DC Fuse Size Calculator

Related Calculators

Table of contents

• How to use
• Common mistakes
• Definitions
• Methodology
• Limitations & assumptions
• Sources

Load current: The device's normal maximum current in amps before any run-time sizing factor is added.

Load power: The device's power in watts. In watts mode, the calculator divides Load power by DC system voltage to estimate current.

DC system voltage (volts): The operating voltage used with "Load power (watts)" to estimate current in "Watts" mode. It is separate from the maximum voltage used for the fuse comparison.

Smallest wire amp limit: The safe amp limit for the smallest wire in the protected circuit section. The fuse mainly protects this wire from overheating.

Fuse size list: The set of common fuse amp ratings the calculator can round up to.

Fuse DC voltage rating (volts): The DC voltage rating specified for the fuse. It must cover the maximum open-circuit voltage, not merely the nominal system label; other ratings and application conditions still apply.

Maximum circuit voltage (volts): The highest circuit voltage relevant to fuse interruption, from the supply or system specifications. Do not invent it from a nominal battery label. This input is used only for the voltage comparison.

Wire headroom after fuse choice: Smallest wire amp limit minus the selected fuse size. A negative value means the fuse is larger than the entered wire limit.

Mistake: Entering an old fuse value as Load current.
Fix: Use the device's maximum normal Load current, or switch I know the load as to watts and enter Load power.

Mistake: Using the same nominal label for two different voltage tasks.
Fix: In "Watts", use "DC system voltage (volts)" for the operating condition of the entered power. Separately enter "Maximum circuit voltage (volts)" for the fuse comparison; if that maximum is unknown, leave it blank.

Mistake: Choosing Less than 3 hours for How long the load runs when the load can stay on for hours.
Fix: Choose 3 hours or more so Current used for fuse sizing includes the 1.25 factor.

Mistake: Entering the largest wire size in Smallest wire amp limit.
Fix: Use the amp limit for the smallest wire in the protected part of the circuit.

Mistake: Treating "Preliminary listed fuse size" as an approved selection.
Fix: If it exceeds "Smallest wire amp limit (amps)", do not increase the entered limit merely to obtain a favorable result. Check the actual wire and installation ratings and redesign the circuit if needed.

Mistake: Treating a nominal-voltage match as proof that the fuse voltage rating is sufficient.
Fix: Supply both "Fuse DC voltage rating (volts)" and "Maximum circuit voltage (volts)" for a comparison. A matching voltage alone does not check interrupt rating, fuse speed, temperature or other installation requirements.

• This is planning math only. Final fuse choice must follow the equipment manual, fuse data sheet, fuse holder rating, and local electrical rules.
• The fuse size is rounded up from the selected Fuse size list. A different fuse family can have different available sizes.
• The calculator checks amp rating and optional DC voltage rating only. It does not check interrupt rating, fuse speed, temperature derating, wire length, voltage drop, terminal quality, or enclosure heat.
• The Smallest wire amp limit must already include the correct wire gauge, insulation rating, bundle conditions, and installation environment.
• "Entered wire-limit comparison" only compares amp ratings; it does not certify wiring or device protection. Fuses mainly protect wiring from overcurrent.
• If the needed current is above the largest value in the selected Fuse size list, the result should be treated as unsupported rather than rounded to an unsafe size.

How the fuse size is found

The calculator first turns the entered load into Normal load current. In amps mode, it uses Load current directly.

load_current_amps = load_amps

In watts mode, it divides Load power by DC system voltage. DC system voltage must be greater than zero.

load_current_amps = load_watts / system_voltage

This simplified model applies a run-time factor of 1.25 for "3 hours or more" and 1.00 for "Less than 3 hours". Neither factor verifies a particular fuse's current-carrying, temperature or pulse performance.

run_time_factor = 1.25 if run_time_type = continuous, else 1.00

The result is the Current used for fuse sizing.

calculated_current_amps = load_current_amps * run_time_factor

"Preliminary listed fuse size" is the first value in "Fuse size list" at least as large as "Current used for fuse sizing". It is a starting estimate, not a recommendation for a particular fuse model.

recommended_fuse_amps = first selected fuse size where size >= calculated_current_amps

Entered-value comparisons

The wire check compares the rounded fuse size with Smallest wire amp limit. The signed headroom is kept so a failing result shows how far over the wire limit the fuse is.

wire_headroom_amps = wire_amp_limit - recommended_fuse_amps

wire_check passes if recommended_fuse_amps <= wire_amp_limit

When both "Fuse DC voltage rating (volts)" and "Maximum circuit voltage (volts)" are supplied, their difference gives "Fuse voltage rating margin". A negative margin means the rating is below the entered maximum. Missing values leave the voltage requirement unverified; no nominal-voltage substitute is used. Eaton explains that fuse voltage ratings must cover maximum open-circuit voltage and that other selection requirements also apply [3].

voltage_rating_margin_volts = fuse_dc_voltage_rating - maximum_circuit_voltage

Mini example

For a 10 amp load with "3 hours or more" selected, "Normal load current" is 10 amps and "Current used for fuse sizing" is 12.5 amps. "Marine/RV DC common sizes" gives a preliminary 15 amp size. With "Smallest wire amp limit (amps)" set to 20, the headroom is 5 amps. This is only an entered-limit comparison; the voltage requirement is unverified until both voltage-comparison inputs are supplied.