Wire Gauge Calculator: Ampacity and Voltage Drop for 12V Runs

Every conductor in a vehicle is sized by two independent tests and has to pass both. Ampacity is a safety limit: how much current the conductor can carry without its insulation exceeding its temperature rating. Voltage drop is a performance limit: how much of the 12V survives the length of the run. A cable that passes the ampacity test can still be badly undersized for a 20 ft run, and a cable sized only for drop can still be protected by a fuse that will never blow. This calculator runs both and reports the one that governs.

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What is ampacity, and where do the numbers come from?

Ampacity is the maximum continuous current a conductor may carry without its insulation exceeding its temperature rating. It is not what the wire survives briefly. It is the figure that keeps the insulation intact over years of heat cycling and vibration.

The figures this calculator uses are ABYC E-11 for copper with 105C insulation, outside engine spaces, with three or fewer current-carrying conductors bundled together, which is the normal condition in a vehicle build. The engine space column applies the roughly 30 percent derate that the higher ambient temperature under a hood demands.

Three other conditions cut ampacity and are worth knowing. Bundling more than three current-carrying conductors together reduces the rating for every conductor in the bundle, because the heat has nowhere to go. Insulation rated below 105C carries less, which is one of several reasons to buy marine grade cable such as 4 AWG Marine Grade Battery Cable, 20 ft Red and 20 ft Black rather than an unmarked reel. And a conductor buried in insulation or run in a sealed conduit behaves like a bundled one. When the conditions are uncertain, size up. The ampacity chart holds the same figures with the bundling derates alongside.

How does voltage drop change the answer?

Ampacity says whether a conductor is safe. Voltage drop says whether it is useful. Every foot of copper has resistance, and current through resistance produces a voltage loss, so the appliance at the far end sees less than the battery voltage.

On a 12V system there is almost no headroom to give away. Five percent lost on a 120V circuit is 6V and nobody notices. Five percent lost on 12V is 0.6V, and a fridge compressor notices immediately: it draws more current at the lower voltage, runs hotter, and on a marginal system trips its own low voltage cutout while the battery is still half full.

The formula is Vdrop = 2 x L x I x R, where L is the one-way run in feet, I is the current in amps, and R is the conductor resistance in ohms per foot. The factor of two is not a safety margin. Current travels out along the positive conductor and back along the negative, so the copper in the circuit is twice the distance between the ends. A battery 10 ft from a fuse block is a 20 ft circuit. Dropping the two halves the calculated loss and produces cable one or two gauges too small.

The target on 12V is 3 percent, which is 0.36V, and it applies to charging runs, the main DC trunk, panel feeds and anything with a motor in it. Ten percent is acceptable only on non-critical lighting. The voltage drop by run length chart shows the whole grid at once, and the wiring and fusing guide works through the reasoning in full.

A worked example: the 2000W inverter run

This is the circuit that catches people out, because the AC side of an inverter looks harmless and the DC side carries the highest current in the entire vehicle.

Step one, find the DC current. DC amps equal AC watts divided by system voltage times efficiency. A Renogy Pro 2000W Pure Sine Wave Inverter 12V to 120V at its full 2000W output, at 90 percent efficiency, is 2000 divided by 10.8, which is roughly 185A. That is more current than a typical starter motor draws while cranking, running continuously through cable you installed yourself.

Step two, check ampacity. Two AWG carries 210A outside engine spaces, so it passes with a modest margin. Note that if any part of this run passed through the engine bay, 2 AWG derates to 147A and fails.

Step three, check voltage drop. At 10 ft one way in 2 AWG: 2 x 10 x 185 x 0.000156 = 0.577V, which is 4.8 percent and well outside target. In 1/0: 2 x 10 x 185 x 0.0000983 = 0.364V, right on the line. In 2/0: 2 x 10 x 185 x 0.0000779 = 0.288V, comfortably inside.

Step four, pick the cable. 2/0 AWG. Ampacity 330A against a 185A load, and 0.288V of drop against a 0.36V budget. If the inverter can be mounted 5 ft from the battery instead, 2 AWG gives 0.289V and passes both tests, which is a concrete reason to put the inverter beside the battery rather than across the vehicle.

