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Wire Size for 100A Subpanel

1973 Views 6 Replies 3 Participants Last post by  epsilono
This is a continuation of my basement wiring topic, this time about the service wire feeding the 100A subpanel.

The previous owner chose to install the subpanel to address the lack of breaker space in the main panel. The subpanel is 4" away from the main panel. I have determined that to effectively use the subpanel, I need to add a ground bar because both existing bars are on left/right side of the box and both will need to be used for the neutral pigtails on all the AFCI breakers (required). The mounting holes for the ground bar kit (GE Powermark Gold Load Center) are, you guessed it, on the opposite side of the panel from where the service feed enters the subpanel and the ground is terminated. So the ground will not be long enough to reach the other side of the panel and I will need to run a whole new feed cable from the main panel to the subpanel.

For new cable, based on what I've read, the chart shows that I need to run 1-1-1-3 copper or 1/0-1/0-1/0-? aluminum to guarantee 100A at 60deg. Am I interpreting this correctly?

What size would the ground need to be in that 1/0 aluminum cable?

These cable sizes don't seem to be big box stocked (by the foot), are these something I can readily find at electric supply houses or online? Any suggested links? I would only need like 4 or 5 feet.


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Why 1-1-1-3 cable for copper? Standard SER cable (THHN) will let you use the 75 degree rating and then 3-3-3-5 will be sufficient. My understanding is that 60 degrees is required to be used only if the cable is to be buried in insulation. While your run is short working with those thick cables and wires is always a pain.

There appears to have been an iteration of NEC that required 60 degrees, but then later that was relaxed to require 60 degrees for cable buried inside thermal insulation only.
That's the difference between 2008 NEC and 2011 NEC right? I saw that in the 2011 code, I think the 60deg requirement was relaxed as you listed, but it would depend on what code cycle my municipality is in correct?
Yes it would, but mostly it would depend on AHJ. Many inspectors will be OK with going by the newer code even if the old version is still the official one. Call your inspector and figure out if he is OK with it.

Copper 2-2-2-4 is easy to find. I do not know about the other sizes.
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