750 MCM Aluminum for 400A Panel?

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l3city

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Hi fellas, I have a 400A/3-phase panel. The length from the Service to the panel is 350 Feet. The voltage system is 120/208V, 4-wire. The feeder runs underground in PVC-40 conduit. The ambient temperature in in this place is about 120F.

The electrician says it is safe to feed this panel with a 750MCM aluminum to cover for the 400A Panel. After my derating factors and votage drop I find this 750 MCM Aluminum feeder to be good for about 356 Amps. (.82 x 435 Amps per NEC table 310.16 and Temperature correction factors)

Is this correct?
Thanks ahead for any input:smile:
 
The ambient temp. is 120F underground?

Secondly the VD is based on the actual load not on the conductor ampacity per se.

It is also possible if this feeder is underground from another structure you would need an EGC run with it if you are under the 2008 NEC.
 
Will the 750's even fit under the 400A CB ....will there be enough wire bending space, also?

IMHO, 750's cannot be correct here.
 
If my calculations are correct the max size of AL required would be 700MCM

I did calculate the voltage drop @ 3% however instead of my normal 2% for a feeder.
 
Dennis Alwon said:
The ambient temp. is 120F underground?

Secondly the VD is based on the actual load not on the conductor ampacity per se.

It is also possible if this feeder is underground from another structure you would need an EGC run with it if you are under the 2008 NEC.
Click to expand...

a)Ambient above ground is anout 120F (feeders are underground)
b)Yes the EGC is run with the (4) 750 MCM (1-set only, no parallel run)

What the electrician says is that we shall not apply the Temperature Correction Factor (under table 310.16) because the feeder is underground.

What you think?
thanks for responding
 
l3city said:
a)Ambient above ground is anout 120F (feeders are underground)
b)Yes the EGC is run with the (4) 750 MCM (1-set only, no parallel run)

What you think?
thanks for responding
Click to expand...

What distance are we talking about that is exposed to the 120F.
 
Dennis Alwon said:
I believe art. 310.15(A)(2) except. unless I interpreted that incorrectly.
Click to expand...

Ok things seems to be a little clear now, but how about Voltage Drop (350 Feet feeder length)
thanks for responding


Ooops I just got your response, thanks tool is good
 
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Ok, I am getting the same value as I got before, and this 750 MCM Aluminum will not be good for the 400A panel, thanks all for your responses
:smile:
 
l3city said:
Ok, I am getting the same value as I got before, and this 750 MCM Aluminum will not be good for the 400A panel, thanks all for your responses
:smile:
Click to expand...

What is the calculated load on the panel? If you forsee the panel being used to it's capacity then it is undersized. Remember the NEC is silent on VD this is a design issue. Check the calculator with the actual load expected and see what you get.
 
Dennis Alwon said:
What is the calculated load on the panel? If you forsee the panel being used to it's capacity then it is undersized. Remember the NEC is silent on VD this is a design issue. Check the calculator with the actual load expected and see what you get.
Click to expand...

Exactly, since we don't have any control as to how much load is going to added in the future for this panel, than we consider the worse case to be 400A in the VD. I have seen panels getting very hot almost at their full capacity, and we try to prevent that.

What we will end-up doing is just reducing the main breaker size at the panel to be 350A and also to accomplish for the 75-deg rated lugs at the panel.
thanks
 
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