Cable and Fuse Size Calculator for 12 V, 24 V and 48 V DC

Enter what you connect and how the cable runs. You instantly get cable size and fuse, calculated as in vehicle engineering: not just by voltage drop, but also by heat and installation.

1What your device needs Voltage drop

35 mm²

95 % of allowed drop

At most 2 % drop (0.214 V) at 206 A: calculated 33.13 mm², rounded up to 35 mm².

2What the cable can take Heat

70 mm² ↑ thicker

35 mm²: 149 % load

At 50 °C, 35 mm² would run at 149 % with 167 A. Only 70 mm² carries that continuously (96 %).

3What the installation leaves Installation

70 mm² ✓ fine

70 mm²: 96 % load

Free, with spacing, devices in the bundle: 1, factor 1. 70 mm² runs at 96 %: fine.

4Which fuse fits Fuse (DC)

MEGA 300 A

In the fuse box at 65 °C it must hold 200 A continuously: MEGA 300 A (holds 205 A there). It protects the cable against short circuit, but not fully against overload: the cable carries 174 A. That is fine if the device limits its own current, like an inverter, charger or DC-DC charger. For distribution or sockets, split the circuit.

Only for DC in vehicles (12, 24, 48 V). Not for 230 V: shore power and inverter output belong in the hands of a qualified electrician.

How to go thinner

calculate
With 24 V instead of 12 V only half the current flows for the same power: 25 mm². calculate
calculate
calculate
Two cables in parallel: 25 mm² each, same length and same terminations so the current splits evenly.
It does not get thinner here without cutting into function or safety.

Lugs, crimps, fuse holders and switches must handle the same current. Most hot cables are really hot connections.

Working, tables and background

Working with your numbers

1 Power with margin: 2,000 W × 1.1 = 2,200 W
1 Design current at the lowest voltage: 2,200 W ÷ 10.70 V = 206 A
1 Cable length: 1 m × 2 = 2 m (positive out, negative back)
1 Allowed voltage drop: 2 % of 10.70 V = 0.214 V
1 A = L × I ÷ (κ × ΔU) = 2 × 206 ÷ (58 × 0.214) = 33.13 mm², rounded up 35 mm²
2 Continuous current for heat and fuse: 2,000 W ÷ 12 V = 167 A. At nominal voltage and without margin, because a cable heats up over minutes and an almost empty battery is not a steady state.
2 Table, 50 °C column: the smallest cross-section carrying 167 A continuously is 70 mm².
3 Factor for Free, with spacing, table row 1: 1. Needed: 167 A ÷ 1 = 167 A in the 50 °C column: 70 mm².
4 Fuse: 167 A + 20 % safety factor = 200 A it must hold at 65 °C. Table: MEGA 300 A holds 205 A there.
= Result: the largest cross-section of steps 1 to 3. 70 mm²

What the result means

Voltage drop with 70 mm²: 0.101 V (0.9 %), at least 10.60 V reach the device.
Heat in the cable at continuous load: 14 W.
Copper: 1.25 kg. The copper core is about 9.4 mm across; cross-section is not diameter.
With 70 mm² the device may be up to 2.1 m from the battery.
Cross-check with the real strand resistance (0.259 mΩ/m at 20 °C) and the current without margin (187 A): 0.097 V (0.9 %). Fine, the margin covers the difference to κ = 58.

What a cable carries continuously (copper, PVC, single, free air)

Cross-section	at 30 °C	at 50 °C	Resistance
0.5 mm²	11 A	7.8 A	37.1 mΩ/m
0.75 mm²	15 A	10.6 A	24.7 mΩ/m
1 mm²	19 A	13.5 A	18.5 mΩ/m
1.5 mm²	24 A	17 A	12.7 mΩ/m
2.5 mm²	32 A	22.7 A	7.6 mΩ/m
4 mm²	42 A	29.8 A	4.71 mΩ/m
6 mm²	54 A	38.3 A	3.14 mΩ/m
10 mm²	73 A	51.8 A	1.82 mΩ/m
16 mm²	98 A	69.6 A	1.16 mΩ/m
25 mm²	129 A	91.6 A	0.743 mΩ/m
35 mm²	158 A	112 A	0.527 mΩ/m
50 mm²	198 A	140 A	0.368 mΩ/m
70 mm²	245 A	174 A	0.259 mΩ/m
95 mm²	292 A	207 A	0.196 mΩ/m
120 mm²	344 A	244 A	0.153 mΩ/m