Step five, size the fuse. The fuse must sit above the working current and at or below the ampacity of the smallest conductor. Working current is 185A continuous, so a 200A fuse would nuisance-blow on any surge. The cable is 2/0 at 330A. A 250A class T fuse sits between the two and has the DC interrupt rating a lithium circuit demands. Fit it within a few inches of the battery positive post, before the cable goes anywhere.

Now consider what happens if you keep 2 AWG and fit that same 250A fuse. The fuse now exceeds the 210A ampacity of the cable, so a sustained fault at 230A would heat the cable indefinitely and never blow the fuse. That is precisely the condition that starts fires, and it is why the cable and the fuse are chosen together rather than separately.

What gauge does each circuit need?

A few of those rows sit one gauge above the strict minimum, and deliberately so. Motor loads and continuously heavy loads get a conductor with margin, because a motor running on sagging voltage draws more current and gets hot, and because a cable that is exactly adequate on paper has nothing left when a terminal starts to corrode. The calculator gives you the minimum that passes both tests. Going one size up is always allowed. Going one size down is not.

Three rows deserve a second look. The DC-DC charger feed runs 18 ft from the starter battery at the front of the vehicle to the house bank at the back, and that length is why a 60A load needs 2 AWG when 6 AWG would carry it on ampacity alone. Voltage drop governs on long runs, always. The 3000W inverter row shows a different constraint entirely: 278A continuous needs a 350A fuse, and a 350A fuse may only protect a conductor rated for at least that much, which forces 3/0 cable even though 1/0 would have passed both the ampacity and the drop tests. And at 278A a 24V house system starts to look cheaper in copper than 12V does, because the same inverter at 24V draws 139A. The 12V versus 24V comparison works out where the crossover sits.

How do you actually terminate this cable?

A connection is where a wiring job fails. A poorly crimped lug on a 185A cable has resistance, and resistance at that current is a heater that will eventually melt insulation and then find something to short against. You cannot hand-crimp a 4 AWG lug properly, and a Brileine 10 Ton Hydraulic Lug Crimping Tool (12 to 2/0 AWG, 9 dies) at around fifty dollars is the cheapest genuine safety item in the build. Crimp tinned TKDMR Copper Wire Lug and Ring Terminal Kit (2 to 12 AWG, 160 pieces) and seal each one with adhesive lined Adhesive Lined 3:1 Heat Shrink Tubing Kit (400 pieces) , because water in the strands is how a good crimp turns green and then hot.

Never stack ring terminals on a battery post. The bottom terminal loses clamping force first as the stack relaxes, and a loose 200A connection is a fire risk on its own. A pair of Joinfworld 12V 250A Busbar Power Distribution Block (4 x 3/8 in studs) , one positive and one negative, gives every cable its own torqueable stud and makes the system serviceable years later. Fit a Blue Sea Systems 5191 MRBF Terminal Fuse Block (30 to 300A) directly on the battery post so the main fuse sits exactly where it belongs, and a Blue Sea Systems m-Series Battery Switch, On/Off with Knob (6006) so the whole system can be isolated by hand.

The negative side is sized exactly like the positive side, because it carries the same current. That is obvious and routinely ignored, usually by someone relying on the chassis as a return path through a rusty seam. Bond the negative busbar to the chassis at a single prepared point, downstream of any shunt so house current is still counted. Protect every conductor that crosses a metal edge with Split Wire Loom Conduit, 3/8 in x 120 ft and a grommet, and support cable every foot or so, because vibration will find the one run you left flapping.

Everything on this page is arithmetic you can check against published ABYC E-11 figures and manufacturer documentation. It is researched guidance rather than an electrical certification, it does not replace ABYC E-11 itself or the installation manual supplied with your components, and any lithium installation should be inspected by a qualified installer before it carries load.

Where to go next

• 12V wiring and fusing guide, the long form version of this page
• Wire gauge ampacity chart
• Fuse sizing chart
• Inverter sizing calculator, which produces the current figure for this page
• Inverter installation guide