Switch Size Calculation

[mstrlucky74]

Looking for help on how I would size the following: Boy I gotta learn some electrical sizing formulas.

120/208 4000A switchboard and I need to feed a 120/208v 3 phase 200A motor. It would be a continuous load? How would I go about sizing the switch in the switchboard? Would it be 125% of the 200A? I think I need a disconnect by the motor so I'd be sizing to the disconnect.

Same switchboard and I need to feed a 120/208v 3 phase 300A panel. How would I go about sizing the switch in the switchboard?

Ok, now a tricky one(for me at least). I want to feed a 120/208 1200a dist board from the 120/208v 4000a board but want to feed it via 277/480. So I'd need a step up xfrmr by the 4000a board then a step down xfrmr by the 1200a board. How would I go about sizing the switch in the 4000A board and the two xfrmrs.

I know this is a alot and appreciate any feedback/overview on how to approach these questions on sizing.

So when sizing, equipment and conductors and you know the size of the load do you then size the OCPD then the feeder?

 

[Jraef]

If it's switching the motor, the switch needs to have a HP rating equal to or greater than the motor HP. HP ratings are different from just simple amp ratings. From a gross thumbnail standpoint, 200A motor current at 208V equates to roughly 75HP, so make sure whatever switch you get has a 75HP rating on it. You'll likely find that will be a 600A rated switch! (looking at the HP ratings of Sq. D disconnect switches)

For the other switches, it's covered in 225.39.

 

[electrofelon]

I'll try taking a stab at it to get things moving........

Looking for help on how I would size the following: Boy I gotta learn some electrical sizing formulas.

120/208 4000A switchboard and I need to feed a 120/208v 3 phase 200A motor. It would be a continuous load? How would I go about sizing the switch in the switchboard? Would it be 125% of the 200A? I think I need a disconnect by the motor so I'd be sizing to the disconnect.

A couple of comments:
1) continuous loads dont apply to motor loads - they are just motor loads
2) Presumably obvious, but you will need not just a switch but a fused switch. You need to protect the motor and the conductors at less than 4000 amps.
3) Note you need to use the FLA values in the NEC charts for your current rating.

Same switchboard and I need to feed a 120/208v 3 phase 300A panel. How would I go about sizing the switch in the switchboard?

Short answer is 300 amps! More complete answer is, you would need to protect the feeder conductors, protect the panelboard at its rating (408.36), and meet or exceed the calculated load. The panel is probably actually a 400 amp buss, so considering that and the "next size up rule", you could have several code compliant scenarios. If you have a main breaker in that panel, you could also use the tap rules and there are even more possibilities!

Ok, now a tricky one(for me at least). I want to feed a 120/208 1200a dist board from the 120/208v 4000a board but want to feed it via 277/480. So I'd need a step up xfrmr by the 4000a board then a step down xfrmr by the 1200a board. How would I go about sizing the switch in the 4000A board and the two xfrmrs.

Your OCPD would need to be coordinated to protect the conductors, and protect the transformer (see 450.3(B)). IF you are looking to supply the full 1200 amps, you would need a 500 KVA transformer. Maybe you could get by with a 300 - not sure what your actual load is. If I may inquire: why the step up and down? I assume long run and/or trying to save wire? It takes a lot of aluminum conductor to exceed the price of two transformers. Your conductors would be about $24 per foot for the 208V run assuming no upsize for VD (4 sets 500 AL).

So when sizing, equipment and conductors and you know the size of the load do you then size the OCPD then the feeder?

Generally, you start with the load you need to supply, then you find conductors that meet or exceed that, then you size the OCPD to protect the conductors (keeping in mind that we get more fudge room for motors).

 

[mstrlucky74]

If it's switching the motor, the switch needs to have a HP rating equal to or greater than the motor HP. HP ratings are different from just simple amp ratings. From a gross thumbnail standpoint, 200A motor current at 208V equates to roughly 75HP, so make sure whatever switch you get has a 75HP rating on it. You'll likely find that will be a 600A rated switch! (looking at the HP ratings of Sq. D disconnect switches)

For the other switches, it's covered in 225.39.

Switches have HP ratings? Even switches within distribution boards?

 

[mstrlucky74]

I'll try taking a stab at it to get things moving........




A couple of comments:
1) continuous loads dont apply to motor loads - they are just motor loads
2) Presumably obvious, but you will need not just a switch but a fused switch. You need to protect the motor and the conductors at less than 4000 amps.
3) Note you need to use the FLA values in the NEC charts for your current rating.



Short answer is 300 amps! More complete answer is, you would need to protect the feeder conductors, protect the panelboard at its rating (408.36), and meet or exceed the calculated load. The panel is probably actually a 400 amp buss, so considering that and the "next size up rule", you could have several code compliant scenarios. If you have a main breaker in that panel, you could also use the tap rules and there are even more possibilities!





Your OCPD would need to be coordinated to protect the conductors, and protect the transformer (see 450.3(B)). IF you are looking to supply the full 1200 amps, you would need a 500 KVA transformer. Maybe you could get by with a 300 - not sure what your actual load is. If I may inquire: why the step up and down? I assume long run and/or trying to save wire? It takes a lot of aluminum conductor to exceed the price of two transformers. Your conductors would be about $24 per foot for the 208V run assuming no upsize for VD (4 sets 500 AL).



Generally, you start with the load you need to supply, then you find conductors that meet or exceed that, then you size the OCPD to protect the conductors (keeping in mind that we get more fudge room for motors).

Thanks ! Really could overview and will be useful to me now and in future. Yes long run so I figure distributing at 480v is a cost saving but I think your right.