Estimating Heater Resistance From the Rating Plate Wattage

Why this matters

The tech sheet is not always on the truck, and the replacement element is not always the right part. One line of arithmetic tells you what a heater should read before you condemn it, and what a new heater should read before you install it. It catches the open coil, the element from the wrong model, the half-burned dual coil, and the wrong-voltage part that would have gone in, run at the wrong power, and come back as a callback. It takes thirty seconds and needs only the wattage and the voltage.

The relationship

A resistive heater turns all its input into heat, so its power, voltage and resistance are tied by P = V squared / R. Turned around for the field:

R = V squared / W, where V is the voltage the heater is rated at and W is its rated wattage.

Two conditions sit inside that formula, and both matter:

• V is the rated voltage of the heater, not the supply you happen to have. A 240 V element on a 208 V supply keeps the same resistance and simply makes less heat.
• W is the heater's own rating, not the appliance's total. A dryer rating plate may list the whole appliance's input, motor included. The element's own wattage is on the element, in the parts list, or on the tech sheet.

The resistance value from the tech sheet for that model is always the pass value. This calculation is the sanity check behind it, and the way to get a working estimate when the sheet is missing.

The worked lines

HEATER RESISTANCE ESTIMATES, R = V x V / W

Dryer element, 5400 W at 240 V
  V x V  = 240 x 240        = 57,600
  R      = 57,600 / 5400    = 10.7 ohms
  check: 57,600 / 10.7      = 5383 W  (rounding only)
  amps:  5400 / 240         = 22.5 A

Bake element, 3400 W at 240 V
  R      = 57,600 / 3400    = 16.9 ohms
  check: 57,600 / 16.9      = 3408 W
  amps:  3400 / 240         = 14.2 A

Dishwasher element, 1000 W at 120 V
  V x V  = 120 x 120        = 14,400
  R      = 14,400 / 1000    = 14.4 ohms
  check: 14,400 / 14.4      = 1000 W
  amps:  1000 / 120         = 8.3 A

The amps line is the live cross-check. Ohms are read with power off and the heater isolated; current is read with a clamp meter on one heater lead while it runs. If both agree with the rating, the heater is good and anything wrong is upstream of it.

Note what the three lines show side by side: a 1000 W element at 120 V reads within a few ohms of a 3400 W element at 240 V. Resistance alone does not identify the part. Resistance plus rated voltage does. That is why a wrong part can read "about right" and still be wrong.

Why a cold reading runs a little low

The rating is the wattage the element makes at operating temperature. The resistance wire in appliance heaters (nickel-chromium and iron-chromium-aluminum alloys) has a small positive temperature coefficient, so its resistance rises somewhat as it heats. A meter reads it cold, so a good element usually reads a little below the calculated figure.

A practical rule of thumb: a cold reading within about 10 percent of the calculated value, and on the low side, is consistent with a good element. The tech sheet governs where it states a range. What the rule of thumb does not cover:

• OL (open). A broken coil. No amount of temperature coefficient explains it.
• Near zero. A short, or the meter is reading across something else in parallel.
• Well above the estimate. A partly broken coil arcing across a gap, a burned terminal adding resistance, or a different part.
• Well below the estimate. A higher-wattage part, a coil shorted turn-to-turn, or a parallel path through the circuit you did not isolate.

The direction matters because it predicts the symptom. Higher resistance at the same voltage means less power: a slow-drying dryer or a slow-preheating oven. Lower resistance means more power and more current: a tripping breaker, a blowing thermal fuse, an overshooting oven.

Taking the reading so the number means something

Power off and verified dead before the leads go on. Unplug or open the breaker, then prove absence of voltage at the heater terminals with a tester proven on a known live source before and after (the NFPA 70E Article 120 practice), per 'Appliance Electrical Safety Check Before Service'. On a range, an element on a 240 V circuit can still carry one hot leg through a switch that is off.

Lift at least one lead off the heater. In circuit, the meter reads the element in parallel with whatever else bridges its terminals. On many electric dryers the motor's centrifugal switch and the timer sit in that path, and the reading comes out lower than the element alone.

Zero the meter leads. On a 10 ohm element, a few tenths of an ohm of lead resistance is a few percent of the reading. Short the leads, note the reading, and subtract it.

