100A Sub #4 Wire?? - Electrical - Page 2 - DIY Chatroom Home Improvement Forum

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Old 03-02-2011, 07:01 AM   #16
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Originally Posted by wirenut1110 View Post
All I can say is, that the 15 localities I work in and the manufacturers of pre-wired transfer switches must be wrong.

310.15(B)(6) says between the main disconnect and the panelboard that supplies, either by branch circuits or feeders, or both, all loads that are part or associated with the dwelling.
310.15(B)(6) would be the correct table if the feeder was carrying all the load. Since the OP has said it is not you must use 310.16 to size the conductors.

Originally Posted by kbsparky View Post
IF you have a 200 Amp service in the house, and install a 100 Amp sub-panel from it, THEN the minimum wire size allowable would be a #3 copper, or #1 Aluminum.

Table 310.15(B)(6) does not apply here, since the sub-panel does not carry the entire load of said dwelling unit.

You will have to use table 310.16 to calculate wire size, using the 75 degree column, due to temperature limitations of your terminations.


Answers based on the National Electrical Code. Local amendments may apply. Check with your local building officials.
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Old 03-02-2011, 08:08 AM   #17
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Thanks one and all. Think I will just change out the #4 to #3. Sounds like more of you believe based on Code sections belive this to be the right selection.

Saturday Cowboy it is a 100A circuit breaker in the Main panel that feed the Sub.

I did once run #4 CU protected by a 90A breaker for my back-up electric coils for my HeatPump. That is why this time I had an issue. I remebered the inspector said I had to buy the way more uncommon/expensive 90A and NOT use the 100A that was half the price at HD.

Saturday Cowboy I just re-read your post and you probably hit it out of the park. "Under Current Code" well I'm here in the middle of Tennessee where they aren't "current" with many things. We don't have building codes they don't inspect the plumbing ( only the septic and field) and I think they turn a blind eye to everything else. I've done a couple posts here already about it. So maybe #4 was fine 30 years ago so it is still fine now. I come from New Jersey so I'm used to doing work to Code cause it make thing easier. That is why I'm glad you all are here.

thanks again


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Old 03-02-2011, 08:13 AM   #18
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Originally Posted by Saturday Cowboy View Post
just to be sure what size breaker is it protected by in the main panel?

a "100a" sub panel can not be serviced by #4 cu under current code! anyone who thinks otherwise please post a code reference.
The original post says:
less than 5 feet from new 100A in main
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Old 03-02-2011, 11:27 AM   #19
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I also agree #3 is required for the sub panel feeder. But nap makes a good point about article 310.15. Looking at 310.15 below its says 3 wire. Feeders today, unless in metal conduit, must be four wire. Services can be 3 wire regardless of the conduit. So I am of the opinion that any panel after the service panel uses artical 310.16 not 310.15.

Table 310.15(B)(6) Conductor Types and Sizes for
120/240-Volt, 3-Wire, Single-Phase Dwelling Services and
Conductor Types RHH, RHW, RHW-2, THHN,
Conductor (AWG or kcmil

OP. Do not fear. Just go and get a 90 amp breaker and install it. I would not allow the other guy back into my house. If he bought the same wire you already had, he either knows something we don't or he is a hack.
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Old 03-02-2011, 03:32 PM   #20
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Hi J.V. I met the guy out front of our community while I was working on the front gate. He said he could use the work and I figured he would be able to help run wire and would know the Codes and how things are done around here. I will give him the benefit of the doubt about this part of Tennessee not using the latest code but he started to scare me when he had trouble "seeing" what a 3 way switch was doing. I had asked him about adding a couple 4 ways into the circuit so I could run the wire upstairs from the garage and have a way to turn the lights on or off from there. He drew a blank and brought over a friend when I was unable to get him to understand what I wanted. the other guy had a problem explaining it as well. I figured how bad could he mess up a sub so I let him work on that while I took down rock and ran some wire. So here we are, the other guy says he can do the job BUT he wants 3 day with a crew of 3 guys to add a couple lines. Guess he thinks he can get over on this "yankee". So I will change out the couple dollars worth of #4 and put in the #3 cheaper than changing the breakers. Then start pulling new wire from the sub to feed some new lines I need and fix what is wrong there already.

thanks everyone


P.S. new post coming up need some new advice


Last edited by walt1122; 03-02-2011 at 03:36 PM. Reason: not done hit wrong key
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