Three-phase power
Stores, schools, and shops often get three-phase power. Each of the three hots peaks a third of a cycle after the one before. You move the meter between the hots and the neutral and change the loads.
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What to try
- 1Set Meter across to A to N. Every hot reads 120 volts to the neutral.
- 2Set Meter across to A to B. Two hots read 208 volts, not 240, because 120 times 1.732 is about 208.
- 3Pick Unbalanced loads. Row C is off, so the feeder neutral now carries 10 amps.
NEC 2026 sections it covers
- 120.5(A)
- 208Y/120 V is a nominal system voltage.
- 408.10(E)(1)
- The bus bars are in A, B, C order from left to right.
- 250.118(A)
- EMT can be the ground path for each row of lights.
Section numbers only. Read the full text in your code book. Temperatures and times in the visual are teaching models.
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More visuals
- 120 and 240 voltsOne coil on the pole makes two legs that mirror each other. Put a meter across any two wires.
- Switch the hotWires are colored by voltage. See why a light can be off and still live.
- From the pole to the panelThe transformer on the pole sends two hots and a neutral to the house. Break the neutral and watch the voltages split.