Electrical

Fuse Size Calculator

Find the right fuse rating from load amps or watts, plus the fuse type and the smallest wire it protects.

Formula Ireq = k × I, then choose the next standard fuse rating ≥ Ireq (k = 1.25 continuous, 1.0 intermittent)Reviewed Sep 19, 2026

A fuse is there to protect the wire, not the device. It has to be big enough to carry the normal load without blowing, and small enough that the wire cannot overheat before it opens. This fuse size calculator takes your load in amps or watts, applies the standard 125% headroom for continuous loads, rounds up to a real fuse rating, and then checks the answer against wire gauge so you know both the fuse to buy and the wire it needs.

Calculation Bench

Load Input Mode

01Load

I · Nameplate or measured current

02Circuit Voltage

V · Optional; adds a fuse voltage-rating check

Load Type

Wire Rating Standard

Your Wire (optional)

Solution

Enter the load to get a fuse rating, fuse type and minimum wire gauge.

Ireq = k × I, then choose the next standard fuse rating ≥ Ireq (k = 1.25 continuous, 1.0 intermittent)

Formula Sheet

I_{req}=k\,I,\quad k=1.25\ \text{(continuous)}

I=\dfrac{P}{V\times PF}

FFuse Rating
I_reqRequired Current
ILoad Current
kHeadroom Factor
PLoad Power
VVoltage
PFPower Factor

Variables & Units

Symbol	Variable	Description	Common Units
F	Fuse Rating	The selected standard fuse rating.	A
I_req	Required Current	Load current after the headroom factor.	A
I	Load Current	Normal operating current of the circuit.	A
k	Headroom Factor	1.25 for continuous loads (3 hours or more), 1.0 for intermittent loads.
P	Load Power	Power draw in watts, used when the current is unknown.	W, kW
V	Voltage	Circuit voltage used to convert watts to amps.	V
PF	Power Factor	Ratio of real to apparent power; 1 for DC and resistive loads.

How to Use This Calculator

01Choose whether you know the load in amps or in watts.
02Enter the load. If you only know a device's wattage, enter the watts and the circuit voltage (12 V, 24 V, 120 V, and so on).
03Pick Continuous if the load runs for 3 hours or more (fridges, lighting, chargers, pumps). Pick Intermittent for short-duty loads.
04Choose a wire rating standard: NEC 60 °C for ordinary building wire, or ABYC 105 °C for marine and vehicle wire that is rated for 105 °C. If you are unsure of the insulation rating, keep NEC 60 °C, which is the more conservative choice.
05Optionally pick the wire gauge you already have to see whether the fuse actually protects it. Then read the fuse rating, the common fuse type for that size, and the smallest wire gauge in the table that the fuse can protect.

How the Formula Works

First the load is turned into current. If you enter amps, that value is used directly. If you enter watts, the calculator uses I = P / (V × PF); for DC circuits and heating loads PF is 1.

Then headroom is added. Continuous loads are multiplied by 1.25, so a 16 A continuous load needs at least 20 A of fuse rating. This is the same 80% rule used for breakers: the load should not sit above 80% of the fuse rating for hours at a time. Intermittent loads use a factor of 1.

Fuses come only in standard ratings, so the required current is rounded up to the next one, never down. 6.25 A becomes a 7.5 A fuse and 34.5 A becomes a 35 A fuse.

Last, the fuse is checked against the wire. A fuse must not be larger than the wire's ampacity, or the wire can overheat without the fuse ever opening. The calculator finds the smallest wire in the copper ampacity table you selected (NEC 60 °C or ABYC 105 °C) whose rating is at least the fuse rating, and compares it with your wire if you entered one. The two tables differ a lot: 12 AWG is rated 20 A in the NEC 60 °C column but 45 A in the ABYC 105 °C table, because ABYC assumes 105 °C insulation in free air.

