Motor Control Center
Motor Control Center
(OP)
I am new to MCC's. I have an existing MCC that has a 575V 1000A bus and most of my equipment is 480V. Do I need to use transformers for each bucket to step down to 480 or is there a better way to do this?






RE: Motor Control Center
RE: Motor Control Center
RE: Motor Control Center
The switch gear line-up was as follows:
1> An incoming section with metering and main breaker at 600 Volts.
2> Several 600 Volt MCC sections.
3> A section with a main switch and two auto-transformers in open delta configuration dropping the voltage to 480 Volts.
4> Several 480 Volt MCC sections.
Notes:
a> Auto-transformers do not provide galvanic isolation, so the section past the auto-transformers is NOT a separately derived system and no special system grounding is required.
b> The available fault current and the ground fault current on two phases will be limited by the auto-transformers. Hopefully some-one with more arc flash experience than myself will assist with arc flash study methods.
c> With an auto-transformer feed, line to neutral or 277 volt loads are not allowed.
To allow 277 Volt loads you will need to use three auto-transformers connected in wye. Do NOT use a three phase auto-transformer with a three legged core. The phantom delta effect may present problems in the event of incoming voltage unbalances or phase angle errors.
d> Prices, quotes and purchase; Consider that from a technical viewpoint, the additional equipment, whether stand-alone or built with MCC sections need not match the existing equipment. Ask for competitive quotes from several manufacturers. Additions to existing equipment are priced as replacement parts. The replacement parts are priced about 400& higher than competitive bid prices.
Anecdote alert!
We once needed to add one section to an existing MCC. The price came in at over $20,000. The small project was about to be scrapped. I suggested that we did not have to use the same manufacturers equipment. We could bolt up any MCC section. The purchasing department went back and asked for competitive bids from 4 or 5 manufacturers. The $20,000+ price dropped to $5,000+ and some prices were $4,000+
We were able to purchase the equipment that we desired and complete the project.
Of course, if this is a 575 rated MCC running at 480 Volts, these points are moot.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Control Center
RE: Motor Control Center
To answer your question again, step down once for the MCC sections feeding 480 Volt equipment.
Cheapest by far is to use auto-transformers.
Note standard auto-transformers will be 120:480 Volt. That will step 600 Volts down to 480 Volts.
If 120:600 auto-transformers are used to step down 575 Volts the resulting voltage will be 460 Volts.
Most motors intended for use on 480 Volt systems are actually rated for 460 Volts so it should be serviceable. If your selected transformers have voltage taps, then set the taps for maximum voltage. +5% taps should give 483 Volts. Close enough.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Control Center
Im going to look into auto transformers and see if this is the cheapest option. Should I be at concerned with noise if im adding 3-4 transformers?
RE: Motor Control Center
The three transformer option looks good on paper. It is symmetrical and the vector diagram is easy to understand, but in practice the two transformer option is often preferred.
By the way, I have worked on plant shutdowns where the plant voltage was 600 Volts and the rental generators for temporary power were 480 Volts.
Auto-transformers were used to boost the 480 Volts up to 600 Volts for the plant equipment. Then some 480 Volt rental equipment came in. A second set of auto-transformers was used to drop the 600 Volts back down to 480 Volts. One set of auto-transformers was two transformer open delta, and the second set of auto-transformers was three transformer wye. No problems with either set.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Control Center
The only POSSIBLE glitch is if each individual starter bucket has its own Cpntrol Power Transformer (CPT) to provide 120V control power. If it was 600V, the primary of those CPTs will be 600V so you will not be able to use them with a 480V input, you will need to change them to 480:120V CPTs. Just match the kVA rating of them.
Another more minor issue will be the overload relays. A motor of the same HP rating will take more current at 460V than it did at 575V. But you should always check the OL heaters and/or adjustments for a new installation anyway. So just get the user manuals for the OL relays to determine the correct way to select the heaters or settings, and do what it says.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Motor Control Center
I believe that the OP is adding 480 Volt equipment to an existing 575/600 Volt system. I may be wrong.
Bill
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"Why not the best?"
Jimmy Carter
RE: Motor Control Center
We get ours special order.