Lessons · Electrical · 120/240 V in a home
Two hots and a neutral: how 120 and 240 both come from one service
The utility transformer has a center-tapped winding: each end is 120 V to the center, the ends are 240 V to each other, and the center is the neutral. Small loads go from one hot to neutral; big loads go hot to hot.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
You are asked why the dryer has a four-wire cord and the toaster a two-wire plug, or why a panel has two columns of breakers. It is all one picture, and the neutral's job in it is the part that gets people hurt when they get it wrong.
How to think about it
Draw the transformer as a line with a dot in the middle. Top to dot: 120 V. Bottom to dot: 120 V. Top to bottom: 240 V. The two hots are out of step, so a load on each hot sends its current back on the neutral in opposite directions, and the neutral carries only the difference.
Worked example
Hot A to neutral: 120 V. Hot B to neutral: 120 V. Hot A to hot B: 240 VThree voltages, one transformer. Any 120 V load is on A or on B.
Leg A carries 22 A, leg B carries 15 ATwo groups of 120 V loads.
Neutral carries 22 − 15 = 7 AOnly the imbalance comes back on the neutral. If both legs were equal the neutral would carry nothing.
A 240 V heater from A to B: 4800 W / 240 V = 20 A on each hot, 0 A on the neutralIt never touches the neutral. A pure 240 V load has no white wire.
Your turn
Leg A carries 30 A and leg B carries 18 A. Write the neutral current.
I = 30 − = 12 A
Solve one, graded on the server
The trap
Believing the neutral is safe to open under load because it carries little. On a shared neutral, opening it puts two 120 V loads in series across 240 V; one gets more than its share, and something burns. Never open a neutral while its hots are live: de-energize, and test it dead, first.