12V Wiring Guide for Camper Vans: Gauge, Drop, Fuses

In a 12V camper van system, size wire for voltage drop first and ampacity second — voltage drop is almost always the binding constraint. Use Vdrop = 2 × I × R × L ÷ 1000, where I is amps, R is conductor resistance in ohms per 1,000 feet, and L is the one-way run in feet. A 3% target on a 12.0 V nominal system leaves you just 0.36 V of headroom, which is why a 20 A circuit on 10 AWG runs out of budget at 7.2 feet one-way, and why van builders end up using cable that looks absurdly thick next to household wiring.

Why 12V systems demand such thick wire

Power is volts × amps, so cutting voltage by a factor of 10 multiplies current by 10 for the same load. A 100 W device draws 8.33 A at 12 V (100 ÷ 12) but only 0.83 A at 120 V (100 ÷ 120). Wire heating and voltage loss both scale with current, so the 12V version of the same appliance needs roughly ten times the copper.

The second squeeze is the percentage budget. Three percent of 120 V is 3.6 V — a generous allowance. Three percent of 12 V is 0.36 V. You are fighting ten times the current with one tenth the allowable loss, a 100× penalty in effective run length for the same conductor. That is the entire reason van electrical looks the way it does.

The consequences are real, not theoretical. Drop 1 V on the feed to a compressor fridge and its low-voltage cutoff may trip while the battery still reads 12.4 V at the terminals. Drop 0.8 V on a solar controller-to-battery run and the controller reads a falsely high battery voltage, ends absorption early, and chronically undercharges the bank.

The voltage drop formula and where the numbers come from

The DC voltage drop formula is:

Vdrop = 2 × I × R × L ÷ 1000

The 2 accounts for the round trip — current flows out on the positive conductor and back on the negative, so a 15-foot one-way run is 30 feet of copper. Skipping that factor of 2 is the single most common DIY error and it makes every wire look half as lossy as it is.

For R, use NEC Chapter 9, Table 8 (Conductor Properties), the same table electricians use. For stranded uncoated copper at 75°C, the DC resistance in ohms per 1,000 feet is:

• 14 AWG — 3.14
• 12 AWG — 1.98
• 10 AWG — 1.24
• 8 AWG — 0.778
• 6 AWG — 0.491
• 4 AWG — 0.308
• 2 AWG — 0.194
• 1/0 AWG — 0.122
• 2/0 AWG — 0.0967
• 4/0 AWG — 0.0608

Two caveats. Tinned marine cable is coated copper and runs slightly higher resistance than these uncoated values — typically 2 to 5% more, which is a reasonable safety margin to absorb. And resistance climbs with temperature, so a cable baking against a hot van floor performs worse than the 75°C table figure. If you want to skip the arithmetic, run your numbers through the voltage drop calculator and check the answer against the table below.

12V wire gauge table: maximum one-way run at 3% drop

Every cell below is computed as L = 0.36 × 1000 ÷ (2 × I × R), using the NEC Table 8 resistances above. Assumptions: 12.0 V nominal system, 3% drop limit (0.36 V), stranded uncoated copper at 75°C, lengths are one-way (the formula already doubles them), and figures are rounded down to one decimal.

Gauge	10 A	20 A	30 A	50 A
12 AWG	9.0 ft	4.5 ft	3.0 ft	1.8 ft
10 AWG	14.5 ft	7.2 ft	4.8 ft	2.9 ft
8 AWG	23.1 ft	11.5 ft	7.7 ft	4.6 ft
6 AWG	36.6 ft	18.3 ft	12.2 ft	7.3 ft
4 AWG	58.4 ft	29.2 ft	19.4 ft	11.6 ft
2 AWG	92.7 ft	46.3 ft	30.9 ft	18.5 ft

Read the table as a length budget, not a permission slip. The 12 AWG and 10 AWG entries in the 50 A column are arithmetically correct but electrically wrong — those conductors should never carry 50 A continuously regardless of how short the run is. Voltage drop and ampacity are two separate tests and a conductor must pass both.

For non-critical loads such as interior LED lighting, ABYC E-11 permits up to a 10% drop, which multiplies every number in the table by 3.33. The 3% limit applies to panel feeders, electronics, navigation lights, and anything where undervoltage causes misbehavior. Charging circuits belong in the 3% category too.

Worked example

A 6 A fridge circuit, 15 feet one-way, on 12 AWG: Vdrop = 2 × 6 × 1.98 × 15 ÷ 1000 = 0.356 V, or 2.97% of 12 V. It just squeaks under 3%. Run the same circuit on 14 AWG: 2 × 6 × 3.14 × 15 ÷ 1000 = 0.565 V, or 4.71% — a 59% increase in loss from one gauge step. Change any variable and recheck with the voltage drop calculator rather than eyeballing it.

Fuse sizing: the fuse protects the wire, not the device

This is the concept most DIY builders get backwards. A fuse exists to stop the conductor from becoming a heating element during a short or overload. Your fridge has its own internal protection. The 30-foot loop of copper running through your wall cavity does not.

