Electrical Fuse Size Calculator
Size a fuse for general, continuous, feeder or motor circuits under the NEC (125% continuous rule, Table 430.52, 240.6(A) ratings) or IEC 60269 / IEC 60364 (gG and aM fuses), with a cable check.
Fuse Size Calculator
NEC: 125% for continuous loads, Table 430.52 for motors. IEC: In ≥ Ib with gG fuses, aM fuses for motors.
How to Use the Fuse Size Calculator
- 1Pick the standard. IEC for gG/aM fuses to IEC 60269 under IEC 60364 or BS 7671; NEC for UL 248 fuses under NFPA 70.
- 2Enter the load. Amps directly, or kW with voltage, phase and power factor. For motors, enter amps: the NEC Table 430.248 / 430.250 FLC, or the nameplate current under IEC.
- 3Choose the application. This sets the multiplier for the selected standard. You can override it.
- 4Optional: enter the cable ampacity Iz. Use the value after derating to check the fuse against the conductor.
How to Calculate Fuse Size
Multipliers by standard and application
| Application | NEC (NFPA 70) | IEC 60364 / IEC 60269 |
|---|---|---|
| Continuous load (3 h or more) | 125% (210.20(A), 215.3) | 100%: gG fuse with In ≥ Ib |
| Non-continuous load | 100% | 100%: gG fuse with In ≥ Ib |
| Feeder | 125% continuous + 100% non-continuous (215.3) | In ≥ Ib, and selective with downstream fuses |
| Motor branch short-circuit | Table 430.52: 175% dual-element time-delay, 300% non-time-delay; next standard size up permitted (430.52(C)(1) Exc. 1); maximum 225% / 400% if the motor will not start (Exc. 2) | aM fuse (IEC 60269-2) with In ≥ motor current, final size from the manufacturer's Type 1 / Type 2 coordination table (IEC 60947-4-1) |
| Rounding | Next rating from 240.6(A) | Next rating in the fuse range |
| Cable check | Fuse ≤ ampacity (240.4), next size up to 800 A (240.4(B)); motor conductors ≥ 125% FLC (430.22) | Ib ≤ In ≤ Iz and I2 ≤ 1.45 Iz (IEC 60364-4-43). gG: I2 = 1.6 In from 16 A (IEC 60269-1), so In ≤ 0.906 Iz |
Example 1 (IEC): 11 kW three-phase heater, 400 V
- Ib = 11,000 / (1.732 × 400 × 1.0) = 15.88 A → gG fuse 16 A.
- I2 = 1.6 × 16 = 25.6 A, so the cable needs Iz ≥ 25.6 / 1.45 = 17.7 A.
Example 2 (NEC): 10 HP, 460 V three-phase motor
- FLC from Table 430.250 = 14 A (use the table, not the nameplate, per 430.6(A)(1)).
- Dual-element time-delay fuse: 14 × 1.75 = 24.5 A → 25 A. Non-time-delay fuse: 14 × 3.0 = 42 A → 45 A.
- Branch conductors: at least 14 × 1.25 = 17.5 A (430.22). Overload relay per 430.32, based on nameplate current.
Example 3 (IEC): 7.5 kW, 400 V motor, direct-on-line
- Nameplate current: read it from the motor; about 15 A is usual for this size. Enter 15 A, Motor (aM fuse).
- Minimum aM rating: 16 A. Check the starter manufacturer's Type 2 coordination table for the contactor and overload relay you use, and follow its fuse size if larger.
Fuse Types: IEC Categories and UL Classes
| Fuse | Standard | Use |
|---|---|---|
| gG | IEC 60269-2 / -3 | Full-range protection for cables and general loads (overload and short circuit) |
| aM | IEC 60269-2 | Motor circuits, short-circuit only; used with an overload relay |
| gR / aR | IEC 60269-4 | Semiconductor protection (drives, rectifiers, UPS) |
| gPV | IEC 60269-6 | PV string and array protection (DC) |
| Class CC | UL 248-4 | Up to 30 A, 600 V; control circuits and small motors |
| Class J | UL 248-8 | Up to 600 A, 600 V; compact, current-limiting; common in UL 508A panels |
| Class T | UL 248-15 | Up to 1200 A, 300 V or 600 V; very compact, current-limiting |
| Class RK1 / RK5 | UL 248-12 | Up to 600 A, 250 V or 600 V; RK1 limits current more than RK5 |
| Class L | UL 248-10 | 601 to 6000 A, 600 V; service and feeder protection |
UL classes CC, J, T, RK1, RK5 and L carry a 200 kA interrupting rating. In both systems, the fuse's breaking capacity and voltage rating must be at least the prospective fault current and system voltage at the point of installation (NEC 110.9).
