What trips a breaker
A breaker can trip in two ways. On an overload, a bimetal strip gets hot and slowly bends. On a short circuit or a ground fault, a coil becomes a magnet and trips the breaker in about one cycle.
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What to try
- 1On Too much load, set Load to 3 heaters. The current is far over the 20 amp rating, so the strip gets hot and slowly bends.
- 2Pick Short circuit and set Hot wire to Touching. Hundreds of amps flow, and the magnet trips the breaker in about one cycle.
- 3Pick Ground fault and set Hot wire to Touching. The current comes back on the ground wire, not through the ground rod.
NEC 2026 sections it covers
- 240.4(D)
- 12 AWG copper gets a 20 A breaker at most.
- Table 250.122
- A 20 A circuit gets a 12 AWG copper ground wire.
- 250.4(A)(5)
- Fault current needs a low-resistance path back to the source, and the earth is not that path.
- 300.16(A)
- At least 6 in. of free wire stays in the box after a repair.
- 210.12(B)
- Most rooms in a home also need AFCI protection.
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
- Breaker and wirePlug in loads and watch the wire heat up when the breaker is too big for it.
- What a GFCI sensesCurrent goes out and comes back. See what happens when a little goes missing through a person.
- What an AFCI sensesA crushed cord arcs. Watch the shape of the current change, and the breaker count it.