Factors for installation and bundling

Devices	Bundled or in trunking	Side by side on wall or floor	Free, with spacing	Under the ceiling, touching	Under the ceiling, spaced
1	1	1	1	0.95	0.95
2	0.8	0.85	0.94	0.81	0.85
3	0.7	0.79	0.9	0.72	0.85
4	0.65	0.75	0.9	0.68	0.85
5	0.6	0.73	0.9	0.66	0.85
6	0.57	0.72	0.9	0.64	0.85
7	0.54	0.72	0.9	0.63	0.85
8	0.52	0.71	0.9	0.62	0.85
9	0.5	0.7	0.9	0.61	0.85
10	0.48	0.7	0.9	0.61	0.85
12	0.45	0.7	0.9	0.61	0.85
14	0.43	0.7	0.9	0.61	0.85
16	0.41	0.7	0.9	0.61	0.85
18	0.39	0.7	0.9	0.61	0.85
20	0.38	0.7	0.9	0.61	0.85

Your cable by installation and count

Devices	Bundled or in trunking	Side by side on wall or floor	Free, with spacing	Under the ceiling, touching	Under the ceiling, spaced
1	70	70	70	95	95
2	120	95	95	95	95
3	120	120	95	120	95
4	> 120	120	95	> 120	95
5	> 120	120	95	> 120	95
6	> 120	120	95	> 120	95
7	> 120	120	95	> 120	95
8	> 120	120	95	> 120	95
9	> 120	120	95	> 120	95
10	> 120	120	95	> 120	95
12	> 120	120	95	> 120	95
14	> 120	120	95	> 120	95
16	> 120	120	95	> 120	95
18	> 120	120	95	> 120	95
20	> 120	120	95	> 120	95

Fuses compared

Type	Rating	holds at 65 °C	protects the cable
Blade fuse (ATO)	– A	– A
MIDI	– A	– A
MEGA	300 A	205 A	short circuit only

Why we calculate this way

Why is voltage drop not enough? Current heats every cable. Above a certain current it gets hotter than its insulation can take continuously, however short it is. Short, thick inverter cables slip through the voltage-drop check and fail on heat.

Why 10.7 V instead of 12 V? For voltage drop the worst moment counts: an almost empty battery. There a device draws the most current for the same power, and most 12 V low-voltage cut-offs switch off around 10.7 V. For heat and fuse, the continuous current at nominal voltage counts instead.

Why a 10 % margin? No device is lossless. An inverter with 1,000 W output draws about 1,050 to 1,100 W from the battery. The margin covers efficiency and disturbances.

Why 2 or 4 %? Chargers, controllers, inverters and electronics need an accurate voltage or they cut out early or charge wrongly: 2 %. Lights, pumps and fans tolerate 4 %.

Why double length? Current flows out and back. If the device is 3 m from the battery, that is 6 m of cable. Only with the negative via the chassis does the positive cable count alone.

What is 58? The conductivity of copper in m/(Ω·mm²). Fine-stranded cable has slightly higher resistance; the cross-check shows the margin covers it.

Why 50 °C? A vehicle stands in the summer sun; under the roof, floor and panels it gets hot. At 50 °C a PVC-insulated cable carries only 71 % of what it carries at 30 °C. For cables with a higher temperature class the values are on the safe side.

What does installation mean? A cable sheds heat through its surface. Free with spacing that works well; in a bundle, corrugated tube or closed trunking, cables heat each other. So a cable may carry less current there. Engineers call this grouping.

What counts? Every device whose cables run in the same bundle and draw current at the same time. Positive and negative of one device count as one, so does a multi-core cable. Eight devices in a bundle: each cable may carry only about half (factor 0.52).

What the fuse is for: it protects the cable, not the device. It must hold the continuous current in a hot fuse box, and it should not be larger than what the cable carries, otherwise it only protects against short circuits. Fuses also weaken in heat: a MEGA 300 A holds only 205 A at 65 °C.

DC only: everything here applies to 12, 24 and 48 V circuits in vehicles. 230 V follows other standards, other protective devices (circuit breakers, RCDs) and other tables.

Cable and fuse for typical loads

Calculated for 12 V, negative as a cable, cable alone and free, 50 °C. Columns: distance to battery. In a bundle it gets more, the calculator above shows how much.

