Feeder wire size

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ktm400sx

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I have a 400 amp breaker that I'm using to feed a MCC. Can I apply 240.4 (B) and use 1x 500mcm per phase?
This will give me 380 amps according to 310.16 at 75 deg c The breaker is rated 40 deg C .....how does this effect the wire size?
Thanks for any help
 
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ktm400sx said:
I have a 400 amp breaker that I'm using to feed a MCC. Can I apply 240.4 (B) and use 1x 500mcm per phase?
This will give me 380 amps according to 310.16 at 75 deg c
Click to expand...

Yes assuming the calculated load is less than 380 amps.

The breaker is rated 40 deg C .....how does this effect the wire size?
Thanks for any help
Click to expand...

It does not.

Your breaker is likely rated 60/75C, the 40 C rating has noting to do with the terminal temp rating for wire sizing
 
ktm400sx said:
I have a 400 amp breaker that I'm using to feed a MCC. Can I apply 240.4 (B) and use 1x 500mcm per phase?
This will give me 380 amps according to 310.16 at 75 deg c The breaker is rated 40 deg C .....how does this effect the wire size?
Thanks for any help
Click to expand...
If the caculated load is 380 amps or less then you can use the 500 kcm.
However, if the breaker is rated at 40C then you have a problem. Todays breakers are rated at 65/75 or 75C. Check the temp rating again. Sorry Iwire. We were burning up the
keyboard at the same time.
 
The 40 degree C rating is for the ambient temperature of the breaker not the terminals.
 
ktm400sx said:
..Can I ..use 1x 500mcm per phase? This will give me 380 amps according to 310.16 at 75 deg c
Click to expand...

Motors require other considerations not mentioned yet.

1. I see rising motor-load currents during V-drop pulling an extra 1.3A at PF ~ 0.85, 1.6A at PF=1, for every 25ft of #500cu feeder @75c.

2. If those motors run continuous, load x 1.25, that #500 may be too small.

3. If cable size is increased, the lower impedance increases the available fault current, perhaps approaching the limits of starters & equipment AIR's.
 
500kCMIL

500kCMIL

I believe that 500's would be ok, yes they are only rated for 380A but if the breaker is a standard molded case breaker are rated for 80% of the breaker rating in this case 320A so 500's would be fine, if this breaker was rated for 100% then 600's might be a better selection.
 
mike0w said:
. . . but if the breaker is a standard molded case breaker are rated for 80% of the breaker rating in this case 320A so 500's would be fine. . . .
Click to expand...
I think not. A breaker is rated for its rated rating. ;)

The total of the non-continuous load plus 125% of the continuous load should not exceed 500 amps, for a 500 amp breaker. If it's all non-continuous, you can load a 500 amp breaker to every bit of 500 amps.
 
ramsy said:
Motors require other considerations not mentioned yet.
Click to expand...
ramsy said:

2. If those motors run continuous, load x 1.25, that #500 may be too small.
Click to expand...

Even if all motors run 24 hours a day, you only have to apply the 1.25 factor to the single largest motor.
 
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