Apartment service wire size

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acolella

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I have a question about the wire size for the service for an apartment. I’m not sure if service is the proper term. Feeder perhaps? Eight apartments on the floor with a meter bank in the mechanical room. The apartment has a 100 amp panel with a 100 amp main breaker fed from a meter socket in the meter bank in the mechanical room that also has a 100 amp breaker. Would that be classified as a service or a feeder? Anyway, the wire feeding the panel from the meter is #2 AL. Now, I don’t do much residential or any multi-dwelling work, but am I incorrect in thinking that this panel should be fed with #1 AL or #3 CU? How is the #2 AL compliant with a 100 amp overcurrent device? If it is compliant, I am just curious as to how it is. Thanks!
 
acolella said:
I have a question about the wire size for the service for an apartment. I’m not sure if service is the proper term. Feeder perhaps? Eight apartments on the floor with a meter bank in the mechanical room. The apartment has a 100 amp panel with a 100 amp main breaker fed from a meter socket in the meter bank in the mechanical room that also has a 100 amp breaker. Would that be classified as a service or a feeder? Anyway, the wire feeding the panel from the meter is #2 AL. Now, I don’t do much residential or any multi-dwelling work, but am I incorrect in thinking that this panel should be fed with #1 AL or #3 CU? How is the #2 AL compliant with a 100 amp overcurrent device? If it is compliant, I am just curious as to how it is. Thanks!
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That would be a feeder. See 310.12
 
Okay, I see now. So, #2 Al or #4 Cu for 100 amp. Why 83% though? What’s the theory behind that? Thanks for the reply!
 
acolella said:
Okay, I see now. So, #2 Al or #4 Cu for 100 amp. Why 83% though? What’s the theory behind that? Thanks for the reply!
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The theory is just the fact that the Article 220 load calculations result in a much oversized service and the conductors will never see anywhere near the calculated load. :)
 
don_resqcapt19 said:
The theory is just the fact that the Article 220 load calculations result in a much oversized service and the conductors will never see anywhere near the calculated load. :)
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Okay, I think I understand the calculated load resulting in an oversized service. What I don’t completely understand is the service rating and OCPD. It is a “100 amp service” but technically, wouldn’t this would be more like a 90 amp service with those conductors? Why the 100 amp OCPD and not a 90 amp? What if 90 amps is exceeded on those conductors?

I’m sure this has probably been asked countless times. Forgive me, like I said, I have very little experience in residential and none with a multi-dwelling situation. This is purely a curiosity thing for me. I have been in commercial/industrial in an HVAC test lab for the last 10 years, primarily involved with motor circuits, controls, etc. I’ve never sized or installed a service so I’m just trying to get a better understanding of it. I appreciate the replies!
 
My understanding is that due to load diversity even in the event that the conductors, protected by a 100 amp device, happened to be loaded to 100 amps it would be for a short duration mainly do to equipmemnt cycling (stove, ranges, water heaters, etc) and with the other safety factors built in there is little likelihood of conductor damage.
 
Yes as Augie said, in dwelling units there is a diversity, IE not all loads are on at the same time. The 83% has been in the NEC since the mid 50's
And even in your area of experience, services are typically oversized and the connected load is much smaller that calculated.
 
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