Minimum size residential service

[pete m.]

I am going to attempt to paint an electrical picture. :roll:

If I were to install a 100amp rated fusible disconnect and fuse it at 70amps (assuming that the load would allow) as my service disconnect at a new home or as a service change would this be considered a violation of the NEC?

 

[wwebb]

Re: Minimum size residential service

230.79(C) just says the disconnecting means shall have a rating of not less than 100 amperes. It does not mention the minimum overcurrent protection, that's in 230.90. I believe this would be determined by the load(s). I would probably OK this type of installation, if the total code load of the house does not exceed 70 amps.

As a postscript: The 2003 IRC, in E3502.1 states that the rating of the 'UNGROUNDED CONDUCTORS' shall not be less than 100 amps, 3 wire. So I guess it depends on which building codes your jurisdiction is under... but I still don't see anything about minimum overcurrent protection, so, so far, it still may be OK....let's keep looking......

[ April 28, 2005, 03:27 PM: Message edited by: wwebb ]

 

[mc5w]

Re: Minimum size residential service

There are some places where a local rule requires a minimum of 200 amps for a dwelling unit.

I consider the minimum for a single family dwelling to be 200 amps and 125 amps for individual apartments. However, a 125 amp main breaker load center frequently costs as much as a 200 amp.

A small house that has central air, and electric stove, and an electric dryer has a tendency to spot weld the circuit breaker lugs in a 100 amp service. This is because most electrcians will not clean each wire strand with silicon carbide abrasive paper and goop it up with antioxidant. Thanks to catalytic converters outdoor copper wiring oxidizes just a s fast as aluminum.

Really, you also need 100 amps of capacity for each 100 feet from the supply transformer to control voltage drop when starting a central air conditioner or heat pump.

 

[George Stolz]

Re: Minimum size residential service

Originally posted by mc5w:
There are some places where a local rule requires a minimum of 200 amps for a dwelling unit.

True. I know the city of Cheyenne, WY, requires a 150 or greater. But now I'm starting to wonder if they worded it correctly. Personally, until yesterday I assumed that the 100-amp requirement was referring to the OCPD rating, not just the disconnect rating.

Really, you also need 100 amps of capacity for each 100 feet from the supply transformer to control voltage drop when starting a central air conditioner or heat pump.

Now that is a generalization and a half!

 

[pete m.]

Re: Minimum size residential service

Does anyone know if it was the intent of 230.79(c) that a dwelling be fed with a capacity of 100 amps minimum?

 

[George Stolz]

Re: Minimum size residential service

Hey-o! Look at 230.42(B). It ties everything together.

Since the conductors must be sized as a minimum to 230.79, then the OCPD is sized to follow suit.

Whew! (I was just about to write a proposal to say that very thing!)

 

[stud696981]

Re: Minimum size residential service

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Really, you also need 100 amps of capacity for each 100 feet from the supply transformer to control voltage drop when starting a central air conditioner or heat pump.
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Maybe someone can help me with this. I realize proper size wiring is important to avoid voltage drop, however, how does increasing the size of a service panel avoid voltage drop due to a distance factor. If a 100amp service is sufficient for the house but there is a voltage drop due to the distance from the transformere, wouldn't increasing the size of the service drop and SE cable be the answer?

 

[hurk27]

Re: Minimum size residential service

how does increasing the size of a service panel avoid voltage drop due to a distance factor.

I guess that extra 100 amps lying around helps crank over that there big air compressor. kind of like haveing a bigger batery in yuin's car crank's it over faster?

 

[George Stolz]

Re: Minimum size residential service

Originally posted by stud696981:
I realize proper size wiring is important to avoid voltage drop, however, how does increasing the size of a service panel avoid voltage drop due to a distance factor.

Stud, you are very wise.

 

[stud696981]

Re: Minimum size residential service

Thanks for the replies.