Residential Services

[Big Guns]

What is the smallest residential service you can install on a sigle family dwelling legally.

Thank You

 

[iwire]

I don't feel like calculating it but I would bet less than 30 amps at 240 if it was all gas appliances and small. (This is as far as the NEC, local power company rules or local amendments could change things.)

 

[dhalleron]

In my area the utility will not do less than 100 amp.

 

[david]

230.79 Rating of Service Disconnecting Means.
(C) One-Family Dwellings. For a one-family dwelling, the service disconnecting means shall have a rating of not less than 100 amperes, 3-wire.

I would have said 100 amps right off the bat but now since bob answered that way im not sure what im missing

 

[renosteinke]

When the NEC was changed to increase the size of the service disconnect, it was intended to require a minimum service size of 100 amps.

I suppose that's reasonable, but that's not what they said. Oops. Yoo're left to 'infer' the minimum 100-amp service by the listing criteria of the service equipment, etc.

Ditto for the meter bases. Until recently, very many meter bases were not UL listed (as the PoCo was the approving authority). Now they are required to be listed, and I don't think any are listed for less than 100 amps.

Putting things in perspective .... my 1000 sq. ft. simple house, as built in 1957, existed quite happily on a 60-amp service. Since then, the NEC has required SABC's, laundry circuits, smoke alarms (fixed appliances!), etc. A basic load calc starts approaching that 60 amp limit, even with gas appliances. Add air conditioniong, and you're past 60 amps.

 

[charlie b]

I would have said 100 amps right off the bat but now since bob answered that way im not sure what im missing

What you might be missing is that that requirement relates to the rating of the disconnecting means. The amount of power you must supply to the building is a different matter entirely.

 

[iwire]

When the NEC was changed to increase the size of the service disconnect, it was intended to require a minimum service size of 100 amps.

I don't believe that is the case.

Given enough time I can pull up the code making panel comments for a proposal a cycle or two ago that indicate they are only concerned with the size of the disconnecting means and that Article 220 is the deciding factor as far as the service rating.

Of course simple economics may drive the service to 100 amps if you already have to provide a 100 amp disconnection means.

 

[pete m.]

You might also check 240.42(B) for sizing of your service entrance conductors.

Pete

 

[david]

230.79
Oh I get it now the argument is going to be a single disconnect only has to be rated for a 100 amps, A 100 amp service doesn?t need to be installed just a disconnect switch with a 100 amp 3 wire rating. So I guess then a two wire service can be installed as long as there is provision for a three wire service. So I guess they missed it on both accounts.

Even though the reference already handled min. size based on calculated load and if this doesn?t mean a min of a 100 amp service all this talk about 100 amp 3wire is meaningless verbiage.

I guess this is what happens to any document when you do not have a hidtoric reference to what was the meaning of the words being written. or why they were written Even our constitution is suffering the same problem.

someone once said they understood the letter of the law but not the spirit off the law.

 

[iwire]

230.42 Minimum Size and Rating.

(B) Specific Installations. In addition to the requirements
of 230.42(A), the minimum ampacity for ungrounded conductors
for specific installations shall not be less than the
rating of the service disconnecting means specified in
230.79(A) through (D).

It is interesting that if we take that literally it seems to preclude using Table 310.15(B)(6).

 

[iwire]

230.79
Oh I get it now the argument is going to be a single disconnect only has to be rated for a 100 amps, A 100 amp service doesn?t need to be installed just a disconnect switch with a 100 amp 3 wire rating. So I guess then a two wire service can be installed as long as there is provision for a three wire service. So I guess they missed it on both accounts.

Even though the reference already handled min. size based on calculated load and if this doesn?t mean a min of a 100 amp service all this talk about 100 amp 3wire is meaningless verbiage.

I guess this is what happens to any document when you do not have a hidtoric reference to what was the meaning of the words being written. or why they were written Even our constitution is suffering the same problem.

someone once said they understood the letter of the law but not the spirit off the law.

I am only telling you what the people that write the code stated.

That section 230.79 applies only to what it says it applies to.

