How is the fuse rating determined, and how does it relate to circuit load?

Select the fuse rating by considering normal load current, cable current-carrying capacity and the protective device's operating characteristics together. A fuse does not set the current drawn by a load; it disconnects the circuit during an overcurrent. Everyday terminology sometimes groups fuses and circuit breakers together, but these protective device types do not have identical selection rules.

How is load current calculated?

Use power in watts, voltage in volts and current in amperes. For electrical input power, the DC formula is I = P / U; single-phase AC uses I = P / (U × PF); balanced three-phase AC uses I = P / (√3 × U_LL × PF). U_LL is line-to-line voltage and PF is true power factor. Under sinusoidal conditions, PF is approximately cos φ. If apparent power S is known in VA, use I = S / U for single phase or I = S / (√3 × U_LL) for three phase. Check units when working with kVA and kW conversions.

Example: a heater supplied at 230 V

For a 2,000 W heater with a power factor close to 1, I = 2,000 / 230 ≈ 8.7 A. A 2,000 W heater and a 1,500 W heater operating simultaneously on the same circuit draw approximately 15.2 A. Adding another 1,000 W load brings the total to approximately 19.6 A. This exceeds a 16 A rating; a delayed trip does not make the loading acceptable. Account for simultaneous operation, and apply demand factors only when operating conditions and applicable design rules support them.

How must the rating relate to the cable?

General overload protection conditions are I_B ≤ I_n ≤ I_Z and I₂ ≤ 1.45 × I_Z. I_B is design load current, I_n is the protective device's rated current and I_Z is the cable's permissible current under actual installation conditions. I₂ is the conventional operating current for a specified time, not the short-circuit current. For adjustable circuit breakers, consider the overload setting I_r. For fuses, check the second condition separately; comparing the ampere rating with cable capacity alone is insufficient.

For example, with I_B = 18 A and corrected I_Z = 24 A, a 20 A device satisfies the initial current inequality, whereas a 25 A device does not. This does not complete product selection; verify the operating curve and remaining conditions. When selecting cable cross-section, conductor material, insulation, installation method, ambient temperature and cable grouping affect current-carrying capacity. A 2.5 mm² cable does not have one universal permissible current. Also assess voltage drop and the ratings of connections and socket outlets.

Example: a three-phase motor

A motor's nameplate kW usually describes mechanical output power. For a balanced 400 V motor delivering 7.5 kW, assuming PF = 0.82 and efficiency η = 0.90, I ≈ 7,500 / (√3 × 400 × 0.82 × 0.90) ≈ 14.7 A. Base actual selection on nameplate current and manufacturer data. Starting current, starting duration and motor overload protection require separate assessment. For high-power equipment or a motor supplied through a drive, this calculation alone does not determine the fuse: check the drive's input current and specified protective device.

What do B, C and D characteristics change?

For AC miniature circuit breakers covered by IEC/EN 60898-1, typical instantaneous magnetic tripping ranges are B: 3–5 × I_n, C: 5–10 × I_n, D: 10–20 × I_n; verify the selected product's curve. B16 and C16 are both rated 16 A; C16 does not permit a higher continuous load. If a breaker trips when an appliance starts, assess starting and fault conditions before changing its characteristic. A C or D characteristic must remain compatible with minimum fault current and required disconnection time.

Rated current and breaking capacity are different

A marking of 16 A describes rated current, whereas 6 kA describes short-circuit breaking capacity. Breaking capacity must suit the prospective short-circuit current at the installation point. Verify rated voltage and AC/DC suitability too. Backup protection by an upstream device requires a manufacturer-verified combination. The 30 mA marking on a residual current device indicates residual-current sensitivity, not overload capacity. This explains why the difference between an MCB, an RCCB and an RCBO matters.

Should you always add 25% to load current?

No single percentage rule applies to every fuse or circuit breaker. Manufacturers may specify derating for continuous loading, ambient temperature or fuseholders. For example, a 75% loading recommendation for certain UL/CSA/ANCE 248-14 supplementary fuses cannot be applied universally to IEC fuses or miniature circuit breakers. Protective device selection must follow product data and applicable standards. If a fuse repeatedly operates, assess the load, connections and installation before increasing its rating.