Need help sizing feeder to sub panel

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crtemp

crtemp

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Wa state
I am going to install a sub-panel directly under the main 200 amp panel in a single family dwelling. I am going to use #4 copper for the feeders. I am looking at the table 310.15(b)(7) and it looks as though it says I can put 100 amps on the #4 wire. But then I look at table 310.15 (b)(16) and it says that #4 is only good for 85 amps in the 75 degree column. I'm pretty sure the circuit breaker that will feed the sub-panel has 75 degree terminations. What is the correct max amperage in my situation?

Thanks
 
I this setup you cannot use 310.15(B)(7) since the sub-panel is not carrying the entire load of the dwelling. If you're are not using NM cable and have 75? C conductors in a raceway then you can use #4 Cu and protect it at 85 amps or in this case the next higher standard size of 90 amps.
 
I'm trying to find in the code where it talks about being allowed to go to the next size breaker if the amp limit is a non standard size. Could you tell me where this is located?

Sorry I found it (240.4)
 
Last edited:
crtemp said:
I'm trying to find in the code where it talks about being allowed to go to the next size breaker if the amp limit is a non standard size. Could you tell me where this is located?

Sorry I found it (240.4)
Click to expand...

Even though you are allowed to go up to the 90A breaker, your load can't exceed the 85A rating of the wire.
 
I didn't come close to using 85 amps. Just a 30 amp heater and a water heater.

I have been wiring houses though and wiring 60 amp furnaces with #6 SEU copper wire. It's only good for 55 amps but have been putting it on 60 amp breakers and no inspector has had an issue with it. Maybe they haven't noticed.
 
If the SE cable avoids contact with thermal insulation is can be used as a 75? C conductor @ 65 amps.
 
1793 said:
I thought SE cable now, 2011, falls under 334 for NM and is lowered to 60 deg.
Click to expand...
I believe it was 60? in '08, then, as noted in the post, allowed to be used at 75? in '11 IF you avoid contact with insulation.
 
This se cable issue has been a nightmare for years. The bottom line is the code in which it was installed but in reality I have never seen a failed se cable due to the issues being discussed. Loose connections yes but not overload. Most all se cables we install have some type of factors built in such as in electric heat or ranges where there is either a demand factor or 125% already figured in.
 
Which temp applies to SE cable has been hard to follow. If using NM cable there is no question it is to be considered 60C conductors.

You have to watch a little carefully the exact ratings on these electric furnaces if using NM cable or if 60C conductors do apply to anything else. The ratings are so close to 60C conductors not being sufficient that they will get you if you don't pay close attention. Often the amount of heat load is fine but if a blower is included it puts you into needing a larger conductor, where if you are able to use 75C conductors 6 AWG is still ok.

If the heater amps is 44 amps and you have to multiply by 125% to get minimum conductor ampacity, you are at 55 amps limit of 60C conductors. Add a blower and you need to increase conductor size even though 60 amp breaker is still fine.
 
Dennis Alwon said:
I am guessing he clicked the '08 instead of the '11
Click to expand...

Understand. I don't have an electronic version of '11 and wonder what he is using. I have to go on-line to view '11.
(We are not on '11 yet so I only reference in regard to the Forum)
 
Dennis Alwon said:
I am guessing he clicked the '08 instead of the '11
Click to expand...

Correct. I have the files next to each other and I hit the wrong one.

augie47 said:
Understand. I don't have an electronic version of '11 and wonder what he is using. I have to go on-line to view '11.
(We are not on '11 yet so I only reference in regard to the Forum)
Click to expand...

When I purchased my 2011 Code book there was a free PDF I could download.
 
augie47 said:
Understand. I don't have an electronic version of '11 and wonder what he is using. I have to go on-line to view '11.
(We are not on '11 yet so I only reference in regard to the Forum)
Click to expand...

Other than opening a pdf file I think using NEC plus is about all you are going to find. It is closest thing you will find to the older CD-ROM versions of NEC, but is web based so you need internet access to use it as well as a paid subscription. But can use from any browser all you need is to know your username and password.

It is somewhat easy to not realize which edition of NEC you are browsing and you could be in anything between 2002 and 2011. You need to look in upper left corner of the page to see which one you are in. (you do have option of toggling on/off the handbook commentary) You also could possibly be looking at one of the last 4 editions of 70E instead of NEC.

There is free demo, like one or two days. Also included is notations and explanations of changes from previous code in the places where changes were made, links to ROP's, links to related UL white book info near different code sections, and the home page has news and other NFPA content.

http://www.necplus.org
 
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