12V Wire Size Calculator: Gauge, Voltage Drop and Fuse Size

Enter your current and your run length and the calculator gives you a gauge — along with the two separate numbers behind it, because voltage drop and ampacity are different questions and on a 12V system they rarely give the same answer. Drop almost always wins, usually by two or three sizes, and that gap is the reason so much RV wiring is thinner than it should be.

It also checks something no general-purpose calculator can: whether the cable it just specified will physically land in the device you are wiring. Victron publishes a maximum terminal size of 6 AWG for the Orion-Tr Smart 12/12-30, and a 30 amp run over 20 feet calculates to 2 AWG. Both facts are published; nobody puts them next to each other.

What the calculator is doing

Four steps, all of it arithmetic you can check by hand:

1. Current. For an inverter it comes from the watts: DC amps = watts ÷ (volts × 0.85). The 0.85 is inverter efficiency, and it is Renogy’s own published “above 85% at full load” rather than an optimistic peak figure. For everything else you enter amps directly.
2. Voltage drop. Drop = 2 × one-way feet × amps × ohms per foot. The 2 is there because current returns along the negative conductor, so a 10 ft run is 20 ft of copper in the circuit. The calculator walks the conductor table from thinnest to thickest and takes the first one that comes in under your target.
3. Ampacity. Separately, the first conductor rated to carry that current at all — derated 15% if any of the run passes through an engine bay, where the surrounding air is hotter and carries less heat away.
4. The larger of the two wins, and the calculator says which one it was.

That fourth step is the part worth having. Every calculator returns a number; almost none tell you which constraint produced it, and the two behave in opposite ways. Ampacity does not care how long the run is. Voltage drop cares about almost nothing else.

The two constraints, and why drop nearly always wins

Take a 30 amp DC-DC charger fed from the alternator over a 20 ft run:

• By ampacity, 14 AWG carries 35 amps and is sufficient.
• By voltage drop at 3%, you need 2 AWG.

That is six sizes apart, and the ampacity answer is the one that will pass a visual inspection while quietly costing you charging performance for years. The cable is not going to catch fire at 14 AWG — it is rated to carry the current. It will lose 3.8 volts getting there, which is 31% of your 12, and a charger trying to hold 14.4 volts at the battery has nothing like the headroom to do it.

The reason is arithmetic rather than opinion. You only have 12 volts to lose from, so 3% is 0.36 volts of total budget — and 30 amps through even modest resistance eats that quickly. At 120V the same 3% would give you 3.6 volts of headroom for a tenth of the current.

There is one common case where it inverts, and the calculator flags it. A 3,000W inverter three feet from the battery draws about 294 amps: voltage drop only needs 2 AWG over that short a run, but nothing below 2/0 may safely carry 294 amps. Never size below the ampacity answer, whatever the drop maths says — that is the constraint that starts fires rather than the one that disappoints you.

Gauge	Max amps	5A	10A	20A	30A	40A	50A	60A	80A	100A	150A	200A	250A	300A
14 AWG	35A	11 ft	5 ft	2 ft	1 ft	—	—	—	—	—	—	—	—	—
12 AWG	45A	18 ft	9 ft	4 ft	3 ft	2 ft	—	—	—	—	—	—	—	—
10 AWG	60A	29 ft	14 ft	7 ft	4 ft	3 ft	2 ft	2 ft	—	—	—	—	—	—
8 AWG	80A	46 ft	23 ft	11 ft	7 ft	5 ft	4 ft	3 ft	2 ft	—	—	—	—	—
6 AWG	120A	73 ft	36 ft	18 ft	12 ft	9 ft	7 ft	6 ft	4 ft	3 ft	—	—	—	—
4 AWG	160A	116 ft	58 ft	29 ft	19 ft	14 ft	11 ft	9 ft	7 ft	5 ft	3 ft	—	—	—
2 AWG	210A	185 ft	92 ft	46 ft	30 ft	23 ft	18 ft	15 ft	11 ft	9 ft	6 ft	4 ft	—	—
1 AWG	245A	233 ft	116 ft	58 ft	38 ft	29 ft	23 ft	19 ft	14 ft	11 ft	7 ft	5 ft	—	—
1/0 AWG	285A	295 ft	147 ft	73 ft	49 ft	36 ft	29 ft	24 ft	18 ft	14 ft	9 ft	7 ft	5 ft	—
2/0 AWG	330A	372 ft	186 ft	93 ft	62 ft	46 ft	37 ft	31 ft	23 ft	18 ft	12 ft	9 ft	7 ft	6 ft
3/0 AWG	385A	469 ft	234 ft	117 ft	78 ft	58 ft	46 ft	39 ft	29 ft	23 ft	15 ft	11 ft	9 ft	7 ft
4/0 AWG	445A	592 ft	296 ft	148 ft	98 ft	74 ft	59 ft	49 ft	37 ft	29 ft	19 ft	14 ft	11 ft	9 ft