Load	Current	0.5 m	1 m	2 m	3 m	5 m	Fuse
LED light	0.83 A	0.5 mm²	0.5 mm²	0.5 mm²	0.5 mm²	0.5 mm²	Blade fuse (ATO) 1 A
tigerexped water pump small (11 l/min)	8.00 A	0.75 mm²	1 mm²	2.5 mm²	2.5 mm²	4 mm²	Blade fuse (ATO) 15 A
tigerexped water pump large (18 l/min)	15.0 A	1.5 mm²	1.5 mm²	4 mm²	6 mm²	10 mm²	Blade fuse (ATO) 25 A
Roof fan	3.33 A	0.5 mm²	0.5 mm²	0.75 mm²	1 mm²	2.5 mm²	Blade fuse (ATO) 5 A
USB-C/USB-A socket TEX4205-EVO	3.75 A	0.5 mm²	0.75 mm²	1.5 mm²	2.5 mm²	4 mm²	Blade fuse (ATO) 7.5 A
convertiger 100 (USB-C 100 W)	8.33 A	0.75 mm²	1 mm²	2.5 mm²	2.5 mm²	6 mm²	Blade fuse (ATO) 15 A
Compressor fridge	5.00 A	0.5 mm²	1 mm²	2.5 mm²	4 mm²	6 mm²	Blade fuse (ATO) 7.5 A
DC-DC charger 30 A	41.7 A	10 mm²	10 mm²	25 mm²	25 mm²	50 mm²	MIDI 70 A
Inverter 1000 W	83.3 A	25 mm²	25 mm²	35 mm²	50 mm²	95 mm²	MIDI 125 A
Inverter 2000 W	167 A	70 mm²	70 mm²	70 mm²	120 mm²	> 120 mm²	MEGA 300 A
Inverter 3000 W	250 A	> 120 mm²	> 120 mm²	> 120 mm²	> 120 mm²	> 120 mm²	none fits

Frequently asked questions

What cable size for a 2000 W inverter on 12 V?

Run free and close to the battery (up to about 2.1 m): 70 mm² with a 300 A MEGA fuse. In trunking with other devices it quickly becomes 120 mm². At 1 m a voltage-drop-only calculator says 35 mm²; that carries only 112 A continuously at 50 °C, while 167 A are needed.

Is calculating the voltage drop enough?

No. Voltage drop only tells you whether enough voltage reaches the device. Whether the cable can carry the current continuously without overheating is a separate limit: its current rating. For short, high-current cables it almost always decides, and it drops with heat and bundling.

Do I count positive and negative in the cable length?

Yes. Current flows out and back, the formula needs the whole path. If the device is 3 m from the battery, calculate with 6 m. Only with the negative via the chassis does the positive cable count alone. Our calculator asks for the one-way distance and doubles it.

Which fuse goes with my cable?

The fuse protects the cable, not the device. It must hold the continuous current plus a 20 % safety factor even in a hot fuse box (65 °C) and should not be larger than what the cable carries in its installation. The calculator picks the matching blade (ATO), MIDI or MEGA fuse.

Why is the fuse so much larger than the device current?

Because fuses weaken in heat. A MEGA 300 A holds only 205 A continuously at 65 °C. Choosing by the printed rating leads to nuisance blowing in summer.

Does the calculator apply to 230 V?

No. It applies to DC (12, 24, 48 V) in vehicles. 230 V AC, i.e. shore power and the inverter output, follows building wiring rules with circuit breakers and RCDs and belongs in the hands of a qualified electrician.

How much voltage drop is allowed?

For sensitive devices such as chargers, solar controllers without sense wire and inverters at most 2 %, for lights and pumps up to 4 %. We take the percentage of the lowest voltage, for 12 V that is 10.7 V: 2 % is 0.214 V.

Why do you calculate with 50 °C?

In summer a vehicle heats up a lot, especially under the roof, floor and panels. At 50 °C a PVC-insulated cable carries only 71 % of its 30 °C rating: 1.5 mm² then manages 17 A instead of 24 A.

What does installation method mean?

How the cable runs: free with spacing, side by side on a surface, or bundled in a tube or trunking. The worse it can shed heat, the less current it may carry. Eight devices in a bundle: each cable may carry only 52 %.

How do I count the devices in a bundle?

Count every device whose cables run in the same bundle and draw current at the same time, yours included. Positive and negative of one device count as one, so does a multi-core cable.

Which is better, 12 V or 24 V?

For the same power, 24 V carries only half the current. A 2000 W inverter needs only 25 mm² at 24 V instead of 70 mm². For large inverters 24 V is often the key to manageable cables.

May I round down to the next smaller size?

No. Always round up to the next size you can buy. A result of 1.08 mm² means 1.5 mm².

Is CCA cable just as good?

No. CCA is copper-clad aluminium and conducts much worse than copper. Our calculation only applies to copper stranded cable, look for FLRY or FLY.

Why does my cable get hot although the size is right?

Usually it is not the cable but a connection: a badly crimped lug, an undersized fuse holder or a loose screw. Every contact point must handle the same current as the cable.