ground size for 25A base heat circuit?

[malachi constant]

Where am I going wrong?:

I have 11.5' of base heating, rated at 300W/ft, so the load is 3450W. At 208/1 this translates to 16.6A.

Since base heat is considered a continuous load I have to size my feeder and breaker at 125%, or 20.7A. This will be a 25A/2P breaker. And using Table 310.16 this can be fed with two #12s.

To size the ground I look at Table 250.122 and determine that a 25A breaker results in a #10 copper ground.

So I have 2#12 w/ #10 cu ground? Does the ground really need to be bigger than the feeder?

 

[ron]

Take a look at 240.4(D).

 

[ron]

If for some wacky reason, the phase conductors turned out to be smaller than the EGC listed in Table 250.122, then look at 250.122(A)

 

[malachi constant]

Thanks. I knew the 250.122(A) clause existed but didn't know where to look. And I totally forgot the 240.4 clause existed.

 

[busman]

What exception are you using to get around the 310.16 requirement that limits #12 to 20A OCPD?

Mark

 

[steve66]

What exception are you using to get around the 310.16 requirement that limits #12 to 20A OCPD?

I think you mean there is no exception for this case. I agree, you need all #10's.

But I have to admit, in this case, I might be tempted to cheat. If the 20.7 amps was 0.3 amps less, you could use 220.5(B) and use #12 and a 20A breaker. I don't really think using 123% instead of 125% is going to cause any problems. But if I used #12, it would definately be a 20A breaker.


Steve

 

[busman]

Steve,

That's what I meant. Thanks for the clarification.

Mark

 

[steve66]

Mark:

Your post didn't really need any clairification. I was just throwing my two cents in that its really close to a 20A circuit.

Steve