Sub Panel Conductors Sizing for Residential

[billsnuff]

google subpanel

google subpanel

408

 

[augie47]

or, more properly

View attachment 3061

 

[raider1]

I encourage electricans to use the corrrect terms. If you want to find the requirements for a sub-panel, where do you look?

I agree, correct terminology is very important when trying to locate code requirements.

Chris

 

[Jhaney]

let me throw a monkey wrench in this topic.
I am getting my notes together for a 100A subpanel installaion in a detached garage/shop, the cabling will have to go overhead (drive paved between house and garage and its either overhead or circle the house)
So since its running over head except for the run up the mast can I use the Table 310.17 to size my cable?

 

[augie47]

you can use the applicable Table for the installation. You mentioned "cable" and 310.17 is individulal conductors.

 

[Jhaney]

Ahh I missed that Gus that will definitly up my cable size, I was mainly lost at whether the 8ft up mast would have any bearing on using Table 310.17 I have been looking and can't find the section that would bear up this, my instinct says if its in conduit then use Table 310.16

Thanks,
Jeff

 

[augie47]

I would say your instinct is correct....310.16 in the conduit,,,
free air you could reduce, probably to 310.20.
Normally here, the aerial ends up being ACSR to reduce sag, etc. and #2 is common so it is often used.

 

[jrannis]

It does say feeder, it says "the main power feeder'

In my mind it is clear that only a conductor that supplies all the power can be a "main power feeder". A feeder to a sub panel does not usually supply all the power so the feeder to it can not be a "main power feeder"

Now that said, many smart people feel differently, enough that the CMP has now changed the wording to make it clearer. :smile:

OK but it does say that the feeder does not have to be larger than what is required for the "main power feeder"

 

[iwire]

OK but it does say that the feeder does not have to be larger than what is required for the "main power feeder"

Yes it does, and that makes perfect sense to me but some have a problem with the use of the word 'larger'.

 

[Dennis Alwon]

Yes it does, and that makes perfect sense to me but some have a problem with the use of the word 'larger'.

What does larger mean. A #2 alum is larger than a #3 copper but which has a greater amp.

We just came from a state meeting and this exact subject came up. The CMP members that were at this meeting said that you must use the ampacity of the cable not the "larger than". Based on what you say-- based on there knowledge- that is all I can add. I agree with them.