Sizing Service Conductors Correctly

Merry Christmas
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George,
've always assumed that 310.15(B)(6) was giving us a few amps leeway off the 75? columns, they are small but reasonable concessions. Giving away 50A on a 200A service with AL seems outside that reasoning, doesn't it?
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The table is based on real world dwelling unit loads and not the inflated loads that you get when using Article 220. The table wire sizes are based on data from utilities that show the actual current on the average dwelling unit service.

However you are correct that it is based on using wire at its 75?C ampacity.

Don
 
Thanks to the ECN forum, I found this link and chart:
http://www.dcengineers.com/downloads.html
There is a 12 page PDF download. Pages 3 to 6 are charts for feeders and services listed by amperages.
Interestingly enough, they list
100 amps = #3
150 amps = #1/0
200 amps =#3/0
They also supply the required conduit size [although several result in 1 1/4"] for each run.

But isn't the size based on the load. For instance, can't you feed a 200 amp panel that has a single 20 amp breaker in it with a black, #12 wire? Also the Code is just a minimum and you could feed the same panel with 3/4" cable [300 MCM]?
Thus, in a sense, the question of what size of wire to use for a given size panel would be meaningless.
~Peter
 
georgestolz said:
A question before my reply:

Does 110.14(C)(1)(b) guarantee that terminations must be rated 75?? If the equipment is not listed, then it could have 60? terminations over 100 amps, right?

I've always assumed that 310.15(B)(6) was giving us a few amps leeway off the 75? columns, they are small but reasonable concessions. Giving away 50A on a 200A service with AL seems outside that reasoning, doesn't it?
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The problem is trying to coordinate 110.14(C)(1) with Table310.15(B)(6).
How do you apply 110.14(C)(1)(a)(2) to a Table that has no temperature columns ?
110.14(C)(1)(a)(2): "Conductors with higher temperature ratings, provided the ampacity of such conductors is determined based on the 60˚C (104˚F) ampacity of the conductor size used."
How do I apply that to a residential service ?
There is no 60˚C column in Table310.15(B)(6).

So far we have been allowing the 60˚C panel in residential at the ampacities listed in Table310.15(B)(6)

David
 
David,
The reduced conductor sizes for dwelling unit service were in the code long before the temperature rules in 110.14.
Don
 
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