A Measurement of Current and Voltage Associated with Portable Heater and Some Safety Considerations.

Cold days are about to set upon us, my wife and I that is. One day, I think Monday 08 February 2021, will have a Low Temperature of 4˚ F. When we bought this house, it was terribly cold. In the bedroom, in addition to the furnace being on, we used 2 heaters, on low, cycling, on different circuits. We had the house re-wired, so we had Dedicated Receptacles installed in each bedroom for an Air Conditioner. The remainder of each room is installed on it’s own circuit. So, plugged into the A/C Receptacle is good for 20 Amps, and 15 Amps plugged into any other receptacle in the room.

Since that winter, we had the attic insulated, and the walls to the home insulated. It’s not only much easier and cheaper to heat, it’s deadened outside sounds. We now use one heater, cycling, on low, on the A/C Circuit, and the furnace is on 68 or 69 at night.

With the coming cold weather, I was curious what the heaters draw, current wise? I used a Clamp-On Ammeter, and measured the Fan Only, Low Heat, Medium Heat, and High Heat settings of the Operating Heater.

My first exposure to Clamp-On Ammeters was the Classic Analog Amp-Probe Black Meter with a Rotating Dial (see the Featured Image at the Top). In some ways, the word Amp-Probe has become synonymous with Clamp-On Ammeters like Channel-Locks are associated with Slip-Joint Pliers.

I used a Fluke Clamp-On Ammeter to measure the current used by the heater on different settings. Only one wire can be clamped to measure, clamping both will read 0 Amps. To facilitate that I used an Amp-Probe Line Splitter. There’s a feature on the Line Splitter that a friend showed me years ago. Using a number of wraps of wire, the Clamp-On Meter measures more current. If I loop the same wire 5 times, then clamp it, I will read 5X the amount of Current. The Line Splitter Does 1X and 10X. 1X is the actual reading, and 10X needs to have that reading Divided by 10 to get the actual reading. It’s useful for meters with limited accuracy after the decimal point. My meter has just one Digit after the Decimal.

I inspected and tested the heater recently, it had been in use for the season without a pre-season check. The heater has Fan Only, Low, Medium, and High Heat Settings. The heater is rated at 1500 Watts, so that, using Ohm’s Law, Current = Power/Voltage. 1500/120 is 12.5 Amps is what we should see, Max.

Heaters with 3 Heat Levels commonly have 2 sets of Heating Elements. This is similar to a 3 Way Bulb having 2 Filaments. In a 3 Way Bulb, a 50 Watt and 100 Watt Filament is found. Use the first for 50 Watts, the Second for 100 Watts, and both for 150 Watts. Same with the Heater. One, the other, or both. Both is the Highest Heat Output and the most ampacity (Current Use).

I noticed the heater needed cleaning when I switched on the Medium Heat. The dog, sleeping on the floor, sat up with a start. Seconds later I found out why. The dust on the 2nd Heating Element burned off, leaving an acrid smell. This is common, in seconds the dust was gone, no smoke, there wasn’t much dust, but enough to make the smell. Looking inside, I decided I’d clean it the next day. Incidentally, I did not use it on medium, but wanted to see how much hotter the air felt, it was a cold night and the room temperature was slowly dropping, but I kept it in low.

We use the Heater Plugged into a Workplace Style Power Block (Strip). The Heater should be plugged into the wall directly. But the Ampacity of the Power Block is sufficient for the Low and Medium Heat Setting. The Heater Plug was replaced, last year, with an Industrial Grade Plug (stress tears in the insulation on the original, no melting or softening was evident). When I unplug an Appliance, such as a Heater or Steam Vaporizer, I feel the Metal Blades or Prongs on the removed plug to see if it is hot. Warm is ok, slightly hot, starting to be a concern, very hot, there is something wrong with the connection, don’t use it until inspected and repaired. Our house was rewired and I gave the electrician Commercial Grade Receptacles to use. They have a tighter grasp on the blades. Loose Fit in the receptacle is cause for overheating and can eventually lead to a fire, if your heater plug looks melted somewhat, something is wrong. That’s why we make sure the heaters cycle (Cool Down Time). That’s why we used to use 2 heaters on low, on different circuits, the plug won’t usually get very warm on low.

See the accompanying images.

