My simple understanding of electricity is that you always need a circuit (from + to -, or from hot to neutral), in order for current to flow.

Now, I recently had to fix my water heater (which is running on 240V), and noticed that the heating element basically consists of two hot legs (at 120V each) coming together. There is no neutral, no ground, just two hots (e.g. like this).

How does that work??? Where's the circuit? Why doesn't it just explode? ;)

P.S. I know I have 240V in my house, and that I get it by combining to 120V legs. But what I don't understand is how a heating element can work by just having two hot legs come in; without any neutral to close the circuit.

For those of you who have never seen those heating elements, this is what it looks like: https://imgur.com/ek6Bxeu Two connectors that basically each receive 120V, with about 10Ω between them.

  • \$\begingroup\$ If this is a US/Canada system, then there's no need for neutral. It's just the two opposite sides of a transformer output. Neutral is the center-tap and the water heater doesn't require the center-tap for power. Using the two opposite sides doubles the RMS voltage and therefore halves the required current, lowering the cost of wiring. \$\endgroup\$
    – jonk
    Dec 29 '19 at 19:10
  • 1
    \$\begingroup\$ Why did you think it would explode? Current just flows between points of different potential. \$\endgroup\$
    – Transistor
    Dec 29 '19 at 19:19
  • \$\begingroup\$ 240V rated heaters use half the current thus easier on wires and breakers when > 15A*120V =1.8kW but with less wire loss 240V* 12A = 3kW... 240V is preferred as it balances L1, L2 current to transformer shared by neighbours \$\endgroup\$ Dec 29 '19 at 20:50


simulate this circuit – Schematic created using CircuitLab

You have two 120V circuits in your house, they stack up to 240V nicely.


Most likely you have two life circuits which are in opposite phase.

This is an image I borrowed from http://www.thecoffeebrewers.com/electricity.htmli:

enter image description here

Each by itself is a complete sine wave against ground. But if you connect between the two you get a sinewave with double the amplitude.


Assuming your picture is accurate, I think you have two heaters - upper and lower - that are in parallel across the 240V input. Each heater has a thermostat in series with it.

Tried to draw and insert a simple diagram using the schematic tool, but crashed and burned!


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