Case Study #177: What Happens When Two Heaters With Different Wattage and Voltage Are Wired In Series?

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Case Study #177

A customer phoned in one day about an issue with two heaters. The two heaters were wired in series on a barrel. One of the heaters, however, was getting red hot and the other was not. He did not know why. Our first thoughts were that maybe the heaters were accidentally wired in parallel rather than in series. After checking with the customer, we found that the heaters were wired correctly. But, why was one of them getting so hot?

One heater was 610W at 230V. The other was 1260W at 230V. The customer applied 460V across these two heaters when they were wired in series.

Hint: to successfully wire two heaters in series, they must be equal wattage AND equal voltage, i.e. they must have equal resistance.

Now to see what happens when we calculate the resistance of each heater:

resistance equals E squared over W

Now when we apply 460V across these two heaters that are wired in series the circuit looks like this:

voltage across the 87 heater equals 310 volts, voltage across the 42 heater equals 150 volts, wattage for heater 1 equals 1,107, wattage for heater 2 equals 536 watts
table showing stamped wattage versus actual wattage of the two heaters and an arrow pointing to the 1,107 watt that got red hot