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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...