Feeder conductors

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augie47

augie47

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State Electrical Inspector (Retired)
Is there any condition other than voltage drop or ambient adjustments that would require an 800 amp buss-duct, protected by a 800 amp overcurrent device, to be supplied with other than 500 kcmil Cu conductors, parallel in seperate conduits?

would a continuous rated OCP device affect this ?
 
Re: Feeder conductors

Originally posted by augie47: . . . require . . . other than 500 kcmil
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That's a bit of an awkward way to say that. I think you are asking if there is anything that would forbid the use of a pair of parallel 500's.

My answer is yes. First you calculate the load, then you pick the conductors, then you pick the OCP. If the calculated load is above 760 amps, then you need a set of conductors that can handle above 760 amps. A pair of parallel 500's can't do that job.

On the other hand, if the calculated load is 750, then a pair of parallel 500's can handle that load, and you can protect those conductors with an 800 amp OCP.
 
Re: Feeder conductors

Originally posted by charlie b:
On the other hand, if the calculated load is 750, then a pair of parallel 500's can handle that load, and you can protect those conductors with an 800 amp OCP.
Click to expand...
We agree on this one. :cool:
 
Re: Feeder conductors

charlie, sorry for the wording...I was trying to indicate that it might be a good idea to use larger even if not required, however, you hit on my problem....
engineering drawings don't show a "load" on the duct, just an overall building load. The "connected" load is well under 760 amps and we don't enforce "what if" situations, but the entire idea of the buss duct is to acomodate added load.
I want to say the conductors must be rated >800 but, on the other hand, the OCP device, unless it has a 100% rating would not sustain a continuous load of 760 amps itself.
 
Re: Feeder conductors

Augie would you require the feeders to an 800 fused disconnect to rated greater than 800 amps if the connected load was 740?

There is good design than there is code minimum.

You can only force compliance with one of those items.
 
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