Fuse Size Chart: Amps, Wire Gauge & Max Watts 2026

How fuse size is worked out

Three steps. First, the load current: watts divided by volts. Second, the continuous-load rule: if the load runs for three hours or more at a stretch, multiply by 1.25 — the fuse and wire have to carry 25% more than the load draws, so they never run at their limit. Third, pick the smallest standard fuse size that covers the result, and read the wire gauge off the table: the fuse protects the wire, so the two always go together. At the defaults — 1,500 W at 120 V, continuous — the load current is 1,500 ÷ 120 = 12.50 A. The 125% rule raises it to 12.50 × 1.25 = 15.63 A (exact 15.6250 A). A 15 A fuse is too small, so the answer is the next size up: a 20 A fuse on 12 AWG wire.

The continuous columns in the chart are the 80% rule in watts: a fuse should carry no more than 80% of its rating continuously, so a 15 A fuse at 120 V is good for 15 × 0.8 × 120 = 1,440 W of continuous load, and a 20 A fuse at 120 V for 1,920 W. The two rules are the same rule seen from opposite ends: 1.25 times the load, or 80% of the fuse.

The worked example, line by line

Setup: load 1,500 W, voltage 120 V, continuous load.

Line 1 — the load current. 1,500 ÷ 120 = 12.50 A.

Line 2 — the continuous-load rule. 12.50 × 1.25 = 15.63 A (exact 15.6250 A). This is the half-cent trap: 15.625 rounds half-up to 15.63, not down to 15.62.

Line 3 — the fuse. The smallest standard size at or above 15.63 A is 20 A. A 15 A fuse would blow on a load that runs all day.

Line 4 — the wire. The table pairs a 20 A fuse with 12 AWG wire. The circuit can carry 20 × 120 = 2,400 W at most, and 20 × 0.8 × 120 = 1,920 W continuously.

Reverse: the most a 15 A fuse can carry continuously at 120 V is 1,440 W; a 20 A fuse at 120 V is 1,920 W; a 30 A fuse at 120 V is 2,880 W and at 240 V is 5,760 W; a 60 A fuse at 240 V is 11,520 W. A 60 A fuse is 4.00× a 15 A fuse, a 30 A fuse is 2.00×, and a 20 A fuse is 1.33× (exact 1.3333).

Every branch, checked

1,800 W at 120 V, continuous: 1,800 ÷ 120 = 15.00 A, × 1.25 = 18.75 A — a 20 A fuse, 12 AWG. A 15 A fuse is too small once the 125% rule applies, even though the raw load is exactly 15 A.

1,200 W at 120 V, continuous: 1,200 ÷ 120 = 10.00 A, × 1.25 = 12.50 A — a 15 A fuse, 14 AWG.

900 W at 120 V, continuous: 900 ÷ 120 = 7.50 A, × 1.25 = 9.38 A (exact 9.3750 A) — a 15 A fuse, 14 AWG. The second half-cent trap: 9.375 rounds half-up to 9.38.

2,400 W at 120 V, continuous: 2,400 ÷ 120 = 20.00 A, × 1.25 = 25.00 A exactly — a 25 A fuse, 10 AWG. An exact tie still takes the fuse of that size: 25 A covers exactly 25 A.

3,000 W at 120 V, continuous: 3,000 ÷ 120 = 25.00 A, × 1.25 = 31.25 A — a 40 A fuse, 8 AWG. There is no 35 A size in the standard set, so 31.25 A jumps to 40 A.

5,500 W at 240 V, continuous: 5,500 ÷ 240 = 22.92 A, × 1.25 = 28.65 A (exact 28.6458 A) — a 30 A fuse, 10 AWG. At 240 V the same watts need half the amps.

9,600 W at 240 V, continuous: 9,600 ÷ 240 = 40.00 A, × 1.25 = 50.00 A exactly — a 50 A fuse, 6 AWG. Another exact tie, stated as such.

7,200 W at 240 V, continuous: 7,200 ÷ 240 = 30.00 A, × 1.25 = 37.50 A — a 40 A fuse, 8 AWG.

