Adjusting Heater Output 208-240 volts


Old 10-26-04, 10:34 AM
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Adjusting Heater Output 208-240 volts

I am about to purchase a shop heater (Marley Engineering Products - Model UH-524TA) that wants to be hard wired into a 220 (or so) circuit. It offers several heat output (wattage) adjustments that appear to be regulated by voltage. At 208 volts, it can offer 1874, 2500, 3123, or 3750 watts. At 240 volts, the wattages are 2500, 3332, 4165, and 5000. All of these variations are described as attainable through jumper settings.

Where I am not clear is in the question of whether the input voltage can be varied by the jumpers or does the heater expect to see a given voltage level (either 208 or 240) coming into the heater jumper box (the tech support folks at the manufacturer were not well versed in electrical service delivery and PG&E has not returned my calls).

I have always been under the impression that so-called 220 service is really somewhere between something less than 220 and up to 250 and that we take what comes with regular variations being the norm. If this is true, it would give some creedence to the notion that the heaters jumper setting will have some effect on what it sees in voltage, but is seems odd to me that any jumper box in a heater would actually regulate voltage.

Any thoughts on whether I can actually expect to see either the 208 or the 240 wattage outputs? Thanks.

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Old 10-26-04, 11:09 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
Your residential voltage is 240 volts.

Decide whether you want 2500, 3332, 4165, or 5000 watts, by setting the jumper accordingly, and size your wire based on those wattages.
Old 10-26-04, 11:09 AM
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The jumpers would not change the input voltage, but would only adjust proportionally how many watts would be consumed at a given input voltage.

Though calculating the exact heat output of the unit probably isn't necessary, you could do it by measuring the input voltage with a multitester.

Most residential services around here are 240V. Like I said, if you want the exact numbers, you need to get a multitester.
Old 10-26-04, 11:58 AM
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Thanks for the replys. Makes sense to me. Only question left is why the heater has a rating of 208 volts. That seems to suggest that there are some significant number of service deliveries that are at 208 or why would they list 208 and the corresponding wattages...

Thanks again.

Old 10-26-04, 12:01 PM
Join Date: Jan 2004
Location: Oregon
Posts: 1,219
You are right that 240V is a 'nominal' number, which could be as low as 220 or higher than 240V. 240V is the nominal voltage for most residential service. In residential single phase service you have two 120V legs of opposite polarity, providing 240V line to line.

208V is a different beastie all together. It is the nominal line to line voltage for most light industrial general purpose lighting circuits. The number is derived from the fact that these systems are _three phase_ systems. By the magic of vector addition, you have _three_ 120V legs with 120 degrees of phase difference, and the voltage that you get line to line is 208V. This is also a nominal value, with the real line to line voltage being anything from 200V to 220V.

You will sometimes see this three phase 120/208V service in larger apartment buildings. It is important to be able to recognize this, because '240V' appliances may not tolerate 208V, and because the 'neutral' conductor will always carry current roughly equal to the line current; the current in the neutral never balances out with only two hots.

sjr is correct about what the jumpers will do: they will change the watts produced at any given applied voltage. If you need about 4000W and you have 240V, then use one jumper setting. If you have about 208V, then you will need the other jumper setting to get roughly 4000W.

Old 10-26-04, 06:38 PM
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Thanks Jon...

It's pretty amazing that one can sit at a computer screen and learn about things in a few minutes that others have taken years to learn and experience. I appreciate the light you shed on the 208 volt thing. Looks like I will have the option to get the maximum wattage out of the heater, should I need it. I just spent the better part of the day putting insulation between the studs of the shop. By the time the heater arrives, I'll have walls up and probably won't need 5KWs. But it is nice to know that it will be there. Thanks again.


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