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#49289 - 03/03/05 03:50 PM Ampacity allowances for 100A and less
rogersan Offline
Member

Registered: 03/01/05
Posts: 23
Loc: La Mesa, CA, USA
I am a little confused on sizing for wires rated 100A or less. I think that you have to use the 60 deg. C tables for allowable ampacities since all terminals under 100A are rated at 60 deg. C (lowest rating of splice or term). Then demand factors/correction etc is applied to this (125% of motor loads etc?) Can someone explain this to me since it seems a little confusing. Also if you use table 310-15(b)(6) for dwellings can you use these straight across the board (say at 80% of calculated load?)

Thanks for the help
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#49290 - 03/03/05 05:11 PM Re: Ampacity allowances for 100A and less
bucketman Offline
Member

Registered: 10/31/04
Posts: 71
Loc: vancouver WA, USA.
How do you know all 100a and less terminals are only rated 60c? are just most of them that way?
I've never looked to see. maybe I shood start looken.

so far as ajustment factores like D-rate with more than three current carrying conductors thats taken off the ampacity of the wire.

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#49291 - 03/03/05 08:13 PM Re: Ampacity allowances for 100A and less
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
You really have to look at the label on the device. I think I have even seen 30a 75c but I know there are 60a 75c breaker lugs.
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#49292 - 03/03/05 10:03 PM Re: Ampacity allowances for 100A and less
Norstarr Offline
Member

Registered: 09/25/04
Posts: 89
Loc: Wi
I may be mistaken since I do not have a lot of terminations in front of me but I have usually found that most terminations are rated at 75 deg. The CH breaker I am holding in my hand right now has a dual rating of 60/75 deg. I will be paying more attention to my terminations in the future for the rating but I'm certain most are 75 deg. I very rarely use the 60 deg column.
In wisconsin we do not have to derate per Table 310.15(B)(2)(a) for residential wiring.
Table 310.15(B)(6) is simple to understand. If you calculate you need a 100 amp service you are allowed to use #4 wire instead of #3 and I believe this is mainly allowed because it is residential and the loads are intermittant and almost all non continuous unlike commercial and industrial.
Base your circuit ampacity on the 60 or 75 deg column (determined by your termination) and your conductor ampacity for derating and correction on the 90 degree column and use thhn wire. Hope this helps.
Ron

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#49293 - 03/04/05 05:05 AM Re: Ampacity allowances for 100A and less
bucketman Offline
Member

Registered: 10/31/04
Posts: 71
Loc: vancouver WA, USA.
"residential and the loads are intermittant and almost all non continuous"

Any thing thak can be on for 3 hours or more is continous. res. or com.

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#49294 - 03/04/05 06:46 AM Re: Ampacity allowances for 100A and less
watthead Offline
Member

Registered: 10/13/01
Posts: 182
Loc: South Carolina
Bucketman
Commercial lighting is considered by code to be continuous and residential is not.

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#49295 - 03/04/05 08:44 AM Re: Ampacity allowances for 100A and less
winnie Offline
Member

Registered: 09/15/03
Posts: 652
Loc: boston, ma
Just a couple of points here:

Terminals for <100A devices can be presumed to be rated at 60C, unless marked otherwise. Many (most??) circuit breakers are marked for 75C conductors. Most 'wire nuts' are marked for even higher temperature. I don't recall any temperature markings on receptacles and the like, however.

Table 310.15(b)(6) can only be applied in a very restricted class of situations. It can only be used with particular wire types and a particular application: the _main_ feed to a residence (including the NEC controlled part of the service entrance), _or_ the feeder supplying the main load of a residence, _or_ downstream feeders in a residence if they are as large as the main feeder. If you do a demand calculation on a residence, and it just happens to come out to 100A, then you can select a 100A conductor from table 310.15(b)(6).

For any load other than those specified in 310.15(b)(6), you have to start with table 310.16 (or the other ampacity tables) for your calculations, even if you are in a residence.

-Jon

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#49296 - 03/04/05 10:10 AM Re: Ampacity allowances for 100A and less
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
Here are some typical SqD breakers and their rating per the label. http://members.aol.com/gfretwell/breakers.jpg
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#49297 - 03/04/05 05:53 PM Re: Ampacity allowances for 100A and less
rogersan Offline
Member

Registered: 03/01/05
Posts: 23
Loc: La Mesa, CA, USA
Ok, I think I found the section finally that answers my question. 110.14(C)(1)(a) states the 60 degree celsius for under 100A and 75 degree for over 100A unless otherwise marked. I think the intent is that the lowest ampacity rating for the entire circuit is the one that has to be used when determining allowable ampacities for conductors. If that sounds right please let me know.

Thanks for all the help.
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