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Old 10-16-2009, 09:52 PM   #1
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Home wiring problem

While going through and checking out all my wiring before I do some work I noticed what appears to be a big issue... I have an outside main panel with a 100 amp and a 60 amp breaker which go on to feed their sub panels. The 60 amp is using #6 service entrance wire which is good, but the 100 amp is running #4 service entrance to the sub panel. #4 is only rated to 80 amps correct? Another problem is the panel is probably overloaded for a 100 amp anyway... There are 9 20 amp breakers, 2 30's, 1 40, and 1 50. Granted most of the time barely half the stuff is running and it's only around 1100 sq ft home.
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Old 10-16-2009, 10:05 PM   #2
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This specific issue seems to come up all the time. I checked the 1999 NEC code, and there is a table (I can't give the cite right now, my table is out of town) that specifies that #4 copper can be used for service entrance 100A wiring. In addition, I came across a curious section of the code that seems to allow branch circuits to be wired using equivalent wire size to the service feed, which I took to mean that if #4 Cu is good for 100A as service entrance, you could use #4 Cu for 100A as a subpanel wire feed.

This seems to be an area of active disagreement, I know that in posts on this forum, at least 1 electrician has noted that you need #3 Cu for 100A, based on the ampacity tables in the NEC. My house is wired for 100A using #4 SE cable, this was done in 1959 I believe (that is when the house was built), and so far as I know all the other houses in my neighborhood also used #4 Cu for 100A service entrance. I don't know what the current interpretation of code is in my town, since I have never upgraded the service.

As for whether 100A is adequate, that is a function of the load distribution, which is discussed at length in the NEC.
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Old 10-16-2009, 10:11 PM   #3
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Thanks for the response. It gives me a little bit of reassurance knowing your house is #4 too lol... Mine house was built in the 60's but the wiring isn't old. The last owner replaced it but why the didn't go higher I have no clue. The wire in question is only around 30 feet in length and from a calculator I just used it said that it is ok for that length.

I really need to pick up a NEC..
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Old 10-16-2009, 10:23 PM   #4
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#4 copper wire is allowable for the service feeder for a dwelling, it was first mentioned in the 1971 edition of the Code.

Otherwise, table 310-16 states it has an ampacity of 85 Amps.

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Old 10-16-2009, 11:37 PM   #5
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Readers I just want to make it clear what Kbsparky mention with 100 amp service with #4 conductors that is for service entrance only but once it become a branch feeder all bet is out of the window it will only have 85 amp rating and again I will mention that is copper conductor verison for alum verison no it will not meet the current code for service appaction ditto with branch feeder.


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Old 10-17-2009, 07:58 AM   #6
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Service entrance cable (as opposed to "plain Romex") has different temperature ratings and could withstand a little warming up when the load got to 100 amps.

Also single service conductors strung through the air to the utility pole can withstand a little warming up.

You would not want to splurge on running #4 wire for 100 amp use because you may get unacceptable voltage drop through the entire system.
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Old 10-17-2009, 11:50 AM   #7
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Adding up the breakers in a panel is a useless exercise. Many of those breakers could have zero current flowing in them. You need to do a demand load calculation to determine if the panel is overloaded.
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Old 10-17-2009, 12:49 PM   #8
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Art. 310.15(B)(6) specifically allows #4 Cu for a 100 amp 3 wire service or feeder. Since the 100 amp panel is part of the service, #4 Cu is allowed.
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Old 10-17-2009, 02:26 PM   #9
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Originally Posted by J. V. View Post
Art. 310.15(B)(6) specifically allows #4 Cu for a 100 amp 3 wire service or feeder. Since the 100 amp panel is part of the service, #4 Cu is allowed.
310.15(B)(6) specifically states it is for when it serves as the main-service feeder. In this case, the main-service feeder ends at the panel which contains the 100A and the 60A breakers.

2008 NEC
(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services

and Feeders.
For individual dwelling units of onefamily,

two-family, and multifamily dwellings, conductors,
as listed in Table 310.15(B)(6), shall be permitted as
120/240-volt, 3-wire, single-phase service-entrance conductors,
service-lateral conductors, and feeder conductors
that serve as the main power feeder to each dwelling unit
and are installed in raceway or cable with or without an
equipment grounding conductor. For application of this section,

the main power feeder shall be the feeder between the

main disconnect and the panelboard that supplies, either by
branch circuits or by feeders, or both, all loads that are part

or associated with the dwelling unit.
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