Fuse Size Calculator

Using NEC Mode

1. Select "NEC" on the toggle switch (default).
2. Choose System Type (AC/DC) and Load Type.
3. For "Continuous Load" or "DC", enter the operating current.
4. For "Motor Load", enter the Full Load Current (FLC) from NEC Table 430.250, not the nameplate.
5. Click "Calculate".

Using IEC Mode

1. Select "IEC" on the toggle switch.
2. The calculator is for motor loads (DOL starters) under IEC.
3. Enter the Motor Nameplate Current (I_n).
4. Click "Calculate" to get the typical sizing ranges for 'aM' and 'gG/gL' fuses.
5. Important: Always verify these ranges against the manufacturer's time-current curves.

Understanding Results

1. NEC: Results are specific ratings. You must round up to the next standard fuse size (e.g., 47.25A -> 50A).
2. IEC: Results are a range. This is a starting point. You must check curves to ensure the fuse can handle the motor's starting current (I_st) for its starting time (t_start).
3. Always verify calculations with local electrical codes and manufacturer data.

NEC Motor Sizing (Rule-Based)

Based on NEC 430.52. Uses FLC values from standard tables (like 430.250), not the motor nameplate, for sizing conductors and branch-circuit protection.

Formulas (Single Motor):

Time-Delay Fuse (TD):
Size = FLC x 1.75 (175%)

Non-Time-Delay Fuse (NTD):
Size = FLC x 3.00 (300%)

If the 175% TD fuse fails to hold during start-up, NEC permits increasing up to 225% of FLC. The Overload Relay (OLR) provides the actual motor protection.

IEC Motor Sizing (Performance-Based)

Based on IEC 60947-4-1. Uses the motor nameplate current (I_n). Relies on manufacturer curves and coordination tables (Type 1 or Type 2).

Formulas (Starting Point):

'aM' Fuse (Motor Protection):
Range ~ I_n x (1.6 to 2.0)

'gG/gL' Fuse (General Purpose):
Range ~ I_n x (2.0 to 2.5)

You MUST check the fuse's curve to ensure the motor's starting current (I_st, often 6-8x I_n) for its start time (t_start) is in the non-melting zone.

NEC Sizing (Other Loads)

For non-motor loads, the NEC provides simpler, standard factors.

Formulas:

AC Continuous Load:
Size = Current x 1.25

A continuous load is one operating for 3 hours or more (e.g., lighting, heaters) [NEC 210.20(A)].

DC Loads (All Types):
Fuse Size = Current x 1.50

A 150% factor is a common, safe practice for DC circuits for both fuse types.

Practical Checks

Always run these checks:

• Start Ride-Through: Verify on the manufacturer curve (I_st & t_start).
• Fault Level: Fuse breaking capacity >= available short-circuit current.
• Coordination: Prefer Type 2 (contactor remains serviceable) using manufacturer's tested combinations.
• Heat/Ambient: If enclosure is hot, consider a step-up and re-check curves.
• Selectivity: Ensure only the branch device clears motor faults, not an upstream fuse.

NEC Checklist (Single Motor)

1. FLC from Table 430.250.
2. Time-delay fuse size = 175% x FLC -> next standard size.
3. If starting trips: increase (up to 225% x FLC allowed).
4. Verify AIC and coordination with OLR/contactor.

IEC Checklist

1. Use nameplate current (I_n) and I_st, t_start.
2. Prefer aM fuses with OLR; start at 1.6–2.0 x I_n.
3. For gG fuses, start 2.0–2.5 x I_n.
4. Confirm curve ride-through at I_st for t_start.
5. Verify breaking capacity and Type 2 coordination.