Voltage drop - demand load assumptions

[lielec11]

I have a client in a bind requesting VD calculations to confirm their switch to AL feeders however it's honestly not something I'm tasked with doing quite often. I have the riser/panel schedules from the engineer, however, how do you decide what type of demand load to put on all of the panelboards? The panel schedules from the engineer don't provide any load calcs or information.

Thanks.

 

[junkhound]

just use 80% of breaker rating?

 

[lielec11]

My first thought was to use something similar like 75% but I was just seeing if anyone had any other suggestions. Seems these results are only as good as the assumptions.

 

[winnie]

Why would voltage drop be any different with Al rather than Cu?

Jon

 

[lielec11]

Why would voltage drop be any different with Al rather than Cu?

Jon

Their rationale is that they can save money by switching to AL feeders but they need to prove there will be no VD issues by switching since it differs than the EORs design.

 

[augie47]

Your post made me curious so I played with some numbers on a VD calculator. 150 amp load at 100 ft resulted in a 2.3 volt drop with 3/0 cu, 3.0 with 4/0AL and 2.54 with 250AL.

 

[oldsparky52]

If you use an AL conductor that has the same or less resistance than the copper conductors on the drawing, ... what VD problem could you have?

 

[winnie]

Your post made me curious so I played with some numbers on a VD calculator. 150 amp load at 100 ft resulted in a 2.3 volt drop with 3/0 cu, 3.0 with 4/0AL and 2.54 with 250AL.

Yes, but in your calculation you are considering conductors that are already oversized vs the minimum required for 150A. Compare instead the minimum required Cu vs Al for a 150A circuit.

Al conductors will have the same voltage drop as Cu as long as they are large enough to have the same resistance. (Approximately, inductive effects will be different. ) But since Al conductors have more surface area for the same resistance they have a bit more ampacity.

Jon

 

[winnie]

Their rationale is that they can save money by switching to AL feeders but they need to prove there will be no VD issues by switching since it differs than the EORs design.

The resistivity of Aluminum is 54% greater than copper.

If you use Al conductors 54% larger in cross section than the designed Cu conductors then voltage drop will be very similar.

Going up 2 AWG steps increases wire cross section 59%.

So if you change from Cu to Al 2 AWG bigger your voltage drop is more or less the same.

Jon