Ohm's Law, Power & NEC 80% Continuous-Load Rule
Priya is sizing a circuit for a new 1,500 W heater on 120 V. She must verify the breaker, the wire gauge, and that she won't trip the panel on cold mornings.
- Multimeter
- NEC ampacity chart
- 12-AWG / 14-AWG romex
- Breaker (15 A or 20 A)
- 1
Pick the right formula
Volts × Amps = Watts. Solve for Amps: I = P / V = 1500 / 120 = 12.5 A.
- 2
Apply the 80% rule
Continuous loads can use only 80% of breaker. 12.5 / 0.8 = 15.6 A → use a 20 A breaker.
- 3
Size the wire
20 A circuit needs 12 AWG copper minimum (NEC Table 310.16).
- 4
Verify with the meter
After hookup, clamp meter on the hot. Reading should sit ≤ 13 A under load.
A 1500 W space heater on a 20 A / 120 V circuit. NEC 210.20(A) limits continuous loads (≥3 hr) to 80% of breaker rating. Compute draw, verify safety margin, then check what happens if the homeowner adds a 600 W toaster oven mid-shift. Wire ampacity, voltage drop on long runs, and resistive heating all interlock.
Tap Show next step to reveal the math one piece at a time.
1500 W heater on a 20 A circuit
Given: P = 1500 W, V = 120 V, breaker = 20 A
- 1
Solve current
I = P ÷ V = 1500 ÷ 120 = 12.5 A
Voltage drop trap
Given: 240 V across a 30 Ω element
- 1
Find current
I = 240 ÷ 30 = 8 A