feeder sizing

[steveng]

the scope is to run a feeder for a genset optional load,
the gen is 35kva 480/277

the feeder run is 330'.
my load is 11 amp on the 480, supplying a 3 ph 5 ton ac with elec heat 10 kw

primary feed 480 to 240/120 1ph xfmr fla 20.8 10kva
secondary amps 1 ph xfmr 10kva, 10000/240 = 41.6a

my load calc is 5000va on 125a 240 1 ph panel = 21a


my question is, concerning the total load on the generator?
the panel load, is calculated at 21 amps or 5000 va 240 1 ph
the primary load is 11 amp on the 480, 3ph

what would the total load amps be on the gen at the calc load?
larrry, jim, bob?

my gen ocpd is 100a 480 3ph
the normal power feed is 70a 480 3 ph

wire size feeder?
max ocpd at panel?



thanks
Steve

 

[LarryFine]

my gen ocpd is 100a 480 3ph

To me, this is the pertinent info. Size all to suit the 100a breaker on the genny. 100a is a nice round number for a supply.

 

[iwire]

To me, this is the pertinent info. Size all to suit the 100a breaker on the genny. 100a is a nice round number for a supply.

That will work. :smile:

Or it could be excessive to run a 100 amp feeder 330' for a 11 amp load.

Could put a 20 amp overcurrent device at the genset and use any size conductor 12 and larger, the choice of size being based on just the load and the amount of voltage drop your willing to give up.

For that matter you might even get away using the outside tap rules and forget about the additional 20 amp breaker at the genset.

 

[Smart $]

...it could be excessive to run a 100 amp feeder 330' for a 11 amp load.

...

It sounds like he wants to power both the HVAC unit AND the 1? 480/240?120 xfmr as well.

 

[steveng]

It sounds like he wants to power both the HVAC unit AND the 1? 480/240?120 xfmr as well.

you are correct, smart$,

i want to tap the 480 3 phase feeder at the xfmr/panel location to supply a 30 amp 3 ph fused disco for the hvac, also power the xfmr 15kva, 1 ph 480/240/120 xfmr.

thanks larry,bob,smart$

 

[Smart $]

you are correct, smart$,

i want to tap the 480 3 phase feeder at the xfmr/panel location to supply a 30 amp 3 ph fused disco for the hvac, also power the xfmr 15kva, 1 ph 480/240/120 xfmr.

thanks larry,bob,smart$

I would start by assuming your total load to be 10+(5?3/2)=17.5kVA... which makes another 2.5kVA available and balances the lines.

17.5kVA/(480*1.732)=21A

From there you calculate voltage drop for the existing load (21A) to determine minimum size wire, and upsize if additional future loads are planned or likely. Whatever size wire you choose, the ocpd at the genset can be at least as high as it's ampacity, but cannot be less than 21A.

 

[steveng]

thanks smart$

thanks smart$

I would start by assuming your total load to be 10+(5?3/2)=17.5kVA... which makes another 2.5kVA available and balances the lines.

17.5kVA/(480*1.732)=21A

From there you calculate voltage drop for the existing load (21A) to determine minimum size wire, and upsize if additional future loads are planned or likely. Whatever size wire you choose, the ocpd at the genset can be at least as high as it's ampacity, but cannot be less than 21A.

thanks smart$, your name says's it all..