Conductor size and why?

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nickelec

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So I'm not that great at the theory end of things. I have been thinking about something maybe someone here can provide an explanation to. So let's say your doing a typical two family service in a residential dwelling 2 100 amp panels and meters . Now let's say we would use a #2AWG as I would . So an apprentice asked me shouldn't you use a 3/0 since you have 2 100 amp panels eaquling 200 amps and I really couldn't provide him with a proper answer. So i guess my question is y don't we add up the combined amperage . I feel like I know why but can't really put it into words.

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From my perspective, the apprentice is thinking properly as far as theory goes.

310.15(B)(7) allows #4 copper for 100A services to individual dwellings and applies to a service entrance conductor set to each of the dwellings. However, a combined service entrance conductor supplying both meters and disconnecting means cannot utilize the smaller size permitted under 310.15(B)(7) and must have a 'regular' ampacity not less than the combined calculated load. This would not necessarily require 200A service entrance conductors. It could be something less. However, assuming the total demand load approaches a 200A combined demand load total, running 3/0 copper would be correct.
 
nickelec said:
So I'm not that great at the theory end of things. I have been thinking about something maybe someone here can provide an explanation to. So let's say your doing a typical two family service in a residential dwelling 2 100 amp panels and meters . Now let's say we would use a #2AWG as I would . So an apprentice asked me shouldn't you use a 3/0 since you have 2 100 amp panels eaquling 200 amps and I really couldn't provide him with a proper answer. So i guess my question is y don't we add up the combined amperage . I feel like I know why but can't really put it into words.

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You don't ever size conductors based on breakers, you size conductors based on load and adjustment factors and then you select the proper breaker.
 
ActionDave said:
You don't ever size conductors based on breakers, you size conductors based on load and adjustment factors and then you select the proper breaker.
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Close but not completely true.

Code, in general, requires the OCPD be rated not less than 125% the continuous load plus 100% the noncontinuous. As a circumstance of this requirement, the circuit conductor may need be the next size larger than otherwise required.
 
Wait a second, it's the overcurrent protective device that is allowed to be the next largest size by 240.4(B) up to 800A, with the feeder meeting 125%C/100%Non-C load and adjustment and correction factors.
 
310 BLAZE IT said:
Wait a second, it's the overcurrent protective device that is allowed to be the next largest size by 240.4(B) up to 800A, with the feeder meeting 125%C/100%Non-C load and adjustment and correction factors.
Click to expand...

That's correct. OCPD does play a role in governing the wire size, but in general, it is the load that matters first, and the OCPD calculation is a secondary criteria. So if you put a 400A breaker in an application where a 200A unit would suffice, that breaker will govern wire sizing. But if you put a 200A breaker, where your 125%/100% mix is only 180A, chances are, you only need 180A worth of wire.
 
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