Sub Panel

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goldstar

goldstar

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Let's make sure we're on the same page with this and that the terminology is correct. The "sub-panel" that you're installing on another floor does not need a "main" breaker inside it. However, the wiring feeding that panel should be protected and that would be via the breaker you install in the panel you're feeding it from. I wasn't sure that was understood by Infinity's lenghty answer.:)
 
infinity

infinity

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Mrfixit said:
The new panel is a 100 amp Square D w/ bolt in breakers. I ran 2" EMT between them with 2 AWG THHN for the feeds and #8 ground.
Click to expand...

Could this be a problem with 250.122(B) or are your terminations only rated for 60? C?
 
M

Mrfixit

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I don't see where the #8 will be a problem. Do you suggest I increase it to a #6? There are 4 other circuits in the feeder panel wired this same way. Not that they are right, I'm just trying to follow code as I see it.
 
Twoskinsoneman

Twoskinsoneman

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infinity said:
Could this be a problem with 250.122(B) or are your terminations only rated for 60? C?
Click to expand...

Mrfixit said:
I don't see where the #8 will be a problem. Do you suggest I increase it to a #6? There are 4 other circuits in the feeder panel wired this same way. Not that they are right, I'm just trying to follow code as I see it.
Click to expand...

Is the idea that for a 100a ampacity you could have used a #3 ungrounded wire? and #8 EGC. But you increased it to a #2, The increase is about a 17% increase in area so you have to use a EGC wire with at least a 17% increase in area also? Forcing you to increase to a #6 EGC?

Am I getting the gest correct?
 
Dennis Alwon

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You don't even need the egc with emt but #2 at 75C is required for 100 amp. If you install an EGC then it only needs to be #8 unless we are talking aluminum and not copper.
 
Twoskinsoneman

Twoskinsoneman

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I just wanted to add this I came across.
the ROP is long so I won't post it but the panel statement explaining the rejection to the proposal is:

5-290 Log #1511 NEC-P05 Final Action: Reject
Panel Statement: The recommended text does not improve the understanding
of the provision of Section 250.122(B). The panel concludes there are
numerous reasons for increasing the size of the ungrounded conductors in
addition to ampacity adjustment and correction factors such as considerations
for voltage drop, overcurrent device performance, and other engineering
factors. The NEC is a minimum requirement and this section is addressing any increase over the minimum required by code.

I think if you increased from the minimum required (#3 cu) than you have to increase the EGC from it's minimum required (#8 cu)

What do you guys think?
 
Dennis Alwon

Dennis Alwon

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Twoskinsoneman said:
My thinking is it is the ONLY example of upsizing... If you COULD use a smaller wire, than you are using a bigger wire right? What else would be upsizing?
Click to expand...

That is the problem with 250.122(B). Do we read the 60C or 75C column. Either size is appropriate so I don't see the issue but who knows...

For instance if I have a load of 55 amps I can use #6 nm on a 60 amp circuit breaker however if I want to allow for a true 60 amp circuit I would have to use #4 NM. So is that upsizing-- remember I can use a DP 70 but they cost a bit more than a DP60.
 
Twoskinsoneman

Twoskinsoneman

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Dennis Alwon said:
That is the problem with 250.122(B). Do we read the 60C or 75C column. Either size is appropriate so I don't see the issue but who knows...

For instance if I have a load of 55 amps I can use #6 nm on a 60 amp circuit breaker however if I want to allow for a true 60 amp circuit I would have to use #4 NM. So is that upsizing-- remember I can use a DP 70 but they cost a bit more than a DP60.
Click to expand...

I'm really not arguing just trying my best to understand. It seems that using the 75C would be the minimum (assuming termination rating and yadda yadda...). So it would seem (at least implied by the panel statement I posted) that any increase from that would invoke 250.122(B).
Also I'm not sure WHY it must be increased. If you run #4 for a 15A recept won't a #14 wire still handle fault current to open the breaker?
 
Little Bill

Little Bill

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Twoskinsoneman said:
Is the idea that for a 100a ampacity you could have used a #3 ungrounded wire? and #8 EGC. But you increased it to a #2, The increase is about a 17% increase in area so you have to use a EGC wire with at least a 17% increase in area also? Forcing you to increase to a #6 EGC?

Am I getting the gest correct?
Click to expand...

I think you are a little off on the increase from #3 to #2.
#2 has a cir. mil area of 66360
#3 has a cir. mil area of 52620
66360/52620=1.26
according to my calculations
 
Twoskinsoneman

Twoskinsoneman

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West Virginia, USA NEC: 2020
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Facility Senior Electrician
Little Bill said:
I think you are a little off on the increase from #3 to #2.
#2 has a cir. mil area of 66360
#3 has a cir. mil area of 52620
66360/52620=1.26
according to my calculations
Click to expand...

Thanks for the correction. My calculation was done extremely hastily and I may not have even had the right kind of wire. Nor am I gonna check to see if yours are right because the point of it was the idea not really the numbers. :)
 
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