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Enter the current your gear draws and how far it sits from the battery. You get the cable size, the voltage drop and every step of the working.
Enter your numbers to see a result.
Uses REDARC's voltage drop formula for copper at 25°C. Copper areas come from the strand counts of the cable we stock, and ratings are the lower of the Narva and Tycab figures. Please note this is just a guide.
The drop each size gives at your current and run length. Useful if you already have the cable and want to check it.
| Cable | Copper | Voltage drop | Rated to | For this run |
|---|---|---|---|---|
| Enter your numbers above. | ||||
Figures are for copper at 25°C. Resistance rises about 0.4% for every degree above that, so a cable at 45°C drops about 8% more voltage. Ratings are for single core cable. Twin core is rated lower, so check the rating on the reel if you are running twin.
It uses the voltage drop formula REDARC publish for copper cable.
Voltage drop (V) = total cable length (m) × current (A) × 0.017 ÷ copper area (mm²)
Turned around, that gives the copper area your run needs to stay inside the drop you picked. The calculator then takes the smallest cable with at least that much copper that is also rated for your current. On most 12V runs voltage drop sets the size well before the amp rating does.
Auto cable is sold by outside diameter, so 6mm cable has 4.58mm² of copper, not 6mm². The copper areas here are worked out from each cable's strand count. Narva and Tycab also print different ratings for the same copper, so the calculator uses the lower of the two.
For the full chart with both brands' ratings and the lug to suit each size, see our B&S cable size chart. Wiring in a charger? Our DC-DC charger guide and dual battery install guide cover the rest of the job.
Narva and Tycab automotive cable from 3mm auto up to 000 B&S, plus the lugs to finish the job.
Multiply the total cable length in metres by the current in amps and by 0.017, then divide by the copper area in mm². Total length means the positive and negative cables together. A 20A load at the end of a 5m run in 6 B&S works out as 10 × 20 × 0.017 ÷ 13.5 = 0.25V. The 0.017 figure is for copper at 25°C.
3% is the usual target, which is 0.36V on 12V and 0.72V on 24V. DC-DC charger makers often size their cable tighter than that. REDARC's recommended cable sizes each work out at about a quarter of a volt. Where the manual for your gear gives a cable size, use it.
Yes, if it runs back to the battery. The calculator doubles the run for a full negative cable. If the gear earths to the chassis close by, pick the chassis option and only the positive cable counts. A painted or corroded earth point adds resistance the formula cannot see, so a full negative cable is the safer choice. For DC-DC chargers and inverters, follow the manufacturer's wiring instructions.
The 50A Anderson plug is made for 8 B&S cable, which has 7.72mm² of copper. Whether that is enough depends on the run. A 10A load through 8 B&S stays under 3% drop out to about 8m with a full negative cable. If the calculator says your run needs heavier cable than the plug accepts, move up to a larger connector rather than trimming strands to fit.
Yes. Copper's resistance rises about 0.4% for every degree above 25°C, so the same cable at 45°C drops about 8% more voltage. Current heats the cable as well, so a cable in a hot engine bay or bundled in a loom runs warmer than the air around it. If your run is close to the limit, go up a size.
Voltage drop formula and temperature coefficient from REDARC. Copper areas calculated from the strand counts of the Narva and Tycab cable we stock. Ratings are the lower of the Narva and Tycab figures for single core cable. Conditions vary, so treat the result as a starting point and go up a size if you are unsure. Prices and availability correct at time of publishing.
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