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Old 09-07-2019, 09:01 AM   #1
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Default 208V or 480V service?

I have a customer with a machine shop that is presently on a 120/240 VAC service. He has a couple 3 phase machines now that we are running with a 100 amp 120/208 service from another building. We were planning to install a 120/208 3 phase service in this building. He now is looking at getting a machine that is 480VAC and has a 25HP and 75HP motor on it. Which is the best approach....put in a 120/208 service, which is what everything else in the shop uses and put a step up transformer for this one machine or put in a 277/480 service and a step down transformer for the main shop equipment? His machines he has now are CNC and likely can't easily be changed to 480 VAC.
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Old 09-07-2019, 09:29 AM   #2
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I'd guess you want to add up the loads and figure out which way you get away with a smaller transformer.

Did you check with the power company about 480 three phase at the location? That can make the decision very easy
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Old 09-07-2019, 11:26 AM   #3
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We would push hard for a 480v service and possibly use a step down transformer for the 208v. Unless the power co is able to give him both voltages, just have to see what monthly meter charges would be.
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Old 09-07-2019, 11:43 AM   #4
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I'd go with 480 and install a 480 - 120/208 transformer.

You'll need a transformer either way and if the service is 208, it'll need to be 112.5KVA or larger. If the service is 480, a 45KVA will very likely be enough. Less $$$.
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Old 09-07-2019, 01:04 PM   #5
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Quote:
Originally Posted by micromind View Post
I'd go with 480 and install a 480 - 120/208 transformer.

You'll need a transformer either way and if the service is 208, it'll need to be 112.5KVA or larger. If the service is 480, a 45KVA will very likely be enough. Less $$$.
I agree, start with the higher voltage and then step it down with a transformer for the 208 machines and receptacles. Take the 277 and use it for lighting and other things as you see fit. The higher voltage means smaller wires to pull. A step down transformer is easier to find also than a true step up transformer.
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Old 09-07-2019, 01:11 PM   #6
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I agree, start with the higher voltage and then step it down with a transformer for the 208 machines and receptacles. Take the 277 and use it for lighting and other things as you see fit. The higher voltage means smaller wires to pull. A step down transformer is easier to find also than a true step up transformer.
Higher voltage would allow for more future expansion (those LV panels will fill up fast).
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Old 09-07-2019, 04:36 PM   #7
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Another consideration for choosing 480, and stepping down, if you go the other way you have either 1. ungrounded delta= no no for VFDs or 2. corner grounded/grounded B phase= about 50% that the VFDs will see a phase loss or imbalance.
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Old 09-07-2019, 04:45 PM   #8
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The Poco's distribution situation is usually decisive.

You didn't mention it.

Does the customer have a pad-mounted transformer, where medium voltage is brought in?

If so, the Poco will be more than willing to swap transformers -- for a fee.

Or is he tapping a pole mounted open-delta scheme -- that's actually tapped out?

(That is, any further demand will require ground mounted transformers.)

The decision will have to be co-ordinated with the Poco, no doubt. You can't bump the load that much without calculations.
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Old 09-07-2019, 06:24 PM   #9
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If you have a choice...

Simple math. Look in your Code book at FLA of say a 50 HP motor for 208 vs. 480. Now consider that heat is proportional to the square of the current. Everything is much, much smaller and cheaper at 480 for that reason. Wiring, breakers, motors, everything you install will be smaller and less expensive.

If you are in Canada I’d even suggest going to 600 V. It’s around but uncommon in the US so on parts availability alone I’d suggest staying away from 600 V in the US. You can still buy 575 V motors but everything is special order.



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Old 09-07-2019, 07:33 PM   #10
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If you have any hope at all of getting 480/277 into your space, it would be very foolish not to install it.
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Old 09-07-2019, 07:35 PM   #11
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Keep talking, I'm listening. I would think the POCO would give us either voltage.
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Old 09-08-2019, 01:30 AM   #12
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Quote:
Originally Posted by John M. View Post
Keep talking, I'm listening. I would think the POCO would give us either voltage.
OP: You are jerking us around.

What is it?

Pad mounted?

Pole mounted -- open delta -- and pretty much maxxed out?

While 480Y277 has many, many advantages -- on the economics everything turns on what the Poco is going to charge you for changing the Service.

It could be acceptable -- or brutal.

I would not favor an open delta scheme for a hefty load, BTW.

Such a load would have the pigs squealing.

Your man needs analysis from a clean sheet of paper.

Then he can move on to a decision tree.
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