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Jason
Jason, Service Technician
Category: Electrical
Satisfied Customers: 4269
Experience:  Over 20 years of experience in all types of installations, troubleshooting, and repairs.
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7.5 kw heat at 208 volt 3 phase equals how many amps

This answer was rated:

7.5 kw heat at 208 volt 3 phase equals how many amps

Jason :

Hello. Welcome to Just Answer.


 

Jason :

This calculates to about 36 amps.


 

Jason :

That being the case, I would feed it with a 50 amp breaker, and #6 copper wire.


 

Jason :

If you are using THHN stranded building wire, it can be #8/


 

Customer:

can you provide the formula that you used


 

Jason :

Sure. Volts X Amps = Watts. We know the wattage is 7500, and the voltage is 208. So, just divide the watts by the volts, 7500/208 to arrive at 36 amps.


 

Customer:

watts divided by volts--x 1.732 equals amps correct ??this is 3 phase


 

Customer:

are u there


 

Jason :

Thank you for your patience. I'm referencing my materials, as it's been a while since I had to determine when to use the 1.73 rule.


 

Jason :

Yes, you are correct. I was mistaken. 36 X 1.73 = 62.28. That would be a 70 amp breaker and #4 copper wire.


 

Customer:

ok---explain this:


 

Customer:

7500 watts at 240 volt SINGLE phase is 31.25 amps (watts divided by volts)---7500 watts / 208 volts=36.06 times 1.732 is= 62.46 amps----shouldn't the amperage on 3 phase be lower than single phase ???


 

Jason :

Now that I look at it again, I believe we are both misapplying the formula.


 

Customer:

what am i doing wrong ?


 

Jason :

We don't need to multiply the original amperage I arrived at by 1.73.


 

Jason :

That is to find watts.


 

Jason :

Wire is sized on amperage, not wattage.


 

Customer:

a 7.5 kw heater at 208 3 phase


 

Customer:

7.5 is 7500 watts


 

Jason :

Yes


 

Customer:

so only 7500 divided by 208 ??


 

Jason :

Yes.


 

Customer:

ok--thanks


 

Jason :

I apologize for being rusty, it's been a while since I've had to use the 1.73 rule.


 

Customer:

gues im done--nite


 

Jason :

If you don't mind standing by for a moment...


 

Jason :

I just found a site that explains it very well.


 

Customer:

so i just close this web site now ?


 

Jason :

I'll post a link after I view it.


 

Customer:

ok--go i go now ?? dinner is waiting


 

Jason :

You can go and come back later. No problem.


 

Jason and other Electrical Specialists are ready to help you
OK, here's a site that explains it well...

http://wiki.answers.com/Q/How_do_you_calculate_amp_draw_if_you_have_3_phase_volts_and_kw

So, according to this info, you need to first multiply your voltage by 1.73. That comes out to about 360 volts. Then, divide your wattage by that number, which comes out to just over 20 amps.

So, I would run #10 wire, and put this on a 30 amp breaker.