Sizing feeders and using the .83 rule

[Michelle1995]

Alrighty, I’ve been a long time lurker on these forums. But I have never signed up. I’m progressing in my career very quickly. So I think it’s time to start straightening out some questions I have.

So when sizing for a residential dwelling. The 83% rule can be applied towards the main breaker size.

Let’s say 150amp OCPD at 83% is 124.5amps. Now I size my wires using table 310.15(B)(16). I’m using XHHW-2. Now I have to use the 75 degree column, correct? So the 2/0 AL will cover me. But what I don’t understand is why we use 4/0 AL wire for the 200amp service wire when it’s rated for 180amps in the 75 degree column.

Can someone clear this up for me? I’m taking my masters in two weeks. I’m want to lock this in. Thanks all.

 

[LarryFine]

Welcome to the forum.

You always start with the calculated load, which may not exceed the wire's actual ampacity.

We can use a 200a breaker because the next lower standard rating is 175a.

Ask what you need more clarification about.