Feeder voltage drop

[relbas]

I have a feeder 900' long 2 sets of 500 Mcm each set in separate 4" underground conduit at 460 Volt and it is protected with 400Amp fuses.
I would like to increase a fuse to 600Amp.
My question: Can I load this feeder at 100% (600Amp) or in need to be protected at 80 % (480Amp) of max load?
This is a feeder to the separate building (school).

 

[iceworm]

I have a feeder 900' long 2 sets of 500 Mcm each set in separate 4" underground conduit at 460 Volt and it is protected with 400Amp fuses.
I would like to increase a fuse to 600Amp. ...

Absolutely, 600A fuses are fine. 700A fuses are fine. Under certain conditions you could even use 800A fuses.

... My question: Can I load this feeder at 100% (600Amp) or in need to be protected at 80 % (480Amp) of max load? ...

You can load it to 125% of the continuous load plus 100% of the non-continuous load.

However, since you are asking, I'd be wondering, what are you doing about voltage drop, coordination, arc-flash. Increasing the fuse size and loading on a feeder without looking at these, i don't know how you know where you are.

Just curious, what does your engineer-of-record say about this increased loading?

ice

 

[relbas]

I am considering a voltage drop.
what I am trying to achieve is to increase the feeder capacity from 400 Amp to 600 Amp.
And engineer-of-record say that I don't have a capacity to do this

 

[infinity]

So let's assume a 600 amp load, 480 volts, 3?. According to this calculator you need 900 kcmil. Sounds like 2-500's might work.

http://www.elec-toolbox.com/calculators/voltdrop.htm

 

[texie]

How you 'gonna put 600 A fuses in a 400 A fuse holder?

 

[relbas]

I do have a600 rated fuse bucket

 

[ramsy]

what I am trying to achieve is to increase the feeder capacity from 400 Amp to 600 Amp.
And engineer-of-record say that I don't have a capacity to do this

Ask the engineer what happens if each conduit its designed with 3 ccc's paralleled per 300.3(B)(1)[Exception]. I'm showing 59?c @600A, vs 90?c if paralleled with 7-9 ccc's per 300.3(B)(1). The voltage drop should be under 3%.

Regardless, your stuck with engineer approval, or perhaps paying extra for a second option.

 

[iceworm]

... what I am trying to achieve is to increase the feeder capacity from 400 Amp to 600 Amp. ...

I think we all got that.

I ... And engineer-of-record say that I don't have a capacity to do this

Well, he has the whole story - we don't.. Did you ask why he thinks there is not capacity?

ice