Basement Subpanel


Old 05-18-19, 02:18 AM
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Question Basement Subpanel


I want to add a 100 amp panel to my basement, which would be fed from the main panel in the garage. The main panel is approximately 40' away and 200 amps. I hae no choice but to run the wire/cable through the attic and if necessary I can run conduit. This is an attic that will be accessed for diffferent storage needs, not to mention the heat, and depending on what code you use sometimes that makes all the difference in how it's done. I was thinking of going with AL SER 1/0-1/0-1/0-2 to get the full 100 amps I need. Am I limited to the type of wire/cable I can use or can I just use THHN 3 # 2's & #6 gnd through conduit?

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Old 05-18-19, 06:11 AM
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Many houses are run off a 100 amp service. Do you really need 100 amps? Are you adding a lot of heavy loads? I would be willing to bet 60 amps will be plenty for your sub panel and you can still use a 100 amp sub panel, it would just be fused at 60 amps where the feeder originates.

If you run THHN in conduit you would need #3's and a #8 ground for 100 amps. (or #6's and #10 ground for 60 amps)
Old 05-18-19, 06:54 AM
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For 100A you can use 1-1-1-3 Al SER. #1/0 Al is good for 120A. If the #2 you spoke of is in Al, it's only 90A. #1 Al is 100A and #3 Cu is 100A.
Old 05-18-19, 08:15 AM
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If you really need 100 amps for the subpanel you may be looking at a service upgrade.
Old 05-20-19, 01:14 PM
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What gauge do you need for a 100 amp sub panel?

Such a simple question. Such a complicated answer.

How much current a wire can carry depends on 4 things:
  1. The gauge of the wire (obviously)
  2. The temperature rating of the wire, terminals, panel and devices
  3. The ambient temperature of the home
  4. Number of current-carrying conductors inside conduit/raceway

We want to solve for #1 (the wire gauge), so we will need to know the other three variables.

Number of Conductors

Let's just hammer down this last variable first, because it's easy. Three. If you use SER cable, there are 3 current carrying conductors + a bare ground. If you use a conduit, you will pull 3 current carrying conductors + a green ground. Don't pull any more. If you keep this at 3, all of the rest of this post is fine. If you try to add more wires to the conduit or raceway, there is an additional de-rating factors not covered in this post. Don't do it!

Temperature Ratings

A wire can carry more or less current depending on its temperature rating. Look at the product listing of the wire you are considering purchasing. It will state a temperature rating. Also look at the temperature ratings for termination lugs, devices, equipment and panels. The overall temperature rating for the circuit is the lowest rating of any of the components.

The 90C rating level is not used in residential. The 75C rating can be used if the panels are rated over 100A or if the equipment is specifically rated at 75C or higher, but most residential work will be at the 60C rating. If a rating is not listed, 60C is the default. If a cable like SER runs through building insulation, the rating is automatically 60C regardless of other factors.

The following is Table 310.15(B)(16) from the NEC.

Here is the pertinent information from that table broken out in a more simple and easy-to-read form. You will be using the 60 C column, or possibly the 75 C column if all the rules above are adhered to. The 90 C column is not to be used.

As you can see, it appears that either #3 AWG copper or #2 AWG aluminum will carry 100 amps. But wait!!!! These numbers are based on an ambient temperature of 30 degrees C (86 degrees F). We need to factor in your ambient temperature...

Ambient Temperature

The NEC provides a temperature de-rating table, based on ambient temperature, but it does not tell you which ambient temperature you must use. For that I recommend applying some common sense, or contacting your town's building department. If you live in a cold climate, you don't need to worry about this. If you live in Arizona or something, this is going to be a big factor, especially in your case where you mentioned the wire is going to run through an attic. Conduits on rooftops are a special case. You need to follow a special set of temperatures which can be found here.

See how in the table above, the row for 26-30 degrees has a multiplication of 1? And every row after has a multiplication factor less than 1? That means that you may need to jump to a bigger AWG if you're living in a warm location. Look at one of the following tables (most likely the 60 C one) for an examples. Locate the column for the ambient temperature relevant to your building location.

*Ambient Temperature Corrections for 60 C Equipment*

*Ambient Temperature Corrections for 75 C Equipment*

*Ambient Temperature Corrections for 90 C Equipment*


You asked about the conductor size needed to carry 100 amps. As you can see from above, you'll need to get #1 AWG aluminum assuming the panel is rated for at least 75C.

If you switched to copper, you could probably get away with #3 AWG in average climates, but you'd need to bump up to #2 AWG in hot climates.

Now, even in a moderate climate, if this is an installation in an attic, you might want to be safe and go up a size because it can get really hot up there. You can see the table yourself and make a logical decision for you. Either that or call your AHJ and what size they want to see when they do the inspection.

Here is a youtube video of the NEC guys discussing this for the NEC 2014:

Here is a very long video discussing the NEC 2014 changes over NEC 2011 in detail, with regard to wire ampacity and temperature derating.

My personal experience

I installed a 125 amp sub panel. The cable run was about 65 feet, running through a basement (a.k.a. not hot). I used 2-2-2-4 Copper SER. As you can see in the product specs, it's rated for 90 degrees C, but can only be used at 75C (115A) in residential installations. When computed ampacity falls between breaker sizes, the code allows a round-up to the next standard breaker size of 125A. The inspector approved it.

(Mod note: changed some details to reflect correct use of temperature ratings)

Last edited by ibpooks; 05-23-19 at 08:07 AM. Reason: Add information regarding temperature ratings
Old 05-20-19, 01:37 PM
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electric_dummy, in all that info you provided you failed to say that 90 deg.C is only used when derating conductors. Under normal conditions 60 deg.C or 75 deg.C is to be used.

Note: #2 Cu SER is 115A @75C, you are not to use it at its flat out 90C rating of 130A. There are also restrictions when SER is run within insulation that limits it to 60C.

Last edited by pattenp; 05-20-19 at 01:55 PM.
Old 05-20-19, 02:10 PM
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I apologize if I gave misleading information, but it was my understanding that as long as your wire and your terminals are all labeled as being rated for 90 degrees C, then you could use the 90 degrees column. Only if a component is not labeled, would you have to use 60 degrees.
Old 05-20-19, 04:21 PM
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I've never seen 90C rated devices or residential panels. They re 60/75C .

Last edited by pattenp; 05-20-19 at 04:38 PM.
Old 05-21-19, 07:21 AM
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Can any mod delete my post?

Last edited by electric_dummy; 05-21-19 at 07:44 AM.
Old 05-21-19, 08:22 AM
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I'll make some edits to reflect consideration of temperature ratings.

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