A code is worthless if each person chooses to ignore the words and tries to guess intent.


It looks like I will have to find it the proposal.

 

[pete m.]

It is interesting that if we take that literally it seems to preclude using Table 310.15(B)(6).

Don't start that ball rolling.. :happyno:

Pete

 

[kwired]

So we are required to have a 100 amp disconnecting means, but if we only have 30 amps of calculated load can we have a 100 amp breaker as the service disconnect and supply it with 10 AWG copper service entrance conductors, or even 8 or 6 AWG?

Even if you can prove that is alright, good luck convincing most inspectors as well as many others that it is acceptable.

 

[pete m.]

So we are required to have a 100 amp disconnecting means, but if we only have 30 amps of calculated load can we have a 100 amp breaker as the service disconnect and supply it with 10 AWG copper service entrance conductors, or even 8 or 6 AWG?

Even if you can prove that is alright, good luck convincing most inspectors as well as many others that it is acceptable.

Take a look at 230.42(B).

Pete

 

[renosteinke]

So ... let's see if I get this right ...

230.72 says a house needs a 100-amp disconnect, and 230.42 says I need to run 100-amp wires (or larger) to it.

That sounds an awful lot like requiring a 100-amp service as a minimum. We really have no way to limit / control what the PoCo can deliver to those wires.

Sure would be more clear if the code simply said "houses get at least 100-amps."

 

[david]

So ... let's see if I get this right ...

230.72 says a house needs a 100-amp disconnect, and 230.42 says I need to run 100-amp wires (or larger) to it.

That sounds an awful lot like requiring a 100-amp service as a minimum. We really have no way to limit / control what the PoCo can deliver to those wires.

Sure would be more clear if the code simply said "houses get at least 100-amps."

No that?s what I believe the section was understood to mean since it first was written. Every test for certification. When I got my associates degree the study materials from the school and continuing education material all say a min of 100 amp service. Even the engineers for the four major utility company believe this and incorporated a min of 100 amp into their regulations?.

But there is one more link to get you to a 100 amp service, overcurrent protection.
What seems to be said here is you need a 100 amp rated switch 3 wire, and a 100 amp wire. But you can still install say a 60 amp 110 volt service. Provide a 60 amp main breaker or fuses to a 110 volt service your good to go.

 

[charlie b]

It is interesting that if we take that literally it seems to preclude using Table 310.15(B)(6).

You make a very good point here. Just based on the fact that you and I agree so often :roll: , I looked for the "hidden language" in the code (i.e., something buried in an explicit Exception, or a sentence phrased as "except as noted . . . ," or a cross reference to another article) that would allow the two articles to coexist (i.e., so that the one does not preclude the other). I could not find it. The best I can offer, and I am not at all confident in this conjecture, starts with the notion that Table 310.15(B)(6) {NEC 2008} is not an ampacity table. The associated text says essentially, "Under these circumstances, you can use this wire size." It does not say, "Under these circumstances, this wire size has the following ampacity." If it did, then the apparent conflict between 230.42(B) and Table 310.15(B)(6) would vanish. Since it doesn't speak of ampacity, we are left to infer that it overrides the requirement in 230.42(B) by simply saying, "You can use this wire size."

 

[iwire]

So ... let's see if I get this right ...

230.72 says a house needs a 100-amp disconnect, and 230.42 says I need to run 100-amp wires (or larger) to it.

That sounds an awful lot like requiring a 100-amp service as a minimum. We really have no way to limit / control what the PoCo can deliver to those wires.

Yeah we do, they are commonly called fuses.

100 amp service conductors, 100 amp disconnect, 30 amp fuses, 10 AWG to a small panel.

Simple.

Can I see me doing that? No, not so much.

 

[kiddrock]

What is the smallest residential service you can install on a sigle family dwelling legally.

Thank You

Why would anyone want to install a service rated at less than 100 amps on an actuall residence anywhere in the US? ( gas appliances or not).

 

[stickboy1375]

Why would anyone want to install a service rated at less than 100 amps on an actuall residence anywhere in the US? ( gas appliances or not).

Don't start adding common sense to the thread. :roll: