310.12 Application

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ProjectDelta

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Michigan
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Electrician
Hello All, I am increasing the size of my service and moving the meter and I wanted to get opinions regarding 310.12 and how I can apply it.

My house is currently fed with a 100A overhead service which I will be increasing to 200A. The meter will be moved to a location that is closer to the pole and it will feed a 200A main panel outside. Now here is where I want to apply 310.12. The current panel that feeds the house will become a sub panel of the same size, 100Amps. Basically the whole point of increasing the service size is to accommodate electric vehicles in the future. Technically, the new sub panel no longer feeds the WHOLE house (and actually I’ll be shedding a load or two with the new 150A sub panel in the garage), but it seems silly to me that I was allowed to reduce ampacity to 83% if it were the service, but now that it’s a feeder that feeds the same or smaller load but now not technically the whole house. It would be nice to be able to run smaller wire and conduit for cost savings. TIA for the feedback.
 
I kind of agree with you to some extent on why is it ok as service but not feeder in this situation. Simple solution is to put a 90 amp breaker on the feeder instead of a 100 amp breaker to be code compliant.

If you in situation where you supplying it with a feeder tap, then you going to have to find a 90 amp replacement for the main breaker in your existing panel, which may or may not be easy depending on what kind of panel it is. But if you have a feeder breaker as part of what is new, just use a 90 amp breaker.
 
kwired said:
I kind of agree with you to some extent on why is it ok as service but not feeder in this situation. Simple solution is to put a 90 amp breaker on the feeder instead of a 100 amp breaker to be code compliant.

If you in situation where you supplying it with a feeder tap, then you going to have to find a 90 amp replacement for the main breaker in your existing panel, which may or may not be easy depending on what kind of panel it is. But if you have a feeder breaker as part of what is new, just use a 90 amp breaker.
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This actually works out well. I’m replacing the old house panel so I can just use a 100amp panel with no main breaker (which are dirt cheap apparently) and feed it from the new main panel outside. Thanks for the advice!
 
ProjectDelta said:
For my own knowledge, In what instance could one size a feeder at 83% in a single or multi family dwelling as this section code allows?
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If the feeder carries the entire load of the dwelling

For a feeder rated 100 through 400 amperes, the feeder
conductors supplying the entire load associated with a
one-family dwelling, or the feeder conductors supplying
the entire load associated with an individual dwelling unit
in a two-family or multifamily dwelling, shall be permitted
to have an ampacity not less than 83 percent of the feeder
rating.
Click to expand...
 
ProjectDelta said:
For my own knowledge, In what instance could one size a feeder at 83% in a single or multi family dwelling as this section code allows?
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Generally it's only used on service conductors for a single family dwelling or a feeder from a service disconnect on a pole or pedestal. It's also used for feeders from the service disconnect to individual units on multi family dwellings if it carries the entire load of that dwelling and is 120/240v single phase.

Your house and detached garage are still considered a single dwelling.
 
SSDriver said:
Generally it's only used on service conductors for a single family dwelling or a feeder from a service disconnect on a pole or pedestal. It's also used for feeders from the service disconnect to individual units on multi family dwellings if it carries the entire load of that dwelling and is 120/240v single phase.

Your house and detached garage are still considered a single dwelling.
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together they are, pretty certain you would need full 100 amp conductor if running a 100 amp feeder to the garage though.
ProjectDelta said:
For my own knowledge, In what instance could one size a feeder at 83% in a single or multi family dwelling as this section code allows?
Click to expand...
As has been mentioned whenever the feeder is carrying the entire load of a dwelling unit.

Outside of the main supply to a single family dwelling, could be a feeder to an apartment, a suite in a hotel, resort, etc. that meets the art 100 definition of a dwelling unit, even could be a feeder to a "mother in law suite" located within what is otherwise considered a single family dwelling, that "suite" must fully meet the art 100 definition of dwelling unit on it's own though.
 
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