjustment

My question is "should #10 wire be used for these circuits instead of #12. And should the derating calculations start with 30 amps or 20 amps for #12 wire per 240.4(D)"?

 

[jbwhite]

Re: conductor amperage adjustment

walton, you have not provided enough information to size the conductors. But what don posted is the answer to your original question.

 

[walton]

Re: conductor amperage adjustment

The breaker is 20 amp 277v rated 75*C and the conductors are #12 thhn. I do not know the total amperage on the circuit but I am concerned about harmonics and warming neutrals with the nonlinear loads.

 

[charlie b]

Re: conductor amperage adjustment

If the conductors have a 90C rating, then you start the derating process at the 90C rating. A set of six #12 THHN conductors would be derated from 30 amps to 24 amps, because of having more than three current carrying conductors in a raceway. 240.4(D) then restricts the overcurrent protection to 20 amps. So the next question is whether a conductor with overcurrent protection set at 20 amps is sufficient for your load.

That is the piece of information you did not give us. What is the load? You told us what kind of load it is, but not how many amps it is.

Please note that you do not "derate" the conductor by virtue of its supplying a continuous load, nor by virtue of its supplying a non-linear load. Rather, you start with the load, and select an overcurrent device on the basis of that load being continuous.

Suppose, for example, that your load is 16 amps, and will be on for more than 3 hours at a time (i.e., it is continuous). That causes you to have to select an OCPD that is 125% of the value of that load. You need an OCPD of 20 amps. I conclude that if your load is 16 amps or lower, you can use the #12s.

On the other hand, suppose your load is 18 amps, continuous. You need an OCPD of 125% of 18, or 22.5 amps. The next higher available is 25 amps. It is 240.4(D), and not Table 310.16, that forces you to upsize to #10.

Does that answer your question?

 

[walton]

Re: conductor amperage adjustment

Yes. Thank you. Looking at these circuits without knowing the loads present a problem during inspection. I can only assume that the loads are less than 16 amps.

 

[charlie b]

Re: conductor amperage adjustment

If you are dealing with single phase, 277 volt, lighting circuits, then a load of 16 amps would equate to 4,432 watts. A standard, overhead, fluorescent fixture will use up 32 watts for each bulb. So that 16 amp load would equate to just under 140 bulbs. During an inspection, you can get an idea of whether the circuit is overloaded by counting the number of lights powered by a single circuit. But it is really up to the designer to make sure that there are not an excessive number of lights on any given circuit.

 

[charlie b]

Re: conductor amperage adjustment

Originally posted by walton: . . . I am concerned about harmonics and warming neutrals with the nonlinear loads.

If you have multi-wire branch circuits that use all three phases of the 277 volt WYE power source, then certain of the harmonics will add up in the neutral. So that is a real concern. One common way to resolve the issue is by doubling the size of the neutral wire.

But if you have single phase, 2-wire (plus ground) circuits, and not multi-wire three-phase circuits, then the total current in the neutral, including the harmonics, will be the same as the current in the hot legs.

 

[infinity]

Re: conductor amperage adjustment

But if you have single phase, 2-wire (plus ground) circuits, and not multi-wire three-phase circuits, then the total current in the neutral, including the harmonics, will be the same as the current in the hot legs.

Charlie, it's been a while so correct me if I'm wrong but if the circuit, consists of a 3 wire multiwire circuit comprised of 2 hot legs and a neutral from a 3 phase wye system than there will no harmonic current present in the neutral? This seems to be what Walton mentioned in the OP.

 

[charlie b]

Re: conductor amperage adjustment

Originally posted by infinity:. . . if the circuit, consists of a 3 wire multiwire circuit comprised of 2 hot legs and a neutral from a 3 phase wye system than there will no harmonic current present in the neutral? This seems to be what Walton mentioned in the OP.

In a single set of 3 wires that form a multi-wire circuit that is served by two phases of a three phase system, there will be harmonics present in the neutral. It just won't be as bad as you get in a 3-phase, 4-wire circuit.