The rule: the fuse rating must not exceed the ampacity of the smallest conductor it protects, and it must exceed the circuit's continuous load with margin. Common practice is to size the fuse at roughly 125% of continuous load, then confirm that number sits at or below conductor ampacity. If it does not, go up a wire gauge — never up a fuse size.

Ampacity depends on insulation temperature rating, whether the conductor is in an engine space, and how many conductors are bundled. Marine 105°C boat cable outside an engine space is rated 60 A for 10 AWG and 120 A for 6 AWG per ABYC Table VI-A — far higher than building-wire figures, because a single conductor in free air sheds heat well. By contrast, NEC 240.4(D) caps overcurrent protection for building wiring at 15 A for 14 AWG, 20 A for 12 AWG, and 30 A for 10 AWG. Bundle your van conductors in loom and the marine numbers derate sharply, so the conservative move is to size closer to the NEC figures unless you have looked up the correct ABYC table and correction factor.

Placement matters as much as rating. ABYC E-11 requires overcurrent protection within 7 inches of the battery, extended to 40 inches if the conductor is sheathed, and no more than 72 inches for a sheathed conductor connected directly to a battery terminal — and even then it must be "as close as practicable." Unfused cable between the battery and its fuse is unprotected by definition.

Real van loads and their actual amp draws

Load	Running draw at 12V	Note
12V compressor fridge	2–6 A while compressor runs	Dometic CRX 110 startup surge ~6.2 A; duty cycle typically 30–50%
MaxxAir MaxxFan Deluxe 7500K	0.2 A (speed 1) to 3.7 A (speed 10)	Manufacturer per-speed figures; 1.2 A at speed 6
Shurflo 4008 water pump	up to 7.5 A max	3.0 GPM, 55 PSI shutoff; intermittent duty
Laptop charging via 12V USB-C PD	6–9 A	65–100 W output plus conversion loss
LED interior lighting	0.2–0.5 A per fixture	10 fixtures rarely exceed 4 A total
2000 W inverter at full output	~196 A	2000 ÷ 0.85 efficiency ÷ 12 V; surge can briefly double

Notice the spread. Everything on the list except the inverter fits comfortably on 12 or 10 AWG at typical van run lengths. The inverter is a different animal entirely.

Battery-to-inverter cable: short, fat, and fused at the battery

At roughly 196 A continuous, a 2000 W inverter moves more current than every other circuit in the van combined. Three rules apply.

Keep it short. At 200 A on 2/0 AWG over a 5-foot one-way run: 2 × 200 × 0.0967 × 5 ÷ 1000 = 0.193 V, or 1.61%. Stretch that same 200 A run onto 4 AWG and you get 2 × 200 × 0.308 × 5 ÷ 1000 = 0.616 V, or 5.13% — over budget on a five-foot run.

Go fat. 2/0 AWG with a 250 A fuse is the common configuration for a 2000 W 12V inverter, applying roughly a 125% margin over the ~196 A continuous draw. Use a Class T or MRBF fuse — ordinary blade and ANL fuses lack the interrupt rating for a lithium bank capable of thousands of amps into a dead short.

Fuse at the battery. The fuse protects the cable, so it goes at the source end. Also torque lugs to spec and use proper crimps; a loose 200 A connection dissipates serious heat at the terminal long before the fuse notices anything.

Why 24V and 48V are winning in bigger builds

Run the numbers on a 3000 W inverter: 250 A at 12 V, 125 A at 24 V, 62.5 A at 48 V. Because allowable run length scales with the square of system voltage, moving from 12 V to 24 V buys 4× the distance on the same conductor, and 12 V to 48 V buys 16×. That translates directly into thinner, cheaper, lighter cable and smaller lugs.

The common rule of thumb is that 48 V starts making sense above roughly 800–1,000 W of solar or a 3,000 W combined discharge rate. Below that, 12 V wins on simplicity: nearly every van appliance, fan, pump, and light is natively 12 V. The 2026 hybrid pattern in larger builds is a 48 V main bank with a 48 V to 12 V DC-DC converter feeding 12 V accessory circuits — big-cable savings where the current is, native compatibility where the devices are. If the deciding factor is whether a larger solar array pays for itself, the solar payback period calculator will tell you where that break-even lands.

Safety and when to hire a pro

A 200 Ah LiFePO4 battery can deliver several thousand amps into a dead short. That is enough to vaporize a wrench, ignite insulation, and start a fire inside a sealed metal box you sleep in. Treat the DC side with more respect than household AC, not less, because there is no breaker panel upstream to save you.

Non-negotiables: fuse every positive conductor at its source, use stranded fine-strand cable rated for vibration (never solid building wire), protect every pass-through with a grommet, support cable every 18 inches, and use adhesive-lined heat-shrink at terminations. Verify your finished build by measuring actual voltage at the load under load, not by trusting the spreadsheet.

Call a licensed marine or RV electrician when your build includes shore power or any 120 V AC circuit, when the inverter exceeds 3000 W, when you need a lithium bank integrated with the vehicle alternator, or when you are unsure how to size grounding and bonding. AC wiring in a metal vehicle involves shock-hazard rules that are genuinely different from residential practice, and it is the part of a van build most worth paying someone to get right.