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Thread: Minimum Size Overcurrent for Multi family dwelling

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    Minimum Size Overcurrent for Multi family dwelling

    Hello folks, I am looking for some clarification on article 230.79 pertaining specifically to the minimum size overcurrent device required for a dwelling unit.

    View 1. Part (C) indicates a 100 amp minimum as the overcurrent device; which would require the smallest feeder to be a 1/0 AL SER.

    View 2. Part (D) can be applied since this is a multifamily, and we can do as low as 60 amps; allowing the smallest SER runs to be #1, or even #2 if the unit load is small enough.

    Thanks in advance
    Kevin

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    That article does not address overcurrent setpoints. It is about the service disconnecting means. But that is not where you need to start. Step 1 is to calculate the load. Then you calculate the service size. Take as an example that you calculated the load for a (small) single family dwelling unit at 80 amps. You can provide a fused disconnect switch for the building's service disconnecting means. The fuse can be rated 80 amps, but the disconnect switch has to be rated at least 100 amps.
    Charles E. Beck, P.E., Seattle
    Comments based on 2017 NEC unless otherwise noted.

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    Thanks for jumping in on this, we will have meter banks, and will be using circuit breakers for the OCPD.

    So for this 80 amp load, what size circuit breaker can I use in the Meter bank?

    I have the EE calculations to determine the decrease loads if we can use a smaller breaker and a smaller feeder.

    We planned to include a 100 amp breaker for each unit along with a 1/0 AL Feeder, and increasing as necessary form there; however if we can reduce to a smaller feeder and breaker that would help.

    Thanks again
    Kevin

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    Quote Originally Posted by KP2 View Post
    Hello folks, I am looking for some clarification on article 230.79 pertaining specifically to the minimum size overcurrent device required for a dwelling unit.

    View 1. Part (C) indicates a 100 amp minimum as the overcurrent device; which would require the smallest feeder to be a 1/0 AL SER.

    View 2. Part (D) can be applied since this is a multifamily, and we can do as low as 60 amps; allowing the smallest SER runs to be #1, or even #2 if the unit load is small enough.

    Thanks in advance
    Kevin
    As mentioned this applies to rating of the service disconnect. For supplying dwelling units 310.15(B)(7) would allow 2 AWG aluminum to be protected at 100 amps, this only applies to a conductor that supplies the entire dwelling unit load.

    For the multifamily dwelling situation, details are important. A service disconnect supplying an individual dwelling unit would have to comply with 230.79 and be rated 100 amps minimum. A feeder to a dwelling in a multifamily unit however is not service equipment and art 230 does not apply to it. Such feeder can be less than 60 amps if the calculated load of the unit is less than 60. I believe art 225 has similar rule as 230.79, but that would be for a feeder to a separate building and would not apply to a feeder for a dwelling within same building.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    As mentioned this applies to rating of the service disconnect. For supplying dwelling units 310.15(B)(7) would allow 2 AWG aluminum to be protected at 100 amps, this only applies to a conductor that supplies the entire dwelling unit load.

    For the multifamily dwelling situation, details are important. A service disconnect supplying an individual dwelling unit would have to comply with 230.79 and be rated 100 amps minimum. A feeder to a dwelling in a multifamily unit however is not service equipment and art 230 does not apply to it. Such feeder can be less than 60 amps if the calculated load of the unit is less than 60. I believe art 225 has similar rule as 230.79, but that would be for a feeder to a separate building and would not apply to a feeder for a dwelling within same building.
    How can a number 2 AL be protected at 100 Amps, wouldn't that be an 80 amp breaker?

    Perhaps as long as the meter bank that holds the breaker is rated at 100 amps or more, I can use a smaller breaker?

    But then, why would so many EE list a 1/0 AL for small apartments?

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    Quote Originally Posted by KP2 View Post
    How can a number 2 AL be protected at 100 Amps, wouldn't that be an 80 amp breaker?

    Perhaps as long as the meter bank that holds the breaker is rated at 100 amps or more, I can use a smaller breaker?

    But then, why would so many EE list a 1/0 AL for small apartments?
    This is only allowed for a conductor supplying a dwelling unit (entire load of an individual dwelling unit). Is nothing new and has been in code for a long time. Used to be a table stating conductor sizes permitted, now is an 83% factor, before any adjustment factors, but still gives same resulting conductor sizes. Is currently in 310.15(B)(7).

    Small apartment doesn't even need to be supplied by 100 amp feeder, though many do so as a default. It can be 2 AWG aluminum per 310.15(B)(7) though. My guess is EE's don't pay much attention to or maybe don't even know about these allowances as they maybe don't deal with a lot of dwellings.

    Load diversity of a dwelling is the reason for this allowance. Even if you have a calculated load on a dwelling of 100 amps, apparently someone has determined it never draws that much at least for any significant amount of time to justify needing a full 100 amp conductor. How they determined this IDK, but can tell you that unless there is electric heat, I can't recall ever clamping a ammeter on a dwelling with even a 200 amp supply and reading 100 amps.
    I live for today, I'm just a day behind.

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    Thanks Again for the feedback.

    My personal best on a residential service is 67 amps on 200 amp, and 126 amps on 400 amp.

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