The heater is rated @ 31.3 amps at 240 volts and 18.1 amps @ 208 volts. Is there a way to calculate the amps @ 220 volts? Assuming, that if it will operate on 240 and 208, it would also operate on 220, probably is not a good idea??

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The heater is rated @ 31.3 amps at 240 volts and 18.1 amps @ 208 volts. Is there a way to calculate the amps @ 220 volts? Assuming, that if it will operate on 240 and 208, it would also operate on 220, probably is not a good idea??

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7,829 Posts

Where is this shop? At your home or is is a commercial shop?

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5,990 Posts

240/31.3 = 7.7ΩThe heater is rated @ 31.3 amps at 240 volts and

18.1 amps @ 208 volts.

Is there a way to calculate the amps @ 220 volts?

208/18.1 = 11Ω

so 220/7.7 = 29A

or 220/11 = 20A.

This doesn't sound right.

Unless you have to flip a small switch in back or reconnect some wires to different terminals inside for each of these voltages.

Your circuit should be rated for 25% more than the heater current rating under the "continuous load versus intermittent load rule". For example the 31.3 amp. heater should be on its own* 39.1 amp (round that to 40 amp.) circuit.

My own intuition is such that you can install the heater for 240 volts per the instructions and also per the "continuous load ..." rule and neither worry about it nor make any changes if it gets 220 volts.

* The heater might have its own maximum breaker rating for the circuit it is on that prohibits installing two of them on one 80 amp. circuit. Also other smaller appliances and also receptacles have maximum current ratings (typically 20 amps.) that preclude your installing just one circuit to power both the heater and other things.

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7,829 Posts

You don't have to worry what the amps will be at 220v. Like Silk says, nominal voltage will be either 208v or 240v......In which case the current draw for 220 volts would be in between.

....

My own intuition is such that you can install the heater for 240 volts per the instructions and also per the "continuous load ..." rule and neither worry about it nor make any changes if it gets 220 volts.

I should have measured my voltage before posting!! My shop is at my home--not a business. Tonight, my voltage measured 122 on one leg and 123 on second leg.

I also was confused about the big difference in amp between the 240 and 208 voltage and probably is a misprint of specs in the ad.

The heater is capable of reducing the wattage by changing the jumpers and instructs one how to do so.

I was going to change the jumper to 6,250 watts and @ 240 volts, that should be 26 amps? I was going to use an existing 30 amp circuit. However if I apply the 25% rule, that equates to 32.6 amps which means I'll have to use 8/3NM--w/gnd ? and a 40 amp breaker?

If that's the case, I'll have to run another circuit anyway for 6250 watts ---just keep heater @ 7,500 watts unless there is a special code that allows a heater to be non-continuous usuage.

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24,958 Posts

208 is three phase industrial voltage.

220-240 is home single phase voltage.

220-240 is home single phase voltage.

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6,039 Posts

Whats the 25% rule? Use the 30 amp circuit you have.I was going to change the jumper to 6,250 watts and @ 240 volts, that should be 26 amps? I was going to use an existing 30 amp circuit. However if I apply the 25% rule, that equates to 32.6 amps which means I'll have to use 8/3NM--w/gnd ? and a 40 amp breaker?

If that's the case, I'll have to run another circuit anyway for 6250 watts ---just keep heater @ 7,500 watts unless there is a special code that allows a heater to be non-continuous usuage.

I have heard of the 125% rule.

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780 Posts

Load 26 amp + 25% of load (6.5) = 32.5

26amp X 125% =32.5

Same results

26amp X 125% =32.5

Same results

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1,125 Posts

Let me put in my 2 cents; The voltage at a single phase, 2-pole service is 120/208. Due to the formula of 1.73. (square root of 3) In other words. The voltage across both "hot" poles will never be 220 or 240v. It'll measure 120x1.73 = 208. So.You'll get 208 volts for a heater that's rated @ 240v. optimum voltage. That is 86.7% of its capacity. (not very efficient) The solution IMHO is to attach a Buck/Boost Transformer to bring up your line voltage to 240v. The size of the x-former should be rated above the maximum load of the heater in KVA. (Kilovolt-Amperes) (a/o KW. Kilowatts) Some time ago I installed the lines for 6 commercial freezers. The line voltage was 208v.3ph. The compressors were rated @ 230V. After attaching buck/boost transformers, they ran at maximum capacity!!!

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2,487 Posts

What!? A single phase service will be 120/240, not 120/208. The only time you get 120/208 is if you are using 2 phases and a neutral of a 120/208 V three phase wye system. In atypical house, the voltage is 120/240.Let me put in my 2 cents; The voltage at a single phase, 2-pole service is 120/208. Due to the formula of 1.73. (square root of 3) In other words. The voltage across both "hot" poles will never be 220 or 240v.

Thanks for all replies.

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1,125 Posts

I don't know about where you hail from (Atlanta, Ga.) but here in NYC the POCO, Con-Ed supplies [even] single phase, multi-wire services off a 3 phase, Wye connection. That's optimal Voltage. I've measured services where it barely measures 200v.What!? A single phase service will be 120/240, not 120/208. The only time you get 120/208 is if you are using 2 phases and a neutral of a 120/208 V three phase wye system. In atypical house, the voltage is 120/240.

p.s.: In a true single-phase system, the measurements are as per your quote.:whistling2:

That's horrible. I've dealt with 208v at work, and it is definitely not a drop-in replacement for 240v... a lot of things just don't work right or need to be rewired.single phase, multi-wire services off a 3 phase, Wye connection. That's optimal Voltage. I've measured services where it barely measures 200v.

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7,551 Posts

does that mean you have 3 phase available at a residential service? I have never seen one myself. All residential services I have ever dealt with are 120/240.I don't know about where you hail from (Atlanta, Ga.) but here in NYC the POCO, Con-Ed supplies [even] single phase, multi-wire services off a 3 phase, Wye connection. That's optimal Voltage. I've measured services where it barely measures 200v.

p.s.: In a true single-phase system, the measurements are as per your quote.:whistling2:

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3,370 Posts

Since you measured 123 Volts on one line, and 122 on the other, you might discover that the no-load available voltage is 245. A bit high it may seem, but normal to be a couple of volts high for most utilities. Did you measure across both incoming lines?

So-called "220 volts" is obsolete, and has not been in service for 50 years in the US.

Is this heater cord-and-plug connected? If so, what is the configuration on the cord?

How many different wattages are available on the heater by changing out jumpers? So far, it would seem you have 7500 and 6250 watts by what you have posted. Did the manufacturer's instructions tell you to change those jumpers if you are connected to a 208 Volt source? Are there other combinations available?

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2,487 Posts

Well, there ya go. 99% of the rest of the U.S. and Canada use 120/240 V single phase in residential applications. You made a blanket statement that does not apply to most readers of this forum.I don't know about where you hail from (Atlanta, Ga.) but here in NYC the POCO, Con-Ed supplies [even] single phase, multi-wire services off a 3 phase, Wye connection. That's optimal Voltage. I've measured services where it barely measures 200v.

p.s.: In a true single-phase system, the measurements are as per your quote.:whistling2:

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Which is 99% of the country. You CANNOT use NYC as a general example for ANYTHING.p.s.: In a true single-phase system, the measurements are as per your quote.:whistling2:

Much more common in residential areas is to use a single phase transformer connected to any one of the primaries and ground, or to any two of the primaries. The secondary voltages will be 120 and 240 volts.

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