what am i doing wrong

[Ponchik]

Question came up in class, Total calculated load for the feeder is 125A continuous, what size wire and OCPD do i need?

125 x 1.25 = 156.25A

wire size 310.16 THWN = 2/0 copper

OCPD = 175A

Looks good so far.

But if I take the 80% of 175 then it comes out to 140A. which means the breaker maximum continuous capacity is 140A but our load is 156A.

what am i missing here or doing wrong?

Thanks

 

[bob]

Question came up in class, Total calculated load for the feeder is 125A continuous, what size wire and OCPD do i need?

125 x 1.25 = 156.25A
So far your calculations are correct.

wire size 310.16 THWN = 2/0 copper
With a load of 156 amps, the next conductor size for this ampacity is 2/0 at 175 amps.

OCPD = 175A
you then choose a breaker for this load and it is 175 amps because it is the next size standard size.. So in this case the breaker and conductor match with regard to ampacity. If a 160 amp breaker would have been a standard size, you could have used it.

Looks good so far.

But if I take the 80% of 175 then it comes out to 140A. which means the breaker maximum continuous capacity is 140A but our load is 156A.

You have 2 ways to calculate this. You can take the continuous load of 125 amps and divide by 0.80 which give you 125 amps/0.80 = 156 amps
or you can multiply the 125 amps by 1.25 which give you 125 x 1.25 = 156 amps. 1/1.25 = 0.80. Actually your load is not 175 amps. It is 156 amps when you multiply by 1.25 You rounded up to 175 because that was the next size breaker. 156 x 0.80 = 125 amps. I hope this clears this up for you. If not, ask again.





what am i missing here or doing wrong?

 

[Ponchik]

Bob Thanks,

OK it just clicked. :slaphead:

my load will not go higher than 125A. My breaker will hold continuous 140Amps.


i got it, thanks again.