Resistance: NEC Chapter 9, Table 8 — uncoated stranded copper at 75°C. Maximum amps: ABYC E-11, single conductor outside engine spaces, 105°C insulation; derate 15% for cable passing through an engine bay. For a non-critical circuit ABYC allows 10% drop, which permits roughly 3.3× these lengths. Verified 22 August 2026 · rvtrove.com/electrical/12v-wire-size-calculator/

The chart above prints. It is oriented the way the question actually gets asked at the rig — you know your current and you know how far you have to go, so each cell is the longest run that gauge holds, rather than a gauge you then have to look up. The print button saves it as a PDF through your browser’s own print dialog, and if you run the calculator first, your own numbers print with it.

The cable in the box is sized for the box

Manufacturers do publish the cable their inverters need, and the figures are honest — but they are quoted for a specific short run, and that qualifier travels badly.

Renogy ships a 3 ft 1/0 AWG pair with the 2000W P2. At full output that inverter draws about 196 amps, and over 3 ft the included 1/0 loses 1.2% — comfortably inside 3%, and a size thicker than the 2 AWG the run strictly needs. Renogy is being generous, not stingy.

Now mount the same inverter 10 ft from the battery, which is an ordinary thing to do when the bank is under a bed and the inverter is near the panel:

One-way run	Gauge for 3%	What the boxed 1/0 actually does
3 ft	2 AWG	1.2% — comfortable
6 ft	1/0 AWG	2.4% — exactly at the edge
10 ft	3/0 AWG	4.0% — past the target

Three sizes up for seven extra feet, and the cable in the box is no longer adequate. Nothing about the inverter changed and nothing in its documentation is wrong. The length qualifier just got dropped somewhere between the manual and the installation.

The 3000W P2 tells the other half of the story: Renogy specifies 4/0 and does not include it, which is right for a run of about 10 ft and is a real cost to budget alongside the inverter. At the kind of price 4/0 reaches per foot, that is not a rounding error on the purchase.

When the cable will not fit the device

The constraint nobody checks is whether the terminals accept what the calculation asks for.

Victron publishes 16 mm² / AWG6 as the maximum cable size for the Orion-Tr Smart 12/12-30. That is a hard physical limit — the lug either goes in or it does not. So:

• At 10 ft, a 30 amp run needs 6 AWG, which is exactly the limit. Fine.
• At 20 ft, it needs 2 AWG, which will not land in the terminals.

That does not make the charger a bad product. It makes it a charger designed to live near the house bank, and the published figure is telling you so. The right answer at 20 ft is to move the unit rather than the wire: mount it close to the batteries, accept the longer leg on the alternator side, or accept the roughly 4.9% drop that 6 AWG gives you over that distance and know you have accepted it.

Renogy publishes no maximum cable size for the DCC50S at all — only an M8 terminal thread. That is worth knowing before ordering 1 AWG lugs, which is what its 50 amps wants over the same 20 ft run. The extra 20 amps costs you a cable size, and over a long run the difference in cable can exceed the difference between the two chargers.

