system voltage drop

[dinkelja]

A senior electrical designer is trying to convince me that on a 480 system voltage drop isn't really a consideration. Here's the situation. We have a feed to an 800A MCC with about 434 TCHP / 514 KVA of motor load. He insists that since the voltage drop over the feeder is on 1-2% that the voltage at the MCC bus is only around 470V. I am trying to explain that the total system voltage drop (%) isn't equal to the sum of each branch circuits voltage drop %. I calculated 9.5% voltage drop at the bus, 434V! The overall substation bus was, 440V. The total design load is 1380 kva on a 1MVA trans. This is a continuous production line, with probably an 80% DF and 10% diversity. Am I the naive one here? What would be a simple explanation / brief example to calculate the overall system voltage with just a few motors?
Thanks j.d.

 

[dpc]

Voltage drop can certainly be an issue at 480 V. You need to limit total voltage drop to around 5% max. For a consistently loaded system, some of the drop can be countered by changing the transformer tap. You haven't really given us enough info to verify your voltage drop calcs, even if someone was inclined to do so.

The Eaton Cutler-Hammer Consulting Engineering guide has some simple tables that will allow you to get a good handle on approximate voltage drop through a feeder, based on expected load current. You also have to factor in the voltage drop across the transformer.

The power factor of the load has a significant effect on voltage drop.

 

[rbulsara]

It depends on under what conditions the the voltate drop is being considered . System reactance matters only during a motor starting. This is important select a appropriate starting method. Once the motor is up and running only the drop in the branch circuit matters, assuming the taps on 480V source tranformers are set to maintain voltage reasonably close to 480V. This is useful for sizing the branch circuit conductors.

 

[stevenal]

Your NEMA motors should have a nameplate voltage of 460, and tolerate +/- 10% around that. 460 motors on a 480 service allows for some drop.