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7,221 Posts

Call an electrician. :laughing:

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I have twenty four 8.1kW (at 208)1ph heaters to run a service for. This is continuos. The incoming power is 120/208 three phase. Can I get away with a 600 amp 3 ph service?

That comes out to 930 amps if I did the math right.

220.51 Fixed Electric Space Heating. Fixed electric space-heating loads shall be calculated at 100 percent of the total connected load. However, in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied.

Exception: Where reduced loading of the conductors results from units operating on duty-cycle, intermittently, or from all units not operating at the same time, the authority having jurisdiction may grant permission for feeder and service conductors to have an ampacity less than 100 percent, provided the conductors have an ampacity for the load so determined.

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13,306 Posts

But spread out over a three phase service can I get away with a smaller amp service? I should only have 547 amps on any one leg. Or am I confused.HARRY304E said:That comes out to 930 amps if I did the math right.

220.51 Fixed Electric Space Heating. Fixed electric space-heating loads shall be calculated at 100 percent of the total connected load. However, in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied.

Exception: Where reduced loading of the conductors results from units operating on duty-cycle, intermittently, or from all units not operating at the same time, the authority having jurisdiction may grant permission for feeder and service conductors to have an ampacity less than 100 percent, provided the conductors have an ampacity for the load so determined.

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13,306 Posts

Thanks Dennis, I was thinking I would see 66% of the load on each leg. That's where I can up with just under 600 amps. I like your number better. I will have to pull out my books and double check, not that I doubt you.Dennis Alwon said:

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3,259 Posts

Are you saying these heaters are on 100% of the time, 24/7? no thermostat?

I did not take that into consideration. How did you get 539.6 amps

Are you saying these heaters are on 100% of the time, 24/7? no thermostat?

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39,618 Posts

Yup,see I knew I messed it up...:no::laughing:

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No, they are storage heaters. They charge at night for up to 8 hrs. Off peak rate, under 5 cents a kilowatt hour. Only cheaper heat here is ground source heat pump.Vintage Sounds said:

Are you saying these heaters are on 100% of the time, 24/7? no thermostat?

That was mine number too. But that was off the top of my head

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319 Posts

I got 540amps too.

24 x 8.1 = 194.4k / 1.73 = 112370watts / 208v = 540.24amps.

Harry beat me to it.

24 x 8.1 = 194.4k / 1.73 = 112370watts / 208v = 540.24amps.

Harry beat me to it.

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I think because there are 8 heaters across each phase pair. So they overlap, that's how he got 16. If you draw it out it makes sense. The rest of his calcs, I don't know. But I trust him, besides I'll put his name on the calc sheet!HARRY304E said:kVA x 1000

1.73 x E

24x8100=194,400/1.73x208=359.84=194,400/359.84= 540.24 amps:blink::laughing:

Okay Dennis how did you come up with 129,600 watts?

I'm missing a key point

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39,618 Posts

Yeah,that's the ticket...:laughing:I think because there are 8 heaters across each phase pair. So they overlap, that's how he got 16. If you draw it out it makes sense. The rest of his calcs, I don't know. But I trust him,besides I'll put his name on the calc sheet!

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13,306 Posts

It's a 120/208 wye.ablyss said:I didn't see where he mentioned anything abut delta/wye

Correct me if i'm wrong but if it's a delta wouldn't it only be 208 off of one leg?

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741 Posts

He's figuring each unit single phase " only using 2 legs" so each phase won't have the full load of all 24 heaters, only 16 maxHARRY304E said:kVA x 1000 1.73 x E 24x8100=194,400/1.73x208=359.84=194,400/359.84= 540.24 amps:blink::laughing: Okay Dennis how did you come up with 129,600 watts? I'm missing a key point

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