Short answer: size 12V cable for voltage drop, not just for how many amps it can carry. Aim for no more than 2.5% drop, which is 0.3 V on a 12V system, counted over the positive and negative cables together. On 12V, voltage drop usually decides the size well before heat does. And check the product's manual first: Victron's manuals all give the battery cable size (and fuse size) to use.
Why does 12V need such thick cable?
Because low voltage means high current for the same power, and current is what causes trouble in a cable: more drop, more heat.
Victron's Wiring Unlimited book has an eye-opening example. A 2400 W inverter on 12V draws 200 A. With two 1.5 m cables of 16 mm², the drop is 0.32 V on each cable, 0.64 V in total. That's 5% of the energy lost, turned into heat in the cable.
Same inverter, same cables, higher system voltage:
| System | Current | Total voltage drop |
|---|---|---|
| 12 V | 200 A | 0.64 V (5%) |
| 24 V | 100 A | 0.32 V (1.3%) |
| 48 V | 50 A | 0.16 V (0.3%) |
The fixes are thicker cable, shorter cable, or a higher system voltage. For big inverters, Victron suggest 12 V systems stay up to 3000 VA, and that going up in system voltage is the first thing to consider if you want to avoid very thick cables. More in our guide to inverters, VA and watts.
How much voltage drop is too much?
Opinions vary, but Victron advise aiming for no more than 2.5%.
| System | 2.5% drop |
|---|---|
| 12 V | 0.3 V |
| 24 V | 0.6 V |
| 48 V | 1.2 V |
And it isn't only the cable. Every connection adds a little resistance: Victron's examples are 0.06 mΩ per cable connection and 0.35 mΩ for a 150 A fuse. Fuses, shunts, isolators and every lug all add up.
What happens if the drop is too big?
- Lost energy, turned into heat.
- Undercharged batteries. Drop during charging means the battery never gets the full voltage. If a battery never seems to get full, check the cables.
- Hot cables. Victron's warning is worth reading in full: hot cables "can cause melting wiring insulation or cause damage to the cable conduits or to the connected equipment. In extreme cases, cable heating can cause a fire."
Why count the cable length twice?
Because current goes out on the positive and comes back on the negative, and both cables have resistance. Victron's table works on total length: positive plus negative.
The Orion XS manual shows it with a DC-DC charger: a 5 m run counts as 10 m of cable. With 16 mm² cable at about 1.1 mΩ per metre, that's 11 mΩ, and at 50 A the cable loses 550 mV. That's about 4.6% of 12 V, nearly double the 2.5% aim. Long runs to the back of a canopy or a camper trailer are exactly where this bites.
How do I pick a cable size?
Victron's table
The maximum current for each cable size at a drop of 0.259 V (about 2.2% of 12 V), by total cable length. Losses over the contacts aren't included.
| Cable (mm²) | Total length up to 5 m | 10 m | 15 m | 20 m |
|---|---|---|---|---|
| 4 | 12 A | 6 A | 4 A | 3 A |
| 6 | 18 A | 9 A | 6 A | 5 A |
| 10 | 30 A | 15 A | 10 A | 8 A |
| 16 | 48 A | 24 A | 16 A | 12 A |
| 25 | 75 A | 38 A | 25 A | 19 A |
| 35 | 105 A | 53 A | 35 A | 26 A |
| 50 | 150 A | 75 A | 50 A | 38 A |
| 70 | 210 A | 105 A | 70 A | 53 A |
| 95 | 285 A | 143 A | 95 A | 71 A |
| 120 | 360 A | 180 A | 120 A | 90 A |
The quick rule of thumb
For cables up to 5 m: current divided by 3 gives the cross-section in mm². So 200 A needs 200 / 3 = 66 mm². Round up to the next size you can buy.
Victron's Toolkit app will also work out the drop over both cables for you; aim below 2.5%.
A worked example (illustrative: check your charger's manual first)
Say a 50 A DC-DC charger sits 4 m from the starter battery.
- Total cable length: 4 m out + 4 m back = 8 m. Use the "up to 10 m" column.
- 16 mm²: 24 A. Not enough. 25 mm²: 38 A. Still not enough. 35 mm²: 53 A. That's the first size that covers 50 A.
If the charger's manual gives a different size for that length, the manual wins. It knows the product.
Isn't a cable's amp rating enough?
No. An amp rating tells you how much current the cable can carry before it gets too hot. It says nothing about how much voltage you lose over a long run.
As an example, Tycab NZ rate these particular battery cables at 30 °C:
- 8 B&S (7.7 mm², part ABC111203): 74 A
- 4 B&S (20.26 mm², ABC128703): 135 A
- 2/0 B&S (64.15 mm², ABC191003): 292 A
Those ratings drop as it gets hotter: Tycab's factors are 0.9 at 40 °C, 0.76 at 50 °C, 0.6 at 60 °C and 0.45 at 70 °C. So that 4 B&S cable somewhere that reaches 50 °C is good for about 135 x 0.76 = 103 A.
Now put it next to Victron's table. That 8 B&S cable (7.7 mm²) is rated at 74 A for heat, but even a 10 mm² cable only carries 30 A over a 5 m total run before it passes 0.259 V of drop. On 12V, voltage drop is usually the tighter limit.
Enclosed cables run hotter too. At 2.5% drop, 1000 W through a cable means 25 W of heat, and inside conduit that heat may not get away.
B&S, AWG and mm²: why don't the numbers match?
Cable is sold by gauge (B&S or AWG) and by cross-section (mm²), and the conversions differ between makers. Victron's chart gives 4 AWG as 21.1 mm² and 2/0 as 67.4 mm² (and 2 AWG as 33.6 mm², 1/0 as 53.5 mm²); Tycab's 4 B&S is 20.26 mm² and their 2/0 B&S is 64.15 mm². Always work from the mm² figure printed for the actual cable you're buying.
Anything else?
- Use the right cable. No coarse-stranded cable, and no rigid or AC cable for DC battery runs. In marine or damp spots, use tinned marine cable (tin-coated copper strands), like our 2 B&S tinned marine cable.
- Two cables can share the load. Two 35 mm² cables equal one 70 mm², but each cable may need its own fuse.
- Fusing goes with cable size. That's its own guide: fusing a 12V system.
- Good lugs and crimps. Every connection adds resistance, so a proper crimp on the right size lug matters. Our connectors and lugs are sorted by cable size.
Is thicker cable worth the cost?
- Thicker cable costs more and is harder to route. But undersized cable costs you every day in lost charge, and at worst it's a fire risk. On 12V, go up a size when in doubt.
- For a DC-DC charger run from the engine bay, the vehicle side is the hard part. If you're not confident routing and protecting cable through a firewall and along a chassis, that's a job for an auto-electrician. Smart alternators add their own wrinkles; see smart alternators and DC-DC chargers.
Our wire range lists each cable's mm² so you can match it to Victron's table.