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#130754 - 12/06/06 01:10 PM i think im gettin it...  
JLC  Offline
Member
Joined: Oct 2006
Posts: 12
pelham manor,NY,usa
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?


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#130755 - 12/06/06 04:36 PM Re: i think im gettin it...  
jraef  Offline
Member
Joined: Jul 2004
Posts: 95
San francisco, CA, USA
Not if your blower motor is running off of this transformer.


JRaef

#130756 - 12/06/06 04:47 PM Re: i think im gettin it...  
ITO  Offline
Member
Joined: Nov 2006
Posts: 348
Texas
240V/3PH?

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


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


101° Rx = + /_\

#130757 - 12/06/06 05:54 PM Re: i think im gettin it...  
JLC  Offline
Member
Joined: Oct 2006
Posts: 12
pelham manor,NY,usa
no the blower motor is running off of the 240V/3PH/60Hz line. the trans is for all the other components.


#130758 - 12/07/06 07:44 AM Re: i think im gettin it...  
rws  Offline
Member
Joined: Dec 2006
Posts: 14
Texas
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).]


#130759 - 12/07/06 09:50 AM Re: i think im gettin it...  
ITO  Offline
Member
Joined: Nov 2006
Posts: 348
Texas
http://electrical-contractor.net/ubb/Forum7/HTML/000543.html

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


101° Rx = + /_\

#130760 - 12/07/06 02:07 PM Re: i think im gettin it...  
BugMan  Offline
Member
Joined: Aug 2006
Posts: 10
Careful, you could have 240 3-phase for a delta fed service.



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