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Let me see if I got this right...

3K views 17 replies 6 participants last post by  sarness 
#1 ·
Upcoming install, four 50A runs and two 20A runs going in conduit, little tight on space.

Was looking at two 1" runs to jbox then splitting to four 3/4" and one 1/2" down to boxes (1" won't make the clearance)

According to my calcs I can run four #6 and 2 #12 in each conduit and just split it at the jbox (I'll nipple the boxes to get both #12 into one 1/2")

Given 75 degree terminations and derating it looks kosher.

On another location I need four 20A circuits but the 4x4 box is fed by one circuit and is in 1/2"

I can fit 8 conductors in the 1/2" but derating them gives me 18A,

There's no possible way to run another conduit except for surface mount and I'm trying to avoid it at the moment.

And I can't do a MWBC either as its four dedicated circuits coming from a UPS. (actually I think 2 are UPS fed and the other two may be panel fed)
 
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#2 ·
for the four dedicated 120 volt circuits I'm confused as how you only have 18 amps after deration?

110.14(C) Temperature Limitations.​
The temperature rating associated
with the ampacity of a conductor shall be selected
and coordinated so as not to exceed the lowest temperature
rating of any connected termination, conductor, or device.
Conductors with temperature ratings higher than specified
for terminations shall be permitted to be used for ampacity

adjustment, correction, or both.

Did you start your deration from 30 amps?

Pete
 
#4 · (Edited)
75 degree terminations and 8 conductors, according to 310.15 B 16 I'm supposed to start at 25 amps.

Its not drawing 20 amps so I could breaker it at 15, just don't have a list of all the equipment being installed.
If the wire in question is THHN then you can, by virtue of the section I put up in post#2, start out at the 90 degree rating (30 amps for 12 awg).

Then referencing 310.15(B)(3)(a) you would derate to 70%. This results in 21 amps. The adjusted current does not exceed the temperature rating of your terminations so a 20 amp OCPD could still be utilized.

Or, as you said, you could use 15 amp OCPD's. You have a choice.

Pete
 
#6 ·
Look at it this way... 12 awg THHN can, by code, carry 30 amps. That is why the NEC permits you to start your deration from that value (90 degrees).

In your case the adjusted ampacity is still within the parameters of the the 75 degree termination limitation.

The only reason you can't use the 12 awg AT 30 amps is due to the temperature limitation of your terminations.

Pete
 
#8 ·
FWIW... the ultimate outcome, using your logic, would still be code compliant but it would typically result in larger wire which equals additional cost.

I wasn't trying to say you did anything wrong. I was just showing the allowances in the NEC.

Pete
 
#9 ·
I hear ya, I moved recently and not sure where my 05 book is at, going by what my app (elec pro) shows.

I don't do much conduit work so some things are a little fuzzy.

Everything on the 50 amp circuits look alright then? I don't have a 1" bender so I'm going to fudge some premade and get it close. According to the calcs I have to upsize to #6 and I can't put 4 of those in 3/4 (with a #10 ground) otherwise I'd run two 3/4" with #8 but it derates to 42a with 4 conductors.
 
#10 ·
Everything on the 50 amp circuits look alright then? I don't have a 1" bender so I'm going to fudge some premade and get it close. According to the calcs I have to upsize to #6 and I can't put 4 of those in 3/4 (with a #10 ground) otherwise I'd run two 3/4" with #8 but it derates to 42a with 4 conductors.
Can't really say... what are the "50 amp" loads? If you are talking about a 440 or 430 application your MOCP may be larger than the wire you actually need.

Also, you can put four - 6 awg conductors in the 3/4 conduit (assuming EMT) and use the conduit as the EGC.

Pete
 
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