Wire Gauge Calculator: Converting AWG to mm² for Camper Builds

Two weeks ago I helped a friend rewire his Sprinter after a “simple” fridge upgrade melted the crimp sleeves. He had followed a forum chart meant for house wiring, not 12V vans, and his 12 AWG run lost almost 14% of the voltage before it even reached the compressor. Once we plugged the real numbers into VoltPlan and upsized to 6 mm² cable, the fridge pulled exactly what the label promised and the wires stayed cool to the touch.

Bottom line: Always calculate amps, round-trip distance, and acceptable voltage drop first, then choose the smallest gauge that meets those targets in both AWG and mm² so every fuse, breaker, and terminal in your build agrees.

A clean electrical cabinet reminds you what you’re aiming for before cutting copper.

Why Hobbyists Need a Repeatable Method

Camper and boat projects rarely follow the neat diagrams you see in catalogs. You snake wires behind cabinets, stretch them under floorboards, and splice in upgrades months later. That means:

• Voltage drop matters more than perfect laboratory specs. Even a 0.5V loss can shut down a LiFePO4 inverter or dim LED strips.
• Components often mix standards. The solar controller spec sheet lists terminals in mm², your wire spool is in AWG, and European combiner blocks only accept ferrules sized in square millimeters.
• Safety agencies keep updating their guidance for small craft. Instead of memorizing a different rule each year, you need a quick process that works for every circuit.

If you can multiply, divide, and measure distance with a tape, you can size wire like a pro, no electrical engineering degree required.

Step 1: Capture the Real Load

Start by listing every device on the circuit. Grab manuals, look at stickers, or use a clamp meter if the equipment already runs.

1. Continuous current: Anything that runs longer than 10 minutes stays at its printed amp draw. Multiply by 1.0.
2. Intermittent current: Motors, pumps, and compressors spike when they start. Multiply by 1.25 to build in breathing room.
3. Future headroom: If you know a second fridge or fan is coming, add its amps now. Wire is cheaper than tearing cabinets apart later.

Example: A DC fridge at 5.0A continuous and a pair of LED strip zones at 1.5A each → 5.0 + 1.5 + 1.5 = 8.0A load. Add a 25% surge factor if the fridge compressor is grumpy on hot days → 8.0 × 1.25 = 10A target current.

For more context on estimating appliance loads, revisit 12V Electrical System Basics where we break down common draws in campers.

Step 2: Measure the Run Length

Voltage drop is based on the round-trip distance (out and back). Pull a string along the exact path you plan to route the wire, measure it in feet or meters, then double it.

• Vertical detours, drip loops, and service slack count.
• For DC harnesses bundled together, use the longest conductor in the bundle as your reference.
• Write both measurements: feet for AWG charts, meters for mm² conversions.

Example: Battery to inverter path snakes 5 ft up the cabinet, 3 ft forward, and 4 ft down to the inverter. Total one-way = 12 ft. Round trip = 24 ft (7.3 m).

Step 3: Set Your Voltage-Drop Budget

Hobbyist builds work best with simple rules:

• Critical loads (inverters, DC-DC chargers, alternator feeds): ≤3% drop.
• General circuits (lights, USB, fans): ≤10% drop.
• Electronics sensitive to brownout: Stay closer to 2-3% so they never false-trip.

On a 12.8V LiFePO4 system, 3% equals roughly 0.38V. That tiny number keeps chargers from shutting off early when they expect at least 12.4V under load.

Need more detail? Our Fuse Sizing and Placement guide explains how drop targets tie directly into fuse choices and heat.

Step 4: Do the Math (It’s Easier Than It Looks)

Use the classic voltage-drop formula for copper wire:

Wire area (circular mils) = (K × I × L) / Vd

• K = 10.75 (resistivity constant for copper at ~20°C)
• I = amps from Step 1
• L = round-trip length in feet
• Vd = allowed voltage drop in volts

Example Calculation

• Current I = 50A
• Round trip L = 24 ft
• Voltage drop Vd = 0.384V

Wire area = (10.75 × 50 × 24) / 0.384 ≈ 33,656 circular mils

Look up the next-largest wire that meets or exceeds that area: 2 AWG (33.6 mm²). Never round down—always step up.