Then read each terminal to the sheath or chassis. That reading should be OL. Continuity from the coil to the sheath is a grounded element: it may still heat, and it is a shock hazard and a breaker-tripping fault. Replace it whatever its end-to-end ohms say.

Restore and prove it. Reconnect the lead to the terminal it came off, refit covers, restore power, run the heat cycle and clamp the current. The finish test is the clamped amps landing near the rated figure from the table above.

Worked example: the dryer that took two cycles

A customer reports a 240 V electric dryer taking two full cycles to dry towels. The element assembly's stamped tag reads 5400 W at 240 V, so the estimate for the whole assembly is 10.7 ohms and 22.5 A. The parts diagram for this model shows the assembly as two coils wired in parallel.

First reading. Power off, verified dead at the terminal block, one lead lifted. The assembly reads 21.2 ohms, less 0.2 ohms of lead resistance, so 21.0 ohms. That is not near 10.7 and not OL. It is 21.0 / 10.7 = 1.96 times the estimate, almost exactly double. Terminal to housing reads OL.

What double means here. Two equal coils in parallel give half the resistance of either one, so each coil in this assembly should read 2 x 10.7 = 21.4 ohms, or 57,600 / 2700 = 21.3 ohms from its half share of the rating. A reading of 21.0 across the pair is one coil's worth, about 1.6 percent under 21.33 and on the low side: one coil good, one coil open. Reading each coil at its own terminals confirmed it: one at 21.0 ohms, one OL.

Live check before ordering. Lead reconnected, covers on, heat cycle running. The clamp on the element feed reads 11.2 A against 22.5 rated. One 21.3 ohm coil at 240 V should draw 240 / 21.3 = 11.3 A, so the live reading agrees with the cold one. The dryer was making about 2700 W, half its rated heat, which matches towels needing about twice the time.

The finish test after replacement. The new assembly read 10.5 ohms cold, about 1.6 percent under the 10.67 estimate, and its two coils 21.0 and 21.1 ohms individually. Running, the clamp read 22.1 A, inside 2 percent of 22.5. One load of towels dried in a single cycle.

The other answer the same arithmetic gives. Had the cold reading come in at 10.5 ohms and the clamp at 11 A, the element would have been cleared and the low current would have pointed upstream, to a supply problem at the receptacle or the panel. That is a different call, walked in '240 Volt Appliance Receptacles and What Each Reads'.

What changes the answer

A 208 V supply. Apartment buildings fed from a three-phase service often deliver 208 V between legs. The element resistance is unchanged, but the power falls to (208 / 240) squared = 0.751 of rated, about 75 percent, and the current to 208 / 240 = 0.87 of rated. On the 5400 W element that is about 4060 W and 19.5 A. That is a normal reading on that service, not a fault, and some makers sell a 208 V element with lower resistance for those installs.

Two coils in series rather than parallel. The example's assembly was parallel. Where the diagram shows two coils in series, one open coil reads OL across the pair and the dryer makes no heat at all, so the same failure gives the opposite symptom. Some range and dryer heaters switch each coil separately; then estimate and read each coil from its own rating.

Hidden and radiant elements. A hidden bake element under an oven floor or a radiant cooktop element still obeys the formula, but read the rating from the part or parts list rather than the appliance plate, and on a radiant cooktop element expect the limiter in the same assembly.

Electronically switched heaters. A board that drives a heater through a relay or triac changes how you take the live reading, not the cold one. The heater is still a resistor.

Where it goes wrong

Using the appliance's total input as the element wattage. A dryer plate figure that includes the motor gives an estimate a few percent low. Small, but enough to make a good element look marginal.

Reading a new part out of the box and not comparing it. A replacement element from the wrong model can read a plausible number. Run the estimate on the new part's own stamped rating against the rated voltage of the appliance before it goes in.

Condemning on a cold reading slightly below the estimate. That is the temperature coefficient, not a failure. The failure modes are OL, a reading to the sheath, or a reading well away from the estimate.

References

• See related: Thermal Fuses, High Limits and Cycling Thermostats in Appliance Heat Circuits
• See related: How to Walk an Electric Dryer Heat Circuit with a Meter
• See related: 240 Volt Appliance Receptacles and What Each Reads
• See related: Appliance Electrical Safety Check Before Service
• NFPA 70E, Article 120, current edition (verifying absence of voltage before and after)