Sheet #1

MTR-HTR 1 of 3

  • Power Block, Replacement Plug on the Heater Cord, Amp-Probe Splitter, and Heater.
  • Heater Specification Tag and Heater Controls. See the Dot on the Thermostat. That’s the sweet spot.
  • Overview of the Meter and Line Splitter.
  • Splitter X1 and X10 Openings.
  • Voltage Check Test Points on the Splitter.
  • Fluke Clamp-On Meter with Category III Markings on the Clamp Position and Test Lead Inputs.
  • Reverse Side of Clamp-On Meter and Line Splitter.
  • Receptacle End of Splitter and Test Lead End of the Clamp-On Meter.
  • Prong End of Splitter and Clamp End of the Clamp-On.

Sheet #2

MTR-HTR 2 of 3

  • The Measured Current Readings.

Fluke Clamp-On Ammeter, Amp-Probe Line Splitter.

The Fluke Clamp-On Meter also measures Voltage, and Resistance/Continuity using the supplied Test Leads. This Ammeter Measures AC (most do) and DC (most don’t) Currents. AC Current has an Expanding and Collapsing Magnetic Field, and the field induces current in the Clamp. This is the way most work. DC Current though, the Meter needs an Electronic Circuit to equate the Steady DC Current Magnetic Field with the Amount of Current. Not all Meters Measure Clamp On DC Current.

The Line Splitter has Convenient voltage Test Points. It has a X1 and X10 Clamp Position to help with Low Currents or Resolution on some meters. And it’s for Grounded Appliances and maintains the Ground well testing.

So, without further ado, the Readings:

  • Fan Only X1 = 0.2 Amps, X10 = 2.1 Amps (2.1/10 = 0.21 Actual Amount)
  • Low Heat X1 = 4.9 Amps, X10 = 47.4 Amps (47.4/10 = 4.74 Actual Amount)
  • Medium Heat X1 = 7.2 Amps, X10 = 71.4 Amps (71.4/10 = 7.14 Actual Amount)
  • High Heat X1 = 11.6 Amps, X10 = 114.4 Amps (114.4/10 = 11.44 Actual Amount)


The current varies slightly over a short time, perhaps the meter, or the coupling to the Magnetic Fields, or conditions in the Heater Coil concerning Resistance. The above X1 and X10 Readings are fine, despite that variance. This is one problem with Digital Meters, a neurotic adherence to a specific value and not the overall concept. Analog Meters were less prone to user fixation on a particular number. On digital, if I saw, without changing anything on the Heater or without it Cycling, 11.6 and 12.8, back to 11.6 then to 10.7, etc., I’d be concerned. But on the High Heat Setting 11.6 and 11.44 are both fine for the purpose of determining what I want to know, and the Specification Plate lists 12.5 Amps Total, so 11.6 or 11.44 are 92.8% and 91.5% of Maximum Value Expected, respectively, so assuming the Meter is Accurate, the wattage is slightly lower and the current somewhat less than Maximum Specified.

So, during the pending cold snap and low Temperatures of 4˚ F, I’ll be using the above Heater on Medium Heat Setting, being sure that the Heater is Cycling often, and hope for the best. The furnace will, of course, be running, but at 4˚ F, either I run the Furnace set to perhaps 71˚ during the Night (we accidentally left it set at 71˚ or so when it was 15˚ outside one night, the wife had turned it up to “warm the upstairs”, and I noticed the furnace was cycling-on every 5 minutes or very slightly longer. I went downstairs and set the Thermostat to 69˚ and all was well, except I noticed that, even with the Heater on Low, the Room Temperature was gradually decreasing (but it stabilized at 74˚+).

How much Voltage is available at the Power Block with the Heater Off, Fan Only, Low Heat, Medium Heat, and High Heat? Well, I used the Test Points on the Splitter with the Test Leads on the Meter, while I operated the Heater in each setting. As Current Increases, there is a Voltage Drop across the resistance of the wiring from the Breaker Panel to the Receptacle, from the Receptacle to the Power Block across its cord (this is why you should not use an Extension Cord or Power Strip, or if you must, use a #14 or #12 Gauge Appliance Extension cord, as increased wire size decreases the Resistance and increases the Voltage available while the appliance is in use) and Across Poor Connection Points due to Loose Screws, Corrosion, or just Loose Clamping Receptacle Components. The Fan only draws a small amount of Current, so the Voltage Drop is very low.

Sheet #3 Measured Voltages.

MTR-HTR 3 of 3

  • Heater and Fan Off = 122.5
  • Fan Only = 122.5 Volts
  • Low Heat = 120.0 Volts
  • Medium Heat = 118.8 Volts
  • High Heat = 116.6 Volts

Author: Dr-Artaud

A Doctor that is not a Doctor, but named after a character in the movie "No Such Thing", as is the Avatar.

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