Quick Reference Table

AWG	mm²	Circular mils	Typical continuous amps*
14	2.08	4,107	15A (3% drop @15 ft)
12	3.31	6,530	20A
10	5.26	10,380	30A
8	8.37	16,510	45A
6	13.3	26,250	65A
4	21.1	41,740	95A
2	33.6	66,360	130A
1/0	53.5	105,600	170A
2/0	67.4	133,100	195A

*Based on ≤3% drop at ~15 ft round trip in a 12V system. Longer runs or higher loads require upsizing.

Conversion Hacks

• Multiply mm² by 1,973.5 to get circular mils.
• Multiply AWG circular mils by 0.0005067 to get mm².
• Memorize three anchors: 6 mm² ≈ 10 AWG, 16 mm² ≈ 6 AWG, 35 mm² ≈ 2 AWG.

Step 5: Validate Against Protection Devices

Wire isn’t sized in isolation. After you pick a gauge, confirm:

• Fuse/breaker rating is below the wire’s ampacity but above the expected load. Example: 2 AWG typically pairs with 125A ANL fuses for inverters.
• Terminal blocks or busbars accept the conductor size. European DIN blocks might top out at 25 mm² even though the wire math points to 35 mm².
• Crimp tools and lugs match the chosen gauge. Mixing AWG lugs on mm² cable leads to loose crimps and heat.

Step 6: Document Both Units in VoltPlan

Inside VoltPlan, label each conductor with AWG / mm² / amps / length. Example: 2 AWG (33.6 mm²) • 50A • 24 ft RT • 3% drop. That note travels with your BOM, so whoever helps you later sees the reasoning immediately.

Real-World Scenarios

Solar Array Combiner

• 3 × 200W panels wired in series → ~9A @ 60V.
• Roof to charge controller = 18 ft one way (36 ft round trip).
• Voltage drop target: 2% (because MPPT controllers love stable voltage).
• Result: 12 AWG (3.31 mm²) barely makes it; bump to 10 AWG (5.26 mm²) for cooler roofs.

Dual DC-DC Chargers

• Two 60A chargers mounted near the alternator feeding the house bank 8 ft away.
• Combined load = 120A, round trip ≈ 16 ft.
• Drop target: 3% → 0.384V.
• Formula outputs ≈ 44,900 circular mils → 4 AWG (21.1 mm²). If you plan to add a third charger, jump directly to 1/0.

Galley Circuits

• Water pump 7A intermittent + LED lighting 3A continuous.
• One-way run = 10 ft, drop target = 10% (0.128V).
• Required area ≈ 5,879 circular mils → 12 AWG (3.31 mm²). That also gives headroom for a future UV sterilizer.

Common Mistakes (and How to Dodge Them)

1. Using automotive charts: Many car-audio tables assume short runs and airflow under a hood. Camper walls don’t provide that cooling.
2. Ignoring return path: People measure only the positive leg. Always double the distance for DC circuits.
3. Mixing copper-clad aluminum (CCA): Cheap spools claim to be AWG but have higher resistance. Stick to pure copper marine-grade wire.
4. Skipping ferrules: When you insert stranded mm² cable into screw terminals, ferrules prevent “cold flow” and loose connections.
5. Relying on “bigger is better”: Oversize wire without updating fuses and you lose the protective weak link.

Tools & Resources for Hobbyists

• VoltPlan Wire Calculator: Enter amps, length, and drop target. It outputs AWG, mm², and recommended fuse size.
• Clamp meter: Measure actual draw once the circuit runs. Klein and UNI-T make reliable hobby-level meters.
• Measuring tape or fish tape: Mark key distances before walls close up.
• Heat-shrink labels: Print AWG/mm² on each end so future you remembers what’s hiding behind panels.

Keeping Up with Trends (Without Getting Buried in Standards)

You don’t need to memorize marine standards, but keeping an ear on the industry helps:

• 2025 van expos showcased dual alternator kits pushing 180A into house banks. Expect more builds that require 1/0 or 2/0 cable even for short chassis runs.
• LiFePO4 vendors now publish recommended wire sizes in mm² only. Converting early prevents mismatched lugs when your battery arrives.
• Solar manufacturers continue shrinking junction boxes, so MC4 pigtails often max out at 10 AWG. Plan on step-up junctions so you can transition to thicker down-leads without stressing the connectors.

Ready to stop guessing wire sizes? Open your layout in VoltPlan, plug in the actual run lengths, and let the wire calculator confirm the perfect AWG/mm² combo before you buy a single meter of copper. Your fridge, inverter, and future self will thank you.