Rules of Thumb and Words of Advice for low vs medium voltage motor selection

[Natfuelbilll]

I am early in a design for a 200 hp pump and have an opportunity to select low 460 or medium 4160v voltage equipment. The pump motor controller would be fvnr or soft start.

I take 4160 from the poco.

Your thoughts?

 

[rcwilson]

200HP is close to a break point that can go either way depending on specific situation. Many things to look at:

Is 4.16kV switchgear or MCC exisitng making it cheaper to add a starter?
Plans for expansion?
How many 4.16kV - 480V transformers are needed to feed 480V loads? Will adding a 4.16kV switchgear allow reducing the 480V conductor costs by placing transformers near the loads- a side benefit of having 4.16 kV distribution.
Will moving the motor to 4.16 kV reduce size and cost of 480V transformer?
Motor at 4.16 kV reduces the 480V transformer losses and may reduce 480V short circuit levels and arc flash if the 480V transformer can be smaller. Also the short circuit contribution from the motor is reduced by the 4.16 kV- 480V transformer.

4000V motor costs more than 460V.
FVNR contactor at 4.16 kV is a little more expensive than a Size 5 480V.
Soft start at 480V is usually cheaper.
Long run from MCC to motor means more copper at 480V, making 4,160V competetive. Short run, go with 480V.

 

[Julius Right]

I agree with rcwilson. The equipment and the cable of 4 KV are more expensive. The short-circuit current at 4 KV could be 30-40 KA and at 480 V 10-15 KA. You need, for instance, 4/0 or 250 MCM copper for 4 KV and only 2 awg for 480 V. However, for 210 A [motor rated], NEC Tab. 310.15(B)(16) for XLPE or EPR [90 dgr.C] requires 3/0 copper only. If you will provide a soft-start 3/0 will be enough for 1000 feet cable length [voltage drop].

 

[rcwilson]

. You need, for instance, 4/0 or 250 MCM copper for 4 KV and only 2 awg for 480 V. ].

Good point about short circuit withstand of the cable. If we use fused MV Contactors we may be able to get a fuse that clears before the smaller cable is damaged. If we depend on the contactor or a circuit breaker to clear the fault, the extra clearing time does force us into larger cables, negating some of the savings.

 

[mayanees]

Arc Falsh Hazards

Arc Falsh Hazards

Consider that arc flash levels for comparable capacity systems are inherently less for Medium Voltage systems than the low voltage counterpart. Depending on the capacity of the source that's supplying the motor, the LV system would likely be HRC Dangeorus, preventing any energized interaction.
John M