Fuse Size Chart (NEC vs IEC)
| Load current / FLC | IEC gG (In ≥ Ib) | NEC continuous (125%) | NEC motor, time-delay (175%) | NEC motor, non-time-delay (300%) |
|---|---|---|---|---|
| 5 A | 6 A | 10 A | 10 A | 15 A |
| 10 A | 10 A | 15 A | 20 A | 30 A |
| 15 A | 16 A | 20 A | 30 A | 45 A |
| 20 A | 20 A | 25 A | 35 A | 60 A |
| 30 A | 32 A | 40 A | 60 A | 90 A |
| 50 A | 50 A | 70 A | 90 A | 150 A |
| 80 A | 80 A | 100 A | 150 A | 250 A |
| 100 A | 100 A | 125 A | 175 A | 300 A |
| 150 A | 160 A | 200 A | 300 A | 450 A |
| 200 A | 200 A | 250 A | 350 A | 600 A |
IEC motor fuses (aM) are not in this table because their size comes from the starter coordination table. Always check the cable as well.
Selection Checklist and Common Mistakes
- Current rating from the calculator, then the cable check (I2 ≤ 1.45 Iz under IEC; 240.4 under NEC).
- Voltage rating at least the system voltage; AC and DC ratings differ.
- Breaking capacity at least the prospective fault current.
- Category or class: gG for cables, aM for motors with an overload relay, aR/gR for semiconductors; UL Class J/RK1/RK5 time-delay for motors.
- Selectivity: upstream and downstream fuses should not both open on the same fault. Use the manufacturer's selectivity ratio or I²t data.
Common mistakes: using NEC motor percentages with IEC aM fuses; sizing a motor fuse from kW without efficiency; forgetting that a gG fuse needs about 10% more cable capacity than an MCB of the same rating (I2 1.6 vs 1.45); using a fuse that is not rated for DC on a DC circuit.
Fuse Size Calculator FAQ
Find the load current. Under NEC, multiply continuous loads by 1.25 and round up to a 240.6(A) rating. Under IEC, choose the smallest gG rating with In ≥ Ib, then check the cable: 1.6 × In ≤ 1.45 × Iz for gG fuses of 16 A and above.
NEC: 175% of the Table 430.250 FLC for a dual-element time-delay fuse, or 300% for a non-time-delay fuse, rounded up to the next standard size (Table 430.52). IEC: an aM fuse at or above the motor current, sized from the starter manufacturer's coordination table, with an overload relay for overload protection.
A gG fuse protects against both overload and short circuit and is used for cables and general loads. An aM fuse only breaks short-circuit currents; it tolerates motor starting current and relies on a separate overload relay.
Use time-delay (dual-element) fuses for motors, transformers and other loads with inrush. Use fast-acting fuses for resistive loads and electronics, and semiconductor fuses (aR/gR) for drives and rectifiers.
Under NEC the rules are the same (240.6(A) lists both), except that fuses have the extra 1, 3, 6, 10 and 601 A ratings and different Table 430.52 motor percentages. Under IEC, a gG fuse needs more cable margin than an MCB because its conventional fusing current is 1.6 × In instead of 1.45 × In.
Standards and References
The formulas and reference values on this page follow these standards. For design work, check results against the current edition that applies in your country and the equipment manufacturer's data.
- NFPA 70 (NEC) – 110.9; 210.20(A) and 215.3 continuous loads; 240.4, 240.4(B), 240.4(G); 240.6(A) standard ratings; 430.6(A)(1), 430.22, 430.32, 430.52 and Table 430.52, Tables 430.248 / 430.250. Published by NFPA.
- UL 248 series – Low-voltage fuses (Class CC, J, L, RK1, RK5, T).
- IEC 60269-1, -2, -3, -4, -6 – Low-voltage fuses: general requirements (conventional fusing currents), industrial gG/aM, household, semiconductor and PV fuses. Published by IEC.
- IEC 60364-4-43 – Protection against overcurrent (Ib ≤ In ≤ Iz, I2 ≤ 1.45 Iz).
- IEC 60947-4-1 – Contactors and motor starters, Type 1 / Type 2 coordination.
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