Fuse Size Calculator
Enter the load, split into continuous and noncontinuous portions, and get the standard NEC 240.6(A) fuse plus the minimum conductor that fuse can legally protect. Applies the 210.20(A) 125% continuous rule, the 240.4(B) next-size-up allowance, and the 240.4(D) small-conductor limits. General-use loads only; motor circuits size fuses under Article 430.
How to use this calculator
- Enter the load in amperes, split into noncontinuous and continuous (3+ hours) portions. Or switch to watts + voltage and the calculator converts for you (single-phase).
- Set the conductor material, insulation and termination ratings, ambient temperature, and conductor count; these drive the conductor recommendation, not the fuse size.
- Read the fuse rating and the minimum conductor. The notes explain any 240.4(B) next-size-up or 240.4(D) small-conductor interactions, which are where most sizing mistakes happen.
- For motors, air conditioners, and welders, use the equipment nameplate and the applicable article instead (430/440/630); see the FAQ and the Motor Breaker Sizing tool.
NEC reference
Protection is sized per NEC 2020 210.20(A): 100% of the noncontinuous load plus 125% of the continuous load, rounded up to a standard fuse rating from 240.6(A). NEC 240.6(A) gives fuses the same standard ratings as inverse-time breakers plus five fuse-only additions (1, 3, 6, 10, and 601 A). The conductor is sized to the same requirement per 210.19(A)(1) using Table 310.16 with ambient correction, conductor-count adjustment, and the 110.14(C) termination cap, then checked against the 240.4(D) small-conductor limits so the recommended fuse can actually protect it, with 240.4(B) next-size-up flagged when used.
Results are for reference only. Verify against the applicable adopted edition of the NEC and consult a licensed electrician for code compliance.
Standard fuse sizes (NEC 240.6(A))
Fuses share the inverse-time-breaker rating list and add five ratings of their own. The fuse-only ratings are shown in bold below.
| Range | Standard ampere ratings |
|---|---|
| Small (fuse-only) | 1, 3, 6, 10 |
| Common branch/feeder | 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100 |
| Feeder/service | 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600 |
| 601 A break point | 601, 700, 800 |
| Large | 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000 |
The 601 A rating exists because several NEC rules (for example the 240.60 and 240.100 fuse classes and some conductor-protection provisions) change at 600 A; 601 A is the smallest standard rating above that break point.
Common fuse sizes: copper conductors, standard conditions
| Circuit | Fuse | Copper conductor | Governing rule |
|---|---|---|---|
| Control circuit, 6 A | 6 A | 14 AWG | 240.6(A) fuse-only rating |
| General lighting/receptacles | 15 A | 14 AWG | 240.4(D): 14 AWG capped at 15 A |
| Kitchen small-appliance, bath | 20 A | 12 AWG | 240.4(D): 12 AWG capped at 20 A |
| Water heater 4500 W / 240 V | 25 A | 10 AWG | 422.13 continuous: 23.4 A required |
| Feeder, 65 A conductor | 70 A | 6 AWG | 240.4(B) next-size-up |
Standard conditions (86°F, 3 or fewer current-carrying conductors, 75°C terminations). Derating, long runs (voltage drop), and aluminum change the conductor; use the calculator above with your actual conditions.
The three rules that decide fuse size
- 210.20(A): the 125% rule. Continuous loads (3+ hours) count at 125% toward the required protection. Miss this and a heater or long-duty circuit ends up undersized.
- 240.6(A): standard ratings. Fuses come in fixed sizes; you round the requirement up to the next one. Fuses add the 1, 3, 6, 10, and 601 A ratings that breakers do not have.
- 240.4(D): small-conductor caps. Regardless of computed ampacity, copper 14/12/10 AWG are capped at 15/20/30 A (aluminum 12/10 at 15/25 A). This is why a 25 A fuse still needs 10 AWG copper, not 12.
Frequently asked questions
What are the standard fuse sizes?
NEC 240.6(A) sets the standard ampere ratings. Fuses use the same list as inverse-time breakers (15, 20, 25, 30 A and up) plus five ratings that are specific to fuses: 1, 3, 6, 10, and 601 A. The small ratings cover control and fractional-horsepower circuits; 601 A sits just above the 600 A break point that several Article 240 rules turn on.
Can I put a 25 A fuse on 12 AWG copper?
No. NEC 240.4(D) caps 12 AWG copper protection at 20 A regardless of the conductor's computed ampacity (25 A at 75°C, 30 A at 90°C). The 240.4(B) next-size-up allowance explicitly does not override the small-conductor rule. A 25 A fuse requires 10 AWG copper.
When does next-size-up (240.4(B)) apply?
When the conductor's ampacity does not correspond to a standard fuse rating, the device is 800 A or less, and the circuit does not supply multiple cord-and-plug receptacles. Classic example: 6 AWG copper at 65 A protected by a 70 A fuse. Above 800 A the allowance disappears; the conductor must be upsized instead. The calculator flags when its recommendation uses this allowance.
Why doesn't this work for motors and AC units?
Motor branch circuits (Article 430) size the branch-circuit fuse from Table 430.52 at 175% to 300% of the full-load current, well above the conductor ampacity, because a separate overload device protects the motor. HVAC equipment (Article 440) puts the answer on the nameplate as MCA and MOP. Feeding those loads through a general-purpose 125% calculation gives fuses that nuisance-trip on inrush, so this tool stays out of that territory. Use the Motor Breaker Sizing tool for motor fuses.
Related calculators
- Breaker Size Calculator: the same load sized for an inverse-time breaker instead of a fuse.
- Motor Breaker Sizing Calculator: motor branch-circuit fuses and breakers per NEC Article 430 and Table 430.52.
- Wire Size Calculator: full conductor sizing with voltage drop and OCPD together.
- Wire Ampacity Calculator: derated ampacity of a specific conductor.
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