Commercial service wire sizing

[Wattstopper]

So what is the reason why 2/0 can be used on a 200a residential service but 3/0 is required for commercial? I do understand that insulation, voltage drop, and raceway installation com into play. A follow up to this is would a three phase 200a be sized differently?

 

[iwire]

Because the code making panels know that the NEC residential load calculations result in services larger than needed and the conductors will not be overloaded.

 

[Wattstopper]

I do understand that but if I am submitting a riser for an irrigation rack and I calculate a 150a service then what says that my wire must be 2/0 with a 150 main? There is not voltage drop and I will be using the proper insulated wire.

 

[Wattstopper]

But max amps are max amps, is there a demand factor in that calculation which results in a different allowable amperage for that size conductors?

 

[hurk27]

I do understand that but if I am submitting a riser for an irrigation rack and I calculate a 150a service then what says that my wire must be 2/0 with a 150 main? There is not voltage drop and I will be using the proper insulated wire.

Nothing says that a 150 amp service has to be a 2/0, if you are using copper table 310.16 says 1/0 is rated for 150 amps @75?c unless you are using aluminum then you would need at least a 2/0 which has a 135 amp rating but as long as the calculated load is less then the 135 amps then 230.90 exception 2 which points you to 240.4(B) will allow you to use the next size up which 240.6 says is a 150 amp breaker, other wise if the calculated load is over 135 amps then you would be required to use a 3/0 aluminum, so make sure you have your calculated load done correctly as being wrong can cost you.


The allowance of the next size down for dwelling services is because of diversity factors that we see in dwellings, like was said it would be very rare to ever have a dwelling that would continuously pull the full rating of the service, if it did someone didn't do their math correctly as per 220 in the NEC and undersized the service, but non-dwelling applications the service can very well be close to the rating of the service.