Feeder Calc

[Alwayslearningelec]

We are pulling wire in existing conduits( one 2" and another 2" spare). The distance is 700'. 500' is underground pvc and 200' is interior overhead 200' RMC.

We are running from an 277/480v 800A breaker in a distribution board to 277/480v 800A panel HH-01-1-EXT-DPH-E-1. They want us to size the wire.

A few questions regarding voltage drop calc I'm using.

1. I'm inputting 480v and not 277 for the panels. Is that correct?
2. Should I really put 800A for current at end of run? I can safely use 80% of that?
3. As I said above they ran one 2" and one 2" spare. Even if I use 640A(80% 800A) I need both sets of 2" and would have ot run 600 MCM.

THoughts? Thanks

 

[charlie b]

1. Yes. You use the line-to-line voltage.

2. Use the calculated service load. It should include room for future growth. I suspect that value is well under 800 amps. Otherwise, you wouldn't select an 800 amp service board.

3. I don't have my NEC handy. But I seem to recall that 500 kcmil conductors have an ampacity of 380 amps. So two sets of them should suffice. I can't check to see if they will fit into 2 inch conduits, nor can I check the voltage drop resulting from using 500's at that distance and that current.

 

[Alwayslearningelec]

1. Yes. You use the line-to-line voltage.

2. Use the calculated service load. It should include room for future growth. I suspect that value is well under 800 amps. Otherwise, you wouldn't select an 800 amp service board.

3. I don't have my NEC handy. But I seem to recall that 500 kcmil conductors have an ampacity of 380 amps. So two sets of them should suffice. I can't check to see if they will fit into 2 inch conduits, nor can I check the voltage drop resulting from using 500's at that distance and that current.

Thanks Charlie. Problem is I'm going be responsible for wire size as I'm bidding so really need to make sure I'm right in the case we get the job. Am I able to get the calculated service load from the panel schedule above?

 

[don_resqcapt19]

I hope the 2" conduit size is a typo. It is not possible to supply an 800 amp circuit using two 2" conduits.

 

[Eddie702]

Agree your pipe is way too small

 

[infinity]

1. I'm inputting 480v and not 277 for the panels. Is that correct?

Yes.

2. Should I really put 800A for current at end of run? I can safely use 80% of that?

You should use the calculated load of everything connected to that distribution board not a random number.

3. As I said above they ran one 2" and one 2" spare. Even if I use 640A(80% 800A) I need both sets of 2" and would have ot run 600 MCM.

2" is not going to work unless you're going to redesign and use 4160 volts.

 

[Seven-Delta-FortyOne]

"But...But...We saved money by installing it ourselves!"

 

[charlie b]

Am I able to get the calculated service load from the panel schedule above?

Using only what you are showing, no. If you have modeled all panels in Revit, and if you have assigned a circuit and a load value to every pump, fan, heater, welder, receptacle, light, chiller, boiler, and whatever, basically everything on all the floor plans, then Revit will give you a total calculated load.

 

[charlie b]

Problem is I'm going be responsible for wire size as I'm bidding so really need to make sure I'm right in the case we get the job.

If you are going to be restricted to using two existing 2" conduits, then I recommend that you decline to submit a bid.

 

[Alwayslearningelec]

Yea I think it has to be a mistake.

 

[Alwayslearningelec]

Ok so conduit size aside( assuming the correct size will be indicated). How would you go about sizing the wire? What current would you use at the end of the run? THanks

 

[infinity]

What current would you use at the end of the run? THanks

This:

You should use the calculated load of everything connected to that distribution board not a random number.

 

[Alwayslearningelec]

This:

So how would I get the calc load for this distribution panel? I would need the panel schedules for the panels it's feeding, correct? What is the formula to figure calculated load? Thanks

 

[infinity]

I would need the panel schedules for the panels it's feeding, correct?

Yes the load information should be on the panel schedules. It should also tell you the calculated load on the distribution board. Someone had to calculate that 800 amps was sufficient for the total load. Or you can just use 800 amps for your VD calculation.

 

[raberding]

The owner just might be expecting to get 800 amps capacity regardless of the present calculated loads. I'd suggest providing 800 amps of conductors. And use 800 for the VD calc.

 

[infinity]

The owner just might be expecting to get 800 amps capacity regardless of the present calculated loads. I'd suggest providing 800 amps of conductors. And use 800 for the VD calc.

They very well could be but then they shouldn't leave the conductor size for the installer to figure out.

 

[Seven-Delta-FortyOne]

Every set of plans I bid has a full panel schedule, calculated load, and wire sizes spec’d.

 

[infinity]

Every set of plans I bid has a full panel schedule, calculated load, and wire sizes spec’d.

For some reason these plans do not include the conductor size for this feeder. Since the NEC does not require VD compensation for this two sets of 500 kcmil would be code compliant if the calculated load is 760 amps or less.

 

[Alwayslearningelec]

So in this example the panel schedule has a total connected load of 279 amps. It being feed from a 400A breaker. I'm assuming wire to the panel should NOT be sized to the connected load because they may( and probably will) add loads. So what would you size the wire to?

 

[raberding]

the conductors are protected by the upstream 400a breaker. So it needs 400a conductors. 500kcmil would work. The DEMAND LOAD is only 77a. So if the upstream breaker could be reduced to 100, you could use 100a conductors.