ECN Forum Forums Technical Discussion Electrical Theory and Applications i think im gettin it...

OK this one is a 240V/3Ph/60Hz

1 - Blower Motor============1200.VA(5A)
4 - single valve Solenoids====40.39VA(10.1W)ea
1 - Dual valve solenoid=======60.VA
2 - drive motors @ 5VA ea=====10.VA
2 - Temp. cntrlrs @ 1A ea====240.VA
2 - Flame Monitors @ 1A ea===240.VA
10 - Lights @.025A ea(7const)==21.VA
1 - Motor Starter==============8.VA
______________________________________
System Total=================1819.39VA
1819.39VA / 240V================7.58A
______________________________________
Trans. Total==================619.39VA
619.39VA / 240V=================2.58A

So I hope i have this stuff down now so i can size these systems on my own. For this system i would use a 240VPri. - 120VSec. Trans. with 750VA? with a 3A fuse, and (3)10A fuses in my disconnect switch, is this right?

Not if your blower motor is running off of this transformer.

240V/3PH?

(E*1.72)/P = I for 3phase.


[This message has been edited by ITO (edited 12-06-2006).]

no the blower motor is running off of the 240V/3PH/60Hz line. the trans is for all the other components.

I = P/(E*1.732)

In 3 ph, the supply voltage would be 208V.

In practicality, we would find on the motor decal the FLA and multiply by 1.25 and if that includes enough current to match the LRA, fine, if not, then use the LRA. Otherwise, we would look in the table in the NEC with the motor's hp, voltage, and motor type.

Example: induction type motor, 208V 3 ph, 5 hp. Whatever the amp draw in the table is, that would get multiplied by at least 1.25 to ensure enough current to accomodate LRA or LRC (same difference. Got to have enough current to meet locked rotor, or complete standstill of the motor.) Usually, if you take the full load amperage and do the 125 % on that, it will be enough to meet LRA.

But those size fuses would still work, especially if they are standard trade sizes.

[This message has been edited by rws (edited 12-07-2006).]

https://www.electrical-contractor.net/ubb/Forum7/HTML/000543.html

[This message has been edited by ITO (edited 12-07-2006).]

Careful, you could have 240 3-phase for a delta fed service.