Minimum Service Size

[Designer69]

I know the NEC says that dwelling units must have a service size no less than 100 Amps.

I just want to break this down a bit:

1) If I have my main service going to a house panel first then off this house panel I have a branch breaker feeding an individual dwelling unit. Does this mean that this branch breaker cannot be less than 100A?

2) Is this 100A min. service size limited to dwelling units only? IE- If I have a warehouse with tiny loads, can I put a 20A service on it?

Thank You

 

[Dennis Alwon]

230.79 is what you are looking for

230.79 Rating of Service Disconnecting Means. The service
disconnecting means shall have a rating not less than the calculated
load to be carried, determined in accordance with
Part III, IV, or V of Article 220, as applicable. In no case shall
the rating be lower than specified in 230.79(A), (B), (C), or
(D).
(A) One-Circuit Installations. For installations to supply only
limited loads of a single branch circuit, the service disconnecting
means shall have a rating of not less than 15 amperes.
(B) Two-Circuit Installations. For installations consisting of
not more than two 2-wire branch circuits, the service disconnecting
means shall have a rating of not less than 30 amperes.
(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.
(D) All Others. For all other installations, the service disconnecting
means shall have a rating of not less than 60 amperes.

 

[electrofelon]

It's worth noting the code says service disconnect rating, not service size or rating, which is a bit odd....

 

[Dennis Alwon]

It's worth noting the code says service disconnect rating, not service size or rating, which is a bit odd....

Yes, but then there is this

230.42(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).

 

[electrofelon]

Yes, but then there is this

Is that new dennis? I recall this debate but don't remember that section

 

[Dennis Alwon]

Is that new dennis? I recall this debate but don't remember that section

It's been there forever but everyone seems to ignore it. I have mentioned it many times that is why I don't bring up the fact that 230.79 only refers to the disconnect. It is true but 230.42 clarifies. I don't know why they don't just say the feeder or service conductors need to be sized to 230.79 because, IMO, that is what it means. I know others will argue it.

 

[kingpb]

Instead of going with minimums, why not go "Bigger is Better".

 

[Coppersmith]

Instead of going with minimums, why not go "Bigger is Better".

Because clients like to spend the minimum amount they can to get what they want.

 

[electrofelon]

Instead of going with minimums, why not go "Bigger is Better".

Because we like to debate theoretical code situations

 

[Dennis Alwon]

Instead of going with minimums, why not go "Bigger is Better".

Sometimes it just a matter of stating what the code states.

 

[kwired]

Because we like to debate theoretical code situations

Bigger is better has more room for interpretations and should allow for endless debating

 

[Designer69]

230.79 is what you are looking for

230.79 Rating of Service Disconnecting Means. The service
disconnecting means shall have a rating not less than the calculated
load to be carried, determined in accordance with
Part III, IV, or V of Article 220, as applicable. In no case shall
the rating be lower than specified in 230.79(A), (B), (C), or
(D).
(A) One-Circuit Installations. For installations to supply only
limited loads of a single branch circuit, the service disconnecting
means shall have a rating of not less than 15 amperes.
(B) Two-Circuit Installations. For installations consisting of
not more than two 2-wire branch circuits, the service disconnecting
means shall have a rating of not less than 30 amperes.
(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.
(D) All Others. For all other installations, the service disconnecting
means shall have a rating of not less than 60 amperes.

Thank you for the info.

So If my main service disc. is at the main house service panel and a dwelling unit is fed off a branch breaker on the house service panel, then can we call this branch breaker the dwelling unit service disconnect?

Thanks

 

[kwired]

Thank you for the info.

So If my main service disc. is at the main house service panel and a dwelling unit is fed off a branch breaker on the house service panel, then can we call this branch breaker the dwelling unit service disconnect?

Thanks

It is no longer service conductors/equipment and 230 doesn't apply. It is now a feeder.

If the dwelling in question is in a separate building there is still similar requriements in 225.39. If dwelling is within the same building there is no minimum feeder size other then you need to be able to handle the calculated load.