Determine size of xformer needed 460 3 phs to 110 2

[jrick]

Sorry, newbee here, I have 460 3 phase panel. I need to operate three 110 devices @20 amp each. What do I need between? xformer (KVA?), switchgear, cabel-wire size?

 

[jbartos]

Suggestions marked ///\\I have 460 3 phase panel. I need to operate three 110 devices @20 amp each.
///110V x 20A = 2200VA
What do I need between?
///Definitely, a three phase transformer of 9kVA minimum if there are no motors or similar loads that have inrush currents. It is a good practice to leave some margin in there so that 9kVA transformer will be a reasonably good there.\\ xformer (KVA?),
///Yes.\\ switchgear,
///120V 3phase 4wire panel, 60A minimum. If not available, use 100A which are usually available. Also, explore area of Mini Power Zone Centers, e.g. Square D at:

http://www.squared.com

http://ecatalog.squared.com/catalog/173/html/sections/12/17312009.html

\\ cabel-wire size?
///#12 AWG copper is a minimum for 20A circuits. If there is a long run, the voltage drop will be high and #10 AWG or higher will be required.\\\

 

[alehman]

Sounds like a job for an electrical engineer...
Assuming the "110 devices" are single phase, you have 6600VA of load (110 x 20 x 3). I would normally provide a 480 to 120/240V 1-phase transformer - about 10kVA assuming you don't want to allow for future loads, a 2-pole circuit breaker in the 460 panel to feed the transformer and a 120/240V 1-phase, 3-wire panel with a 2-pole main breaker and branch breakers for your loads. Circuit breakers and wire must be sized per local codes. The transformer neutral must be grounded per local code as well.

 

[jrick]

Thank you for you help. obviously so not my field but this should be info I was asked to locate.

 

[jrick]

back to this project, moving from 6600va to 10kva is that 6600\1000 rounded to next available standard size? or are there some factors between? the reason is Project has been changed to seven circuits of 20 amps which would either be 2.3333X10 or 23kva or 110*20*7 =15400va/1000=15.4kva up to next size?

 

[busbar]

Most likely 120V should be used for calculations as oppesed to 110V.

A 15kVA 1ø transformer won’t quite cover the new proposed load. You might consider using a (possibly more readily available) 25kVA 480?—208Y/120 3ø drytype transformer—seven to nine 20-ampere 120-volt loads could be served—three per 120V winding. At any rate, NEC Article 450 primary and secondary overcurrent protection must be applied.

Be aware of the significant heat generated from a 150°C-rise transformer.

 

[alehman]

My thought on going from 6.6 to 10kVA was just as you suggested, to get to the next standard size - and allow a little room for future growth. For 7 circuits, I agree with busbar's suggestions. Standard sizes for 3ph transformers are 15, 30 and 45 kVA. Be careful to balance loads so as not to overload any winding.

 

[skiier]

What's the duty cycle of your equipment? Are all your loads energized at once? Are you overdesigning?

 

[jbartos]

Suggestion: Even though, the three-phase transformers start with 15kVA per industry standards, several manufacturers offer less rated 3-phase transformers, e.g.

http://www.acmepowerdist.com

A good advantage of the lower than 15kVA three-phase transformer is that the transformer gives a chance to balance the load more evenly on the three-phase power distribution system, and one phase down will not blackout all loads on the three-phase transformer secondary.