wiring calculation

[dm9289]

I have a 520a load and I wanted to calculate wire size by tray and by conduit ithink I am correct please confirm

Either case this is a continuos load so 520 x 1.25= 650a cable required

for conduit paralleled conductors i determined 2x700kcm thhn =1040a based on 650a needed

I multiplied 1040a x .82 temp correction 114degrees=852.8
then 852.8 x .80 for 6 current carrying conductors in raceway= 682.24 which is good for 650A ????



In cable tray still 650A required
used 2-500kcm thhn cables with their free air rating 2*700=1400A
Per 3392.80 A(2)b derated by.65 1400 x .65=910
for temp 114 degrees derated by.82 =746????

I believe I am correct could someone please confirm or correct me
Thanks

 

[dm9289]

the load is a 520amp 3 phase load so i will use 6 wires either way

Also the code section should read 392.80 (A)(2)(b)

 

[Smart $]

Assuming copper, 75?C terminations, 90?C rated conductors...

SIZE:
Parallel... 520A ? 125% = 650A... ? 2 = 325A... 2 sets 400kcmil @ 75?C, 335A each, minimum.

AMPACITY:

• (1) Conduit... 520A ? 2 = 260A ... ? .82 ? 80% = 396A... 2 sets 500kcmil @ 430A each

Actual ampacity 430A ? 2 ? .82 ? 80% = 564A

​

• (2) Conduit... 520A ? 2 = 260A ... ? .82 = 317A... 2 sets 300kcmil @ 320A each

Not applicable because minimum size, but 400kcmil @ 380A is okay.​

Actual ampacity 380A ? 2 ? .82 = 623A

​

• Cable Tray (not spaced)... 520A ? 2 = 260A ... ? .82 ? .65 = 488A... 2 sets 300kcmil @ 500A each

Not applicable because minimum size, but 400kcmil @ 615A is okay.​

Actual ampacity 615A ? 2 ? .82 ? .65 = 656A

​

• Cable Tray {spaced 1 diameter [392.80(A)(2)(c)]}... 2 sets 400kcmil @ 615A

Actual ampacity 615A ? 2 ? .82 = 1009A​

 

[dm9289]

in the numbered calculations you used 520load not the 650 continuos load is there a reason that was omitted

also i was hoping the equipment would have 90c terminations I will check with provider

also i assume you meant 3 sets of 2wire. 2 wires per phase on this 3 phase circuit

 

[Smart $]

in the numbered calculations you used 520load not the 650 continuos load is there a reason that was omitted

Factoring continuous 125% is for determining minimum SIZE conductor. You do not factor continuous when sizing for AMPACITY.

also i was hoping the equipment would have 90c terminations I will check with provider

Even if the equipment has 90?C terminations, it is quite likely the equipment itself is only rated 75?C. Termination rating is based on the lowest equipment rating.

also i assume you meant 3 sets of 2wire. 2 wires per phase on this 3 phase circuit

No. If you run 3 sets of 2-wire, your sizing will be based on 300A per conductor (not 260A). When you get to the load end with 2 sets of 3-wire, you have to tap both sets. Will have to refigure for 2-wire... (exercise for you )

 

[dm9289]

thank you very much for the reply I did not understand the continuous part till you explained it i think I understand let me use an example an let me know if it is correct

If i have a continuos load of 80a min wire size is 80X1.25 =100A wire size min 3thhn

But if I have correction factors say .82temp .8 #wires
80/.82/.8=121.95A wire size 2 thhn

So with these 2 stated I would use 2thhn cause it covers both cases


Sorry about the wire set question I misunderstood your original reply

 

[Smart $]

thank you very much for the reply I did not understand the continuous part till you explained it i think I understand let me use an example an let me know if it is correct

If i have a continuos load of 80a min wire size is 80X1.25 =100A wire size min 3thhn

But if I have correction factors say .82temp .8 #wires
80/.82/.8=121.95A wire size 2 thhn

So with these 2 stated I would use 2thhn cause it covers both cases

Correct.

But I now have to throw in the 'bone us'. You have to do the actual ampacity calculation because the conductor must be protected at its ampacity by the OCPD (breaker/fuse). That's why my earlier 'figgers' show actual ampacity calculation.

#2THHN@130A?.82?80%=85A
Minimum OCPD 80A?125%=100A

#2@85A actual is not protected by 100A OCPD. Ampacity must be at 91A per 240.4(B), 240.6. Minimum will have to be #1THHN@145A allowable ?.82?80%=95A actual

 

[dm9289]

Ok I see so I have adjustable trip switchgear so should iset it at 520x1.25 or 650 and size the wire to follow instead of at 520

 

[Smart $]

Ok I see so I have adjustable trip switchgear so should iset it at 520x1.25 or 650 and size the wire to follow instead of at 520

Depends on how you want to run it. Note in my first reply both cable tray methods have an ampacity over 650A. And the two conduit method has an ampacity over 600A. 240.4(B) permits the next higher standard OCPD rating, which is 700A, and also permits a lower adjustable trip setting (650A). As such, you'd only have to recalculate the one conduit method to have an actual ampacity over 600A, if that's how you want to run it.