Why a circuit has a limit

A breaker isn't there to protect you from the appliance — it's there to protect the wire from overheating. That one idea explains every circuit limit.

The breaker matches the wire

Wire can only carry so much current before it gets hot. So each wire size is paired with a breaker that trips before that happens:

• 14-gauge wire → 15-amp breaker
• 12-gauge wire → 20-amp breaker
• 10-gauge wire → 30-amp breaker

Put a 20-amp breaker on 14-gauge wire and the wire can overheat before the breaker ever trips. That's why the pairing is fixed, not a preference.

The 80% rule

A breaker carries its full rating only in short bursts. For a continuous load — anything running three hours or more, like lights or a heater — you may use only 80% of it:

• 15 A circuit → 15 × 0.8 = 12 A usable continuously
• 20 A circuit → 20 × 0.8 = 16 A usable

So that 1,500 W heater (12.5 A) is over the 12 A continuous limit of a 15-amp circuit — it needs a 20-amp circuit. That's exactly the check the circuit-load calculator runs for you.