Worked Example 01

12 V fridge, 60 W continuous

Known

Load: 60 W
Voltage: 12 V
Load type: Continuous

Formula

I = P / (V × PF)

Substitution

I = 60 / 12 = 5 A; Ireq = 1.25 × 5 = 6.25 A

Result

7.5 A fuse

The 5 A load becomes 6.25 A after headroom. The next standard rating above that is 7.5 A, a blade fuse. It is protected by 14 AWG, the smallest wire in the table.

Worked Example 02

16 A continuous load

Known

Load: 16 A
Load type: Continuous

Formula

Ireq = k × I, then choose the next standard fuse rating ≥ Ireq (k = 1.25 continuous, 1.0 intermittent)

Substitution

Ireq = 1.25 × 16 = 20 A

Result

20 A fuse, 12 AWG minimum

16 A is exactly 80% of 20 A, so a 20 A fuse just qualifies. A 20 A fuse needs wire rated for at least 20 A, which is 12 AWG in the NEC 60 °C column.

Worked Example 03

30 A continuous load on 12 AWG wire

Known

Load: 30 A
Load type: Continuous
Wire: 12 AWG

Formula

Ireq = k × I, then choose the next standard fuse rating ≥ Ireq (k = 1.25 continuous, 1.0 intermittent)

Substitution

Ireq = 1.25 × 30 = 37.5 A → 40 A fuse; 12 AWG ampacity = 20 A

Result

40 A fuse; 12 AWG is not protected

The load needs a 40 A fuse, but 12 AWG can only carry 20 A. The wire has to be upsized (8 AWG in this table) before that fuse is safe to use. Shrinking the fuse to protect the wire would make it blow under normal load.

Applications

01Choosing a fuse for a 12 V or 24 V accessory in a van, RV, boat, or off-grid solar system
02Finding the main fuse next to a battery or inverter
03Checking whether an existing wire is protected by the fuse in the holder
04Sizing a fuse for a 120 V or 240 V resistive load from its wattage

Typical Loads at 12 V (Use as a Starting Point)

Load	Typical draw	Approx. current	Fuse (continuous)
LED lighting circuit	30 W	2.5 A	5 A
12 V compressor fridge	60 W	5 A	7.5 A
Water pump	100 W	8.3 A	15 A
Roof vent fan	50 W	4.2 A	7.5 A
Diesel heater	120 W	10 A	15 A
300 W inverter (at rated output)	300 W AC	about 28 A DC	35 A
1000 W inverter (at rated output)	1000 W AC	about 93 A DC	125 A

Copper Wire Ampacity: NEC 60 °C vs ABYC 105 °C (Free Air)

Wire gauge	NEC 60 °C column	ABYC 105 °C, outside engine space
18 AWG	not in table	20 A
16 AWG	not in table	25 A
14 AWG	15 A	35 A
12 AWG	20 A	45 A
10 AWG	30 A	60 A
8 AWG	40 A	80 A
6 AWG	55 A	120 A
4 AWG	70 A	160 A
2 AWG	95 A	210 A
1/0 AWG	125 A	285 A
4/0 AWG	195 A	445 A

Standard Fuse Ratings Used by This Calculator

Range	Ratings	Common family
1-40 A	1, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 35, 40 A	Blade (ATC/ATO, mini)
50-80 A	50, 60, 70, 80 A	MAXI blade or ANL
100-500 A	100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500 A	ANL or Class T
600 A	600 A	Class T or engineered

Assumptions

01Continuous loads use a 125% factor; intermittent loads use 100%.
02Rounds up through a list of common ratings from 1 A to 600 A. Not every fuse family makes every rating; check the family you plan to buy.
03NEC 60 °C option: copper ampacity from the NEC 310.16 60 °C column with the 15/20/30 A limits for 14/12/10 AWG. It is the conservative default and suits ordinary building wire.
04ABYC 105 °C option: ampacity from ABYC E-11 Table 6A for single conductors, not bundled, sheathed or in conduit, with 105 °C insulation, outside engine spaces at 30 °C ambient. Use it only for wire that is actually rated for 105 °C, such as marine-grade tinned copper.
05The NEC table starts at 14 AWG and the ABYC table at 18 AWG. Smaller wire is common on low-current DC circuits but is not covered, so a very small fuse still shows the first row of the table as the smallest wire.
06The wire check requires the fuse rating to be no larger than the wire ampacity. It does not use the NEC 240.4(B) allowance to round up to the next standard fuse size.
07Watts mode assumes the load is at the circuit voltage you enter and, for AC, that the power factor is accurate.

