Derating?

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joebell

joebell

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New Hampshire
I am rewoking circuits into an existing 120/208 panel to accomadate for a future generator.
The existing branch circuits are fed into the panel from a 2" pipe. I calculate that I need at least 42 current carrying conductors in the 2" pipe. I think these should be #10awg but my boss says that #12 awg is ok.I work in Massachusetts and Table 310.15(B)(2)(a) has been amended to allow 21-42 conductors to be derated to 60%. Any thoughts?
 
Re: Derating?

I can?t imagine that much of an adjustment: using 60 % instead of 35%. But I do not work in MA.

A #10 THHN starts with an ampacity of 40, and a 35% derating would bring that down to 14 amps. Unless MA has a different version of that table, as you suggest, I don?t even think a #10 is good enough.
 
Re: Derating?

I didn't believe it until I checked the Massachusetts amendments.

310.15(B)(2)
4 through 6 80 percent
7 through 24 70 percent
25 through 42 60 percent
43 and above 50 percent

I do not agree with this at all but it is to the Massachusetts code :confused:

Justin W
 
Re: Derating?

Charlie it is my understanding the table we use in MA was used for quite some time by the NEC and takes into account 'load diversity' which in this context I believe means "everythings is not fully loaded at the same time".

In many application I believe this to be true.

However there are always those applications that all circuits are fully loaded 24/7 which could be a problem using the MA derating table.

You can see the Table we use in the back of the 2002 NEC

Table B.310.11 Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable with Load Diversity
 
Re: Derating?

Originally posted by charlie b:
I can?t imagine that much of an adjustment: using 60 % instead of 35%.
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You have a hard time imagining it, try working with it from the other direction. :)

I have hard time when I leave MA and work in RI or CT under the standard NEC table. :D

I really have to be careful to work from the NEC and not the MEC. :eek:
 
Re: Derating?

Why don't you use two 3"x4" Wiremold sections and put 21 conductors in each one? This would get you out of derating or at least to the point where #12 is legal for 20 amp circuits and #10 would keep it from getting hot to the touch.
 
Re: Derating?

Originally posted by joebell: so using the Ma. amendment what size condutor do you think I should use #10 or #12
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A #12 THHN starts with a rating of 30 amps. A 60% derating would bring that to 18 amps. You can protect that with the next higher breaker (i.e., 20 amps). So you MIGHT be able to use a #12. The only remaining issue is that the calculated load on each circuit must be less than the 18 amps, or you will have to go with a #10.
 
Re: Derating?

I would go with the #12 AWG THHN. I've never been a big fan of the very prohibitive NEC derating. Load diversity almost always come into play in everyday circuit loading so I can see why the Mass. code would make allowances for it. As far as the fill requirement, if your conduit were 2" EMT than you would be allowed 101 #12 THHN conductors, which is well below the 40% maximum fill requirement.
 
Re: Derating?

Originally posted by infinity: I would go with the #12 AWG THHN.
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So would I, as I said above, but only if the load were less than 18 amps on each and every circuit.
Load diversity almost always come into play in everyday circuit loading . . . .
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Load diversity is a tricky subject. Where safety is concerned, if the NEC (or another authority) does not specifically allow it, and if we cannot prove that it applies, then we should not take advantage of it.
 
Re: Derating?

A 60% derating would bring that to 18 amps. You can protect that with the next higher breaker (i.e., 20 amps).
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Not if the conductors serve receptacles for cord and plug connected equipment.
Don
 
Re: Derating?

To really answer this you need to describe the ampacity of the circuits involved.
If they are all 15 & 20 amp circuits, #12 is suitable. If there are larger circuits it can have an effect on the answer.

As an aside, the derating tables are multipliers. As someone else said, to derate #12 for this application, you multiply the ampacity by the factor (30 X .60 = 18). The tables do not mean that you deduct 60% of the ampacity of the conductors.

Also, the maximum continuous (legal) load allowed on a 20 amp circuit is 16 amps, so the 'next higher' trip value above 18 amps would apply. Therefore #12 will work in this application.
 
Re: Derating?

These circuits are for existing receptacles in the tenant space so I wasn't sure if you could use the next higher size CB. Art.240.4(B) says in order to go to the next size CB all 3 of the conditions listed must be met and #1 has to due with multioutlet branch circuits which makes me think that the next highest CB is not an option and in increase in the conductor size is the only alternitive.
 
Re: Derating?

Joe, I would agree that if all of the conditions of 240.4(B) aren't met than the #12 THHN would have to be protected at 15 amps or #10 THHN would be needed for a 20 amp CB.
 
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