Article 230.79(C)

Eddie702

Licensed Electrician
Basically for a dwelling unit(s) only the Code has decided over many years of calculations and load reports from dwelling units that regardless of the size of the service installed the conductors are almost never loaded to more that 83% of the service size.........just something somebody dreamed up I guess. So for a dwelling the over current protection must be 100 amps minimum size and the code making panels have decided that a wire rated at least 83% of the over current rating is in a dwelling only suitable for the service or a feeder that carries the entire load.

Eddie702

Licensed Electrician
I think @texie explained it better than I did

Davisteam

Member
But doesn't 230.42(B) say we need wire rated for 100A?

texie

Senior Member
But doesn't 230.42(B) say we need wire rated for 100A?
If you meet the requirements of the 83% rule then it is "100 amp wire". Conductor ampacity is not just what the table says-the table is just the basis you use to determine final ampacity depending on a number of factors such as the 83% rule.

Dennis Alwon

Moderator
Staff member
But doesn't 230.42(B) say we need wire rated for 100A?

In general this is true however the 83% rule is an exception for dwellings. Why? Because there is so much diversity in a home there is no way you will see 100 amps on a 100 amp service if you have done the calculations. If it ever got close it would only be for a short time.

Davisteam

Member
In general this is true however the 83% rule is an exception for dwellings. Why? Because there is so much diversity in a home there is no way you will see 100 amps on a 100 amp service if you have done the calculations. If it ever got close it would only be for a short time.
It just seems weird. I understand why we do this for motors, but in every other situation I always thought the protection should be the weakest link in any circuit. Definitely not the conductors.

Fred B

Senior Member
In general this is true however the 83% rule is an exception for dwellings. Why? Because there is so much diversity in a home there is no way you will see 100 amps on a 100 amp service if you have done the calculations. If it ever got close it would only be for a short time.
Also, business/commercial customers have a lot of purpose driven loads creating a higher constant load demand. What would a residence calculations be if most circuits calculated as small appliance circuits,

Dennis Alwon

Moderator
Staff member
Also, business/commercial customers have a lot of purpose driven loads creating a higher constant load demand. What would a residence calculations be if most circuits calculated as small appliance circuits,
Most circuits are involved in the same area of demand as the small appliance branch circuit in the standard calculation.

Davisteam

Member
Also, business/commercial customers have a lot of purpose driven loads creating a higher constant load demand. What would a residence calculations be if most circuits calculated as small appliance circuits,
Makes sense, but again, I just always, logically, thought the weakest link in any circuit should be the protection.

LarryFine

Master Electrician Electric Contractor Richmond VA
Wire ampacity is based on avoiding insulation damage, which occurs over time, not from instantaneous events.

That's one reason undersized EGCs can provide effective OCPD operation: faults are short-duration events.