Smaller conductors with a transformer.. Worth it??

Old 03-27-14, 09:28 PM
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Smaller conductors with a transformer.. Worth it??

Hello everybody,

first of all i'm a DIY and love to understand what i don't and learn how to do things by myself, so please don't give me the usual "find an electrician"
I'll find an electrician for the installation, but i want to have it all figured out before it happens

So here is the deal, i want to wire 2 industrial buildings with 600 Amps each, 120/208 volts Wye, but here is the problem the length from the meter to the distribution panel is 120 feet at least.

Voltage drop calculations give me 500MCM aluminum wire 2 conductors per phase in parallel with 1.67% VD . Quite an enormous amount of connectors for 120 feet.
8 x 500kcmil on 120 feet would be worth $8k of wires...

Could Using smaller conductors and a transformer reduce the amount of connectors and give better results?
If yes what kind of transformer (auto, multiple winding, buck boost, etc..) and what conductor size could be used?

Thank you
Old 03-27-14, 09:49 PM
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I can buy 500MCM XHHW for $2.21/ft. 350MCM copper is more expensive.

I would do a load calculation. I doubt if you'll find that you need a full 600A.

A transformer probably will cost money, not save it.
Old 03-28-14, 12:43 AM
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Check out the prices on transformers (you would need two) rated at 125kVA and then compare that to the cost of the wire. Remember that you can only go as high as 480 volts nominal as the rules change significantly for 600 volts and above.
Old 03-28-14, 05:09 AM
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To stay under 600 volts you can go with 2-1/2 times the voltage (520 volts) although most likely you will only get transformers for 2 times the voltage (to 416 volts).

For 2x the voltage you will be putting 1/2 times the amperes through the high voltage line. Use the same allowable percentage voltage drop you chose which at 416 volts will be twice as many volts you can lose. Then calculate the wire size you need.

The bad news cost wise: You need 3-phase transformers because you have a 3 phase feed. Or arrays of ordinary transformers, three at each location for a total of six. Recommended is some kind of fine tuning of the output voltage, usually in the form of multiple lugs on either the primary or secondary, to account for voltage drop within the transformer. This cost is probably enough to scrub your transformer mission.

Last edited by AllanJ; 03-28-14 at 05:27 AM.
Old 03-28-14, 05:16 AM
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8 x 500kcmil on 120 feet would be worth $8k of wires...
You'll also need a 600 amp fusible disconnect at the meter and a full sized ground in each conduit.
Old 03-28-14, 10:12 AM
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We do not know what the 600 amp load-per-building consists of , but the 600 amp value suggests motor-loads, or welder loads, or a combination of both.

If this is a new design , start-to-finish , then consider a 480 / 277 , 3-phase , 4-wire system. The KVA load-per building = 215900 KVA. With the 480 system, the line-current = 260 amps.

Note the proportions--- 208 volts / 480 volts = .4333 = 260amps / 600 amps

You can use the 480 system for both power and lighting ; you will need 480v / 220v transformers for 120 v loads.

But all of this is highly speculative.
Old 03-28-14, 05:17 PM
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Voltage drop on a 120' run is a non-factor so I see no reason for a transformer. 1.67% is nothing. Also remember that voltage drop depends on load. If there is no load, there is no voltage drop.

The cost of adding four transformers, two for each building, will greatly exceed the cost of the larger wire.

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