adding 400 A branch on 400 A main

[cryomark]

Must supply 3Ø, 480V to 200HP screw compressor. Starter is westinghouse autotransformer, closed transition. Motor is 209 FLA.
The expensive option invloves lots of new conduit and cable run through a heavily plumbed space.

Would like NEC refferents and opinions on legality/ advisability of the following proposal.

Adjacent to the starter cabinet is a square D panel board;
400A main breaker in position 2, magnetic pickup on this breaker adjustable to 6x;
The panel has a number of branch ciruits already; however, most are trivial or obsolete. total steady load would not excede 100A.

Can we install a 400A adjustable instantaneous trip CB in the panelboard to feed the starter?

Calculated start I should be ~700A at the present auto transformer tap. Motor overload protection is integral to the starter.

 

[dpc]

Other than a concern about the loading on the panelboard, I don't see any Code violations, provided this breaker is within sight of the starter. Normal thermal-magnetic breaker sizing for a 200 hp 480V motor is 350 A.

Whether it will actually carry the load is another question. The 400A main breaker is good for only 320 A continuous. With your 100A of other load plus the 209 A from the motor, this is getting close to the 320 A.

 

[ScottyUK]

I can't comment on the NEC aspects, but consider what effect the starting current will have on the voltage delivered to other loads fed from the board. The board doesn't sound like it was ever intended for this sort of duty, so the feeder cable may well be undersized from a volt-drop point of view even if it is within its thermal limit.

If you already have a route to the board for the existing feeder, you may have the option of uprating the feeder without installing new conduit. Sorry for all the speculation - only you know how sensitive your loads are and the size of the conduit to the board.



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If we learn from our mistakes,
I'm getting a great education!

 

[mc5w]

National Electrical Code 230.71(A) allows you to use 2 to 6 service disconnects, so you coould just as easily use a 400 amp service switch for the motor and a 200 amp for the other loads. Same for the secondary of a transformer. The only times that you really want to use a single BIG disconnect is when doing cogeneration or when there is quite a bit of load diversity.

Since you would need to use 200 class J fuses or a 200 amp main breaker to get a decent short circuit rating for the smaller branch circuit breakers, this is a very good option. Most residential 400 amp services that I have seen use two 200 main breakers because that is the least expensive option. A 400 amp circuit breaker of fusible switch costs about 3 or 4 times as much as a 200 amp.

 

[advidana]

you need to check the motor starting curve with the circuit breaker coordination curves to see if this will work. nec looks ok.

 

[cryomark]

Thanks to all for your attentions.

The panel currently feeds 2-45KVA x-formers(70A CBs), a 60A welding plug, a 25HP vacuum pump/blower skid(60A CB), 2-20A area heaters, a 5HP process air compressor(20A CB)

The 480/208-120 xformers are lightly loaded-area lighting,utility 120V, 2 small ventilation fans; one xformer will probably be removed and loads consolidated; welding plug will be removed when plumbing is completed.

Anticipated steady load ~175A max (the 200HP screw compressor will be idling at about 1/2 its capacity under the present application)

panel feed is 3x500 KCM ~120'