Service disconnect rating

Status
Not open for further replies.
pete m.

pete m.

Senior Member
Location
Ohio
The following is commentary from the 2005 NEC handbook which is found directly after the actual code text in 230.79:

"Three-wire services that supply one-family dwellings are required to be installed using wire with the capacity to supply a 100-ampere service for all single-family dwellings.
A conductor ampacity of 60 amperes is permitted for other loads. Smaller sizes are permitted down to 14 AWG copper (12 AWG aluminum) for installations with one circuit. Two-circuit installations must have a rating of at least 30 amperes. Exhibit 230.27 illustrates the conductor sizing requirements of 230.79 for ungrounded service-entrance conductors. A single service disconnecting means is required to have a rating of not less than the load to be carried."

Perhaps I have become confused in the way I am reading the code... I am seeing that 230.79 talks about the "minimum rating of the service disconnecting means". I am not seeing where 230.79 addresses the size of the service CONDUCTORS.

In particular 230.79(c) requires a service disconnect rated for 100 amps for a dwelling. What would stop an electrician from installing a 100 amp rated fused disconnect and installing 70 amp fuses with 6awg copper conductors to feed a dwelling as long as the calculated load was met (of course this would be impracticle due to cost).
 
Those conductors would be feeders. I think you will find in Article 215 that the feeders can't be smaller than the disconnect you mentioned.

The service conductors are usually sized by the utility. See the definition of "service conductors".

Steve
 
Sorry Steve, I was referring to the service entrance conductors on the line side of the disconnect after the service point. The commentary to 230.79 found in the handbook says that theses conductors have to have an ampacity for 100 amps....... I don't think 230.79 has anything to do with conductor ampacity. Am I missing something?
 
I think the service disconnect rating is based on the size fuse/breaker installed and not what it is possible to install.

A rule that affects wire sizing is 230.90. This says the wires have to have the ampacity greater than or equal to the overcurrent rating or setting. But there are some important exceptions in that section to read.
 
I should have read a little closer. 215.2(A)(2) and (3) is what I was thinking of for feeders. But I think Mark has it right for the service conductors.
 
230.90(A) ex 5 does a real nice job of directing you to an easy solution, for a start.
 
Maybe I need to state my question in a different way.

Would it be NEC compliant to install a 100 amp rated fused disconnect for a one-family dwelling that was fused at 70 amps with 6awg copper conductors, as the service entrance and feeder, if the calculated load for the dwelling was 62 amps with respect to 230.79(c)?

Just looking for opinions because the commentary found in the 2005 NEC handbook seems to differ from the text of the code section in that the commentary is suggesting that the conductors must also be rated for 100 amps.
 
pete m. said:
Would it be NEC compliant to install a 100 amp rated fused disconnect for a one-family dwelling that was fused at 70 amps with 6awg copper conductors, as the service entrance and feeder, if the calculated load for the dwelling was 62 amps with respect to 230.79(c)?
Click to expand...

Very well stated and in my opinion yes that would meet all the requirements.


Just looking for opinions because the commentary found in the 2005 NEC handbook seems to differ from the text of the code section in that the commentary is suggesting that the conductors must also be rated for 100 amps.
Click to expand...

The handbook is not code and is simply other peoples opinions of what the code requires.
 
iwire said:
A 100 amp disconnecting means could have any size OCP 100 amps or less.
Click to expand...

Is this stated in the code somewhere? This issue has caused me grief for feeders when I don't want a full 60A to an outbuilding. NEC 230.80 says to add the size of the breakers installed when you have a grouped disconnect. I see no guidance how to arrive at a disconnect's rating when there is only one (not a group).

My panel (disconnect enclosure) was rated at 125A, but they wanted at least 60A worth of breakers to qualify as a 60A disconnect (using the 6 disconnect rule). In this particuler case it was a feeder to an outbuilding, and there is no equivalent of 230.80 in article 225.

I agree that the rating of the enclosure or largest allowed overcurrent device you can install SHOULD be the rating of a disconnect. But I don't see that stated anywhere.
 
What you are describing is different than what Bob is referring to. A panelboard by itself is not a disconnecting means, where individual circuit breakers are. Bob is referring to a fused disconnect. As long as the SWITCH is rated per the NEC minimum, it doesn't matter what the overcurrent device rating is.. A circuit breaker on the other hand is both the disconnect and an overcurrent device all in one.

Also, I doubt a typical panelboard installed at a separate building or structure can qualify for the 6 disconect rule. Most are only listed for use as service equipment when a main breaker is installed OR when not more than 6 circuit breaker are installed AND not used as a lighting and appliance panelboard. Also consider 408.36(A)...
 
Doesn't 230.42(B) prohibit the #6 wire to a single family dwelling?

"....the minimim ampacity for ungrounded conductors shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D)."
 
Your proposed installation violates Section 230.42(B) which requires the conductors to have an ampacity rating not less than the rating of the disconnecting means required by Section 230.79.
 
I don't think 100-amp conductors feeding 100-amp switch with 70-amp fuses violates the code.
 
RB1 said:
Your proposed installation violates Section 230.42(B) which requires the conductors to have an ampacity rating not less than the rating of the disconnecting means required by Section 230.79.
Click to expand...

I stand both corrected and mystified.

I have no idea why 230.42(B) exists or for that mater 230.79.

Both seem to go against 90.1(A) and 90.1(C).
 
Status
Not open for further replies.
Top