480/277V supplies 480-480/277V Lighting transformer 1

[metta0810]

Dear friends,

I have two 500 kVA Lighting transformers (4160V Delta - 480/277V Wye) coming through a three pole transfer switch to feed a 480V Lighting MCC. The transformer neutral is terminated at the MCC ground Bus.

Is it allowed to supply a 480-480/277V lighting transformer from the 480V lighting MCC? If so, is there any advantage or disadvantage of supplying a 480-480/277V Transformer from 480/277V supply?

Thanks

 

[alehman]

Why do you have two transformers feeding through one ATS to one MCC? And what does that have to do with the 480-480/277 lighting transformer? What is a 'Lighting MCC'? The neutral should normally be grounded upstream at the main disconnect, not at the MCC ground bus. Can you post a diagram of your system?

Short answer - yes you can supply a 480-480/277 transformer from a 480V MCC. Why do you want to do that?

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[rcw retired EE]

There are a few good reasons for using a 480- 480/277 volt lighting transformer to supply 277 volt lighting circuits:

1. Keeps all 277 V loads off the main 480 V system, allowing it to be converted to a high-resistance grounded system with fault finding features in the future.

2. Reduced fault currents from the small transformers' impedance may allow use of less costly breakers in the 480/277V lighting panels.

3. Flash hazard may be reduced with the reduced fault current. Maintenance people will spend significantly more time working on 277V light fixtures than other 480V loads. (Motors don't burn out as often as lamps). Reducing the fault levels may provide some arc flash and other hazard benefits.

4. Only 3 power wires are needed on the circuits to the transformer.

5. If it is a large facility with long runs, the transformer taps allow some compensation for steady state voltage drop. (Downside is transient voltage drops will be higher.)

 

[dpc]

Yes, this is fairly common as rcwilson describes. It eliminates the need to carry a large neutral for the main 480 V system and simplifies ground fault protection. It also reduces the risk of tripping off a major portion of the 480V system for a ground fault in a 277 V lighting fixture.



David Castor

http://www.cvoes.com

 

[jraef]

Learned something new today. I thought this sounded like something from the Department of Redundancy Department, but I guess not.

Lighting MCC though? MCC is short for Motor Control Center, so a Lighting Motor Control Center? I suppose you could have an MCC structure that was filled with nothing but Feeder Circuit Breakers in individual buckets, so you would have sort of a poor-man's low voltage low current draw-out switchgear. But that's kind of overkill in my opinion.

Unless the lighting is so critical as to require individual breaker change out without de-energizing the entire system? Hmmm... that might actually fit with the redundant transformer scenario too. Are we growing pot plants or some other critical lighting application like that where any brief down time results in major financial losses?


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
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[waross]

Thanks for the explanation rcwilson. LPS. May I point out that that in some instances the saving by not needing a neutral conductor may be somewhat offset by the need to upgrade the equipment grounding conductor to a system grounding conductor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Good point on equipment grounding, Waross. We overkill the EGC anyway. Also, the savings from the lower AIC breakers and fewer neutrals never fully cover the transformer costs.

jraef- We used to buy enough MCC's that I could get 480V breakers in an MCC configuration cheaper than in a panelboard or switchboard plus have the advantage of being able to add or change breakers without shutting down the loads. (Before arc flash rules). The pricing didn't make sense, but it was to our benefit.

 

[metta0810]

Dear Friends,

Thank you all for your responses. I should've clarified from the beginning that the term "Lighting MCC" is the MCC for Lighting Loads purposes.

 

[alehman]

My question pertained to the two 500kVA 4160-480Y/277 transformers feeding the MCC. Are they connected in parallel? The grounding seems wrong if there's an ATS between the transformers and MCC.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[metta0810]

Alehman,

The two 500 kVA Transformers are supplied by two separate 4.16 kV buses. The transformers feed the MCC through a three pole transfer switch (not a separate derived system).

Therefore, grounding the neutral at the upstream main disconnect are not required in accordance with NEC 250.20 (Exhibit 250.6, 250.7).

 

[alehman]

Ah, now I understand. This is off-topic, but I'm not sure I agree with your interpretation. The Handbook figures you reference apply where one or the other sources is grounded at the source and the other grounded by interconnection of it's neutral with the grounded source neutral.

If this is a true double-ended switchboard with the MCC, ATS and main disconnects grouped at one location, then I think you are fine. Or if the sources are considered services and grouped in one location, the NEC would permit a common grounding point per 250.24.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.