Voltage Drop

[Alwayslearningelec]

Not sure if I'm calcuting this correct for primary(277/480v) side.

Have RGS conduit and copper XHHW wire going from a 100AF/70AT breaker in switchboard 850' to 70A enclosed breaker and then primary side xfrmr.

They are saying to run 3#350 & #3/0. Seems oversized. I come up with 4/0 to keep under 3% VD.

BTW when plugging in valued to VD calcutor how would I know whether to use 277 or 480v? Thanks

 

[infinity]

You would use 480 in the calculation. What are you using for the current value?

 

[electrofelon]

Hmm I get 1/0 using the full 70 amps. If I were doing it I would probably use at most 80% of the 70 which would be #1.

 

[Julius Right]

If I use NEC Table 9 [Effective Z for steel conduit] I get for 1/0 2.79% drop.

 

[Alwayslearningelec]

Hmm I get 1/0 using the full 70 amps. If I were doing it I would probably use at most 80% of the 70 which would be #1.

Wow ..maybe they are figuring aluminum??? THey don't say CU or AL for the insulation?

 

[ptonsparky]

Wow ..maybe they are figuring aluminum??? THey don't say CU or AL for the insulation?

So YOU made it Cu for the Calculations?

 

[ptonsparky]

Aluminum would still be closer to 3/0.

Inrush for an assumed 50 Kva transformer shouldn't be a problem at 850', otherwise a 70 amp breaker is marginal for inrush.
Then again, it may be with 350s.

eta: What is the transformer size?

 

[electrofelon]

Wow ..maybe they are figuring aluminum??? THey don't say CU or AL for the insulation?

You did say this:
"I come up with 4/0 to keep under 3% VD"
So how did you come up with 4/0?. Ask yourself and get back to us when you gets back to you on that

 

[PD1972]

It looks like the transformer is going to be 45kVA, so I would use 54A at 480V for the calculation.

Based on the 350kcmil (at copper) the drop at full load of the transformer is ~1%VD. Is that what the designer is trying to do by leaving 4%VD for everything fed from that panelboard?

 

[augie47]

It looks like the transformer is going to be 45kVA, so I would use 54A at 480V for the calculation.

Based on the 350kcmil (at copper) the drop at full load of the transformer is ~1%VD. Is that what the designer is trying to do by leaving 4%VD for everything fed from that panelboard?

Forgive my "stupid" but I don't understand that.....
the transformer appears to be the only load on the 480 v feeder and if he keeps that to 3% at full current that should not affect the panleboard load should it ??

 

[PD1972]

Forgive my "stupid" but I don't understand that.....
the transformer appears to be the only load on the 480 v feeder and if he keeps that to 3% at full current that should not affect the panleboard load should it ??

Total allowable combined VD between feeder and branch circuit per adopted state energy codes I am familiar with has been 5%.

OP mentions the feeder is coming from a switchboard, which I am assuming to be the service. You can definitely keep the feeder VD to 3% between switchboard and transformer as it wouldn't technically affect panelboard loads; however, branch circuit conductor sizes of circuits fed from the panelboard may be larger to achieve a 2% branch circuit VD.

A 1% feeder VD between switchboard and panelboard would allow for branch circuit conductor sizes to be sized to 4% max VD. I do not know if the increased cost in a larger feeder would offset the cost of larger branch circuit conductors, but that is (in my opinion) what would drive the decision.