240 volt electric heater low power

Discussion in 'Electrical Forum discussion & Blog' started by genmaster, Dec 17, 2019.

  1. genmaster

    genmaster New Member

    Sep 25, 2017
    Master Electrician
    I recently installed a 240 volt 7500 watt forced air electric heater in my garage. It seems to take longer to heat than I would expect, so I clamped it with my amprobe and it's pulling 13.5 amps on each leg. I would expect it to pull 31.25 amps at 240 volts. I called the manufacturer, but haven't been able to contact a living person yet. Am I missing something? My actual voltage is 248 v. Based on my measurements, it looks like it's only putting out 3240 watts. Any ideas?
  2. Reach4

    Reach4 Well-Known Member

    Sep 25, 2013
    Your math is right.

    Make sure the heater gets the 240 volts and the heater was not wired with 120 somehow. I don't know where you measured the 248.

    It seems the heater is defective.
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  4. Dana

    Dana In the trades

    Jan 14, 2009
    Is the voltage still measuring 248V while the heater is active?

    Whether 3240W or 7500W, it still going to take time to raise the temperature of a garage by 10F.

    If the garage is uninsulated & air leaky (garage doors are as leaky a sieves, as are many garage walls/roofs) the heat loss on a cold day could easily exceed 7500W (= 25,590 BTU/hr) if the goal is to keep the garage at 65F or warmer.
  5. WorthFlorida

    WorthFlorida The wife is still training me.

    Oct 28, 2009
    Orlando, Florida
    31.25 amps is the total, not on each leg. Your total is 27 amps, not far off (86%) of the rating plate. With that current draw voltage drop can be impacting the performance. I also doubt it is 248 volts. Is there a BTU rating for this heater.

    What is the voltage for each leg to ground. If you can post a picture of the heater label and what brand of multimeter/clamp meter is it? When the heater is cold, unplug it, turn up the thermostat and switch it in the ON position. Please give us a reading on the resistance across L1 and L2.
  6. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Sep 2, 2004
    Retired Systems engineer for defense industry.
    New England
    On a 240vac circuit, all of the current that goes 'out' one line, goes 'back' on the other. you do NOT add the current on each leg together...it is the same current just flowing through. Same idea if you had a 120vac circuit, you could measure the hot or the neutral and get the same current in/out of the device.

    Power = volts * amps, so if it's 240v*13.5A, it's drawing the 3240W you calculated.

    Does the heater have a hi/lo setting? Could be you only hooked to the lo leads?

    If hi/lo was using two 120vac heating coils, it would need a neutral connection, either powering one or both of them. In this case, it might be running one heating coil off of each leg separately.

    Resistance of heater coils can be misleading, as the resistance changes when they heat up.
  7. ImOld

    ImOld Octogenerian

    Jun 1, 2013
    In the rumble seat
    How about a link or info on this heater. Is it hard wired or did you install a receptacle or use one already installed. 248 volts is well within tolerances. Where is it mounted? As already mentioned, large space heaters come with output settings. Could be a mis-wire. Remember, this is two hot wires and a neutral.
    Last edited: Dec 24, 2019
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