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#77308 - 05/20/01 12:24 AM Derating and adjustment factors  
doc  Offline
Member
Joined: Nov 2000
Posts: 119
Texas
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


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#77309 - 05/20/01 05:14 PM Re: Derating and adjustment factors  
sparky  Offline
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Joined: Oct 2000
Posts: 5,311
doc,
is there a NEC reason to not derate? and why 310-8(a) , i'm a little slow, sorry..... [Linked Image]


#77310 - 05/20/01 09:16 PM Re: Derating and adjustment factors  
doc  Offline
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Joined: Nov 2000
Posts: 119
Texas
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


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#77311 - 05/20/01 09:42 PM Re: Derating and adjustment factors  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
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?
[Linked Image]


#77312 - 05/21/01 01:15 AM Re: Derating and adjustment factors  
Scott35  Offline

Broom Pusher and
Member
Joined: Oct 2000
Posts: 2,707
Anaheim, CA. USA
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


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

#77313 - 05/21/01 06:52 AM Re: Derating and adjustment factors  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,311
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 [Linked Image]

which have you doc?


#77314 - 05/21/01 08:49 AM Re: Derating and adjustment factors  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
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)


Don(resqcapt19)

#77315 - 05/21/01 05:18 PM Re: Derating and adjustment factors  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
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


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