Sizing conductors

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jeffwh28

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What size conductor (paralleled in two raceways) is required for an 800A breaker that supplies a 600A continuous load?
(a) 250 kcmil
(b) 300 kcmil
(c) 500 kcmil
(d) 600 kcmil
I am struggling with the answer and how it was figured out.
I came up with (c) because 600/2 =300X125%=375A (75 deg) =500 kcmil @380A

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600 amp continuous load = 750 amps parallel conducti9rs = 750/2 = 375.. 250 kcmil is good for 405
 
jeffwh28 said:
What size conductor (paralleled in two raceways) is required for an 800A breaker that supplies a 600A continuous load?
(a) 250 kcmil
(b) 300 kcmil
(c) 500 kcmil
(d) 600 kcmil
I am struggling with the answer and how it was figured out.
I came up with (c) because 600/2 =300X125%=375A (75 deg) =500 kcmil @380A

Help
Click to expand...
You have the correct answer but the way you arrived at it is wrong.
 
You need to size the conductors according to the 800 amp OCPD and then they must also be large enough for the load. If the load were 610 amps continuous then the 500's are too small with the next size up rule. In the Op's example it didn't change anything.
 
infinity said:
You need to size the conductors according to the 800 amp OCPD and then they must also be large enough for the load. If the load were 610 amps continuous then the 500's are too small with the next size up rule. In the Op's example it didn't change anything.
Click to expand...
Why would you have to size to the breaker? I thought you calculated the load and then use the breaker size to accordingly. Load is 600*1.25 or 750. 750 is not a standard breaker so 240.4(B) allows the 800A breaker.
 
I got that answer right (500kcmil) but didn't work problem like answer showed and that is why I wanted to know how to work problem right, because next question is worked the same way and I didn't get it right, because I did math off of Continuous load not circuit breaker size. Here is the next question!
What size conductor (paralleled in three raceways) is required for a 1200A breaker that supplies a 900A continuous load?
(a) 250 kcmil
(b) 300 kcmil
(c) 500 kcmil
(d) 600 kcmil
 
jeffwh28 said:
I got that answer right (500kcmil) but didn't work problem like answer showed and that is why I wanted to know how to work problem right, because next question is worked the same way and I didn't get it right, because I did math off of Continuous load not circuit breaker size. Here is the next question!
What size conductor (paralleled in three raceways) is required for a 1200A breaker that supplies a 900A continuous load?
(a) 250 kcmil
(b) 300 kcmil
(c) 500 kcmil
(d) 600 kcmil
Click to expand...

d

240.4(C)
 
oldsparky52 said:
I think most people would do all the calculations then divide by 2 where the OP divided by 2 first and used the 1/2 load in the calculations. I think it looked odd, but the math was correct.
Click to expand...
Yes the book divide 800 instead of 600 but answer was the same either way,
How about this question because answer is not the same?
What size conductor (paralleled in three raceways) is required for a 1200A breaker that supplies a 900A continuous load?
(a) 250 kcmil
(b) 300 kcmil
(c) 500 kcmil
(d) 600 kcmil
 
jeffwh28 said:
Thanks I see that now, but other question divided 800A OCPD instead of 600A continuous load, why? The answer was the same anyway, just want to know why it was calculated that way
Click to expand...

800A is the largest rating where you can use 240.4(B), and take credit for the next size up rule. 600A*125% = 750A, and as long as it is less than 760A, the ampacity of two parallel 500's can "round up" to 800A, and be considered protected.

Once it is over 800A, or if it is a non-standard size in the first place, you need at least as much amps of conductors/terminals as you have OCPD. So add 50% more continuous load, and it is not enough to use 3x 500's. But now it is necessary to use 3x600's, given that the OCPD is 1200A.
 
oldsparky52 said:
Why would you have to size to the breaker? I thought you calculated the load and then use the breaker size to accordingly. Load is 600*1.25 or 750. 750 is not a standard breaker so 240.4(B) allows the 800A breaker.
Click to expand...
I was slightly off on a tangent. I meant to say that you need to consider both the OCPD size and the load when calculating the conductor size. You could end up with a question where the conductors were sized properly for the load but are too small for the OCPD size.
 
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