600 W at 120 V, continuous: 600 ÷ 120 = 5.00 A, × 1.25 = 6.25 A — a 15 A fuse, 14 AWG. 15 A is the smallest standard size on this chart, so small loads still get a 15 A fuse.

1,500 W at 120 V, non-continuous: 1,500 ÷ 120 = 12.50 A, no 125% rule — a 15 A fuse, 14 AWG. Compare with the default: the same 1,500 W needs a 20 A fuse when it runs continuously and a 15 A fuse when it does not. The load type, not the watts, decides.

4,000 W at 240 V, continuous: 4,000 ÷ 240 = 16.67 A, × 1.25 = 20.83 A (exact 20.8333 A) — a 25 A fuse, 10 AWG.

10,000 W at 240 V, continuous: 10,000 ÷ 240 = 41.67 A, × 1.25 = 52.08 A (exact 52.0833 A) — a 60 A fuse, 4 AWG.

12,000 W at 240 V, continuous: 12,000 ÷ 240 = 50.00 A, × 1.25 = 62.50 A — above this chart. The largest fuse here is 60 A, so a 62.50 A load needs a licensed electrician to size the circuit, not a bigger fuse from this table.

Reverse — the lines that matter: a 15 A fuse carries 1,440 W continuously at 120 V; a 20 A fuse carries 1,920 W; a 30 A fuse carries 2,880 W at 120 V and 5,760 W at 240 V; a 60 A fuse carries 11,520 W at 240 V.

The safety rule: never replace a blown fuse with a larger one. The fuse protects the wire: a 20 A fuse on 14 AWG wire lets the wire carry more than it is rated for, and the wire overheats instead of the fuse blowing. Repeated blows mean find the fault — the load, the wiring or the appliance — not a bigger fuse.

What the defaults assume

The model assumes a single resistive load at the voltage you enter, copper wire at the standard ampacities, and the 125% rule for loads that run 3 hours or more. It does not model motor starting current (motors draw several times their running current at start-up), derating for hot rooms or bundled cables, voltage drop on long runs, or aluminium wire. This is NOT a breaker-panel sizing or full load-calculation chart: panel work, service upgrades and anything above 60 A need a licensed electrician. Rounding convention: amps are rounded half-up to two decimals, the fuse is picked from the exact required amps before rounding, and all figures are computed from unrounded values, rounded only for display.

Output · formula · meaning

Who uses this

• The homeowner replacing a fuse: they learn a 1,500 W heater needs a 20 A fuse on 12 AWG wire — so they know a 15 A fuse will keep blowing, and they check the 1,800 W case (18.75 A, still 20 A) before buying a box of fuses.
• The DIYer wiring a workshop: they learn a 5,500 W heater at 240 V needs 28.65 A, a 30 A fuse on 10 AWG — so they know what wire to buy, and they check the 10,000 W case (52.08 A, 60 A, 4 AWG) before sizing the subpanel feed.
• The renter checking a circuit: they learn a 15 A circuit carries only 1,440 W continuously — so they know a 1,500 W heater plus anything else will trip it, and they check the 20 A figure (1,920 W) before moving the heater to the kitchen circuit.
• The student learning the 80% rule: they learn the 125% load rule and the 80% fuse rule are the same rule — so they know why 1,500 W at 120 V needs 15.63 A of fuse, and they check the exact-tie cases (25.00 A and 50.00 A) before trusting a rounded answer.

Accuracy & limitations

The math is exact for the numbers you enter: amps are watts divided by volts, the continuous rule multiplies by 1.25, and the fuse is the smallest standard size at or above the exact result. What the model cannot do is know your installation: check whether the load is really continuous, because a heater that cycles on and off is not the same as one that runs for hours; check motor loads, because starting current is several times running current and needs a time-delay fuse; and check the wire already in the wall, because the fuse must protect the wire that is there, not the wire the chart says. This is NOT a breaker-panel sizing or full load-calculation chart: panel work, service upgrades and anything above 60 A need a licensed electrician. For the wire ampacity behind the gauge column, see AWG wire ampacity chart.