ECN Forum Forums Code Related Discussion NEC & other Code issues Derating and adjustment factors

lets say u are running 440 volt 3 phase equipment.now inside conduit u run 21 wires all thhn #12 stranded these wires are going to 440 volt 3 phase motors do they get derated just like any other wire and if you get to a j-box and cut them into u now have 42 wires in that box would u then use a different derating # or would u still stay with 21 article 310-8{a} hope this makes since to u guys

doc,
is there a NEC reason to not derate? and why 310-8(a) , i'm a little slow, sorry.....

SORRY FOR THE MIX UP I am looking in article 310 and in the 96 code book on page 196 u have 8 Adjustment factors then u have {a} at that point it shows 21-30 wires if I have 21 then I would derate them to 45 percent.Now i go to a junction box i cut them into at that point I now hve 42 wires in the j-box would I derate those wires using 41 or more which would be 35 percent or even though I did cut them in the j-box it would still be 21 wires hope this helps

oh, ok,,,
the 'pipe fill' bears derating,
30A(T310-16) x 45%(T310-15(b)(2))= 13.5A,
so next 240-6 would be 15A

the 'jb' needs only to meet fill requirements. As the wires have been cut, instead of passing thru, i beleive your fill requirements are doubled.
21 x 2.25= 47+ cubes
42 x 2.25= 94.5 cubes

ok doc?

doc,
Derate like this:

Table 310-16: #12 THHN cu @ 90 degrees = 30 amps.
Table 310-15(b)(2)(a): Number of current carrying conductors: 21-30 = 45%
Multiply 30 amps x .45 = 13.5 Amps

This is for 30 degrees C Ambient Temperature. If higher than 30 C, adjust maximum amperage of 310-16 [30 amps for #12 THHN cu] by the corection factor, then derate for 21-30 conductors.
If the loads are LCL, then derate the maximum amperage [30 amps for #12 THHN cu] by 80%, making it 24 amps - then apply temperature corrections [if any], then finally derate for 21-30 current carrying conductors.

The conductors in the raceway [conduit] are the ones being derated. Box fill is another area.

Steve had already pointed out the corrected value of 13.5 amps - using 15 amp OCPD, so this message is just to explain how he came up with that figure.

Scott SET

ok guys...
that's the normal routine.
doc's got motor circuits here, so let's apply 430, part D if branch circuits, part F if control circuits

which have you doc?

Guys,
Remember that with motor circuits the dreated ampacity of the conductors only has to equal 125% of the motor current as shown in Tables 430-147 through 430-150. It does not have to equal the rating of the branch circuit short-circuit and ground fault protection device.
Don(resqcapt19)

Another reminder that doesn't apply to this post, but seems to be a common oversight, you cannot round up to 15 amps if the conductors are part of a multi-outlet branch circuit that supplies receptacles for cord and plug connected portable loads.

Tom