Where This Model Stops

01Does not handle motor, compressor, transformer, or capacitor inrush. These loads need time-delay fuses and code-specific sizing.
02The wire ampacity values assume free air. Bundling lowers them (ABYC applies a 0.7 factor to DC cables bundled together), and so do hot locations: ABYC rates 105 °C wire about 15% lower inside engine spaces. Long runs also need a voltage-drop check.
03Does not check voltage drop, fuse interrupt rating (AIC), or DC voltage rating. Battery banks, especially lithium, can deliver very high fault current and often need Class T fuses.
04Does not replace local electrical code, equipment listing instructions, or a licensed electrician for permanent installations.

References

[1]
ABYC E-11 Table 6A allowable amperage of single conductors
American Boat and Yacht Council (table reproduced by BoatHowTo with ABYC permission)
Source of the 105 °C, outside-engine-space ampacity column used for the ABYC wire option.
[2]
NEC 240.6(A) standard ampere ratings
National Electrical Code (NFPA 70)
Standard ratings for fuses and breakers that the required current is rounded up to.
[3]
NEC 310.16 allowable ampacities and 240.4(D) small-conductor limits
National Electrical Code (NFPA 70)
Source of the copper 60 °C ampacity column used for the wire check.
[4]
NEC 210.20(A) continuous loads
National Electrical Code (NFPA 70)
Basis for the 125% headroom applied to loads that run 3 hours or more.

Frequently Asked Questions

How do I calculate fuse size?

Find the load current (amps = watts ÷ volts), multiply by 1.25 if the load runs 3 hours or more, then round up to the next standard fuse rating. Finally check that your wire can carry at least that fuse rating.

Can I use a bigger fuse than the calculator recommends?

Only if the wire can carry it. A fuse protects the wire, so it must never exceed the wire's ampacity. A bigger fuse than needed also opens more slowly on an overload and can leave the wire hot.

What size fuse for a 12 V device that uses 100 W?

100 W ÷ 12 V is about 8.3 A. As a continuous load that becomes 10.4 A, so a 15 A fuse is the next standard size. Enter 100 W and 12 V in the calculator to confirm.

How is this different from the Circuit Breaker Size Calculator?

Both use the same 125% continuous-load idea. This page is built for fuses: it rounds through fuse ratings, suggests a fuse family (blade, MAXI, ANL, Class T), shows the minimum wire gauge and checks your wire. The Circuit Breaker Size Calculator adds non-continuous and continuous loads together and rounds through breaker ratings.

What fuse size for a 12 V inverter?

Use the inverter's DC input current, not its AC watts. At about 90% efficiency a 1000 W inverter draws roughly 93 A from a 12 V battery, so enter 1000 W ÷ 0.9 = 1111 W at 12 V. That gives a 125 A fuse. Use the manufacturer's recommended fuse if it is larger, and use a Class T fuse on lithium banks.

Do motors need a different fuse size?

Yes. Motors and compressors draw several times their running current for a moment at start-up, so they need time-delay fuses sized by motor code rules. Use the nameplate and the code rules for those loads instead of this calculator.

Should I choose the NEC or the ABYC wire option?

Choose ABYC 105 °C only if your wire is marked for 105 °C, as marine-grade tinned copper usually is, and it runs in free air outside an engine space. Ordinary automotive primary wire is often rated only 80 or 90 °C, and household-style wire is rated 60 to 90 °C. For those, or if you are unsure, use the NEC 60 °C option, which gives a larger wire and a safer answer.

Which fuse type should I buy?

As a rule of thumb, blade fuses cover small circuits up to 40 A, MAXI fuses cover roughly 20 to 80 A, and ANL or Class T fuses cover larger battery and inverter circuits. Whatever type you choose, its voltage rating must be at or above your system voltage.

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