If a battery monitor is going into the same negative run, its shunt is sized against the same peak current you just calculated — and it carries a second rating, in millivolts, that decides whether the readings mean anything. That is what size shunt do you need.

Our numbers differ from most calculators, deliberately

Two tables decide every answer on this page, and which ones you pick changes the result by nearly a factor of two.

Resistance comes from NEC Chapter 9, Table 8 — uncoated stranded copper at 75°C. This is uncontroversial; resistance is a property of the metal.

Ampacity comes from ABYC E-11: single conductor, outside engine spaces, 105°C insulation. Most general-purpose calculators use NEC Table 310.16 instead, and the two disagree sharply:

Gauge	NEC 310.16, 75°C	ABYC E-11, 105°C
10 AWG	35A	60A
6 AWG	65A	120A
2 AWG	115A	210A

Neither is wrong. They describe different installations: NEC 310.16 is building wire bundled in conduit at 30°C ambient, ABYC is 105°C-rated cable run singly in free air. RV and marine battery cable is the second thing, so using the NEC column would push nearly every reader one to two sizes up. At several dollars a foot for large conductor, that is not caution — it is an expensive error in the other direction.

The free fix is almost always distance

Every extra foot costs the same as the last, and cable price rises steeply with gauge. Which means the cheapest improvement to any DC run is nearly always to shorten it rather than to thicken it.

The 2000W inverter above needs 2 AWG at 3 ft and 3/0 at 10 ft. In current copper prices those are not comparable purchases. Moving the inverter seven feet costs an afternoon.

Two related levers, in the order they are worth trying:

• Relocate the device, not the battery. An inverter, a DC-DC charger and a solar controller all want to be near the bank. Loads can be fed by AC or by thinner DC downstream.
• Rewire the solar array before you resize its cable. Panels in parallel multiply current and need the copper to match; the same panels in series draw a fraction of it. Which way you can go is decided by the controller’s voltage limit on a cold morning — what size solar charge controller works that out, and it is the input most sizing guides never check.
• Raise the system voltage if you are still designing. At the same power, doubling the voltage halves the current and doubles the drop allowance, which is four times less copper. This is why 24V and 48V systems exist, and it is a decision you can only make once.

This calculator is for the 12V side only. The same resistance table sets the drop on the 120V side too, but with different currents and a different allowance — for shore cords and extension cords, our RV extension cord calculator uses these same figures, and says what a portable EMS at the pedestal cannot see.

Fusing, briefly

The fuse protects the cable, not the device. That single sentence settles most of the questions about it:

• It goes at the battery end, within a few inches of the positive terminal, because everything downstream of that point is what it is guarding.
• Size it at the first standard rating above 125% of continuous current.
• Check the rating sits below the cable’s ampacity. If the fuse is rated higher than the wire can carry, the wire is the fuse.

The calculator does this arithmetic alongside the gauge, and tells you when the combination does not work — which usually means the cable wants to be one size larger rather than the fuse one size smaller.

Five ways this goes wrong

1. Entering the round-trip length. Enter one way. The formula doubles it. Getting this backwards makes the answer wrong by a factor of two, in the direction that undersizes.
2. Sizing on ampacity alone. The wire will be safe and the system will underperform, and nothing will look broken.
3. Trusting the cable in the box past its stated length. It is correct for the run the manufacturer quoted and for no other.
4. Using the inverter’s surge rating for the cable. Size for continuous output; the surge lasts a second and the cable’s thermal mass absorbs it.
5. Ignoring the terminals. A gauge you cannot land is not an answer, and the limit is usually published.

The devices this points at

Four products where the cable question actually changes the decision:

Related guides

• Do I need a DC-DC charger? — whether to fit one at all, before you work out how to wire it.
• Proportional vs time-delay brake controllers — a brake controller is a 30 A DC device fed from the battery, and an undersized or overlong feed shows up as weak trailer braking that gets blamed on the controller. Size that run here first.
• Trailer brake controller not working — the same voltage drop this calculator predicts is one of the things that shows up as weak braking, and that page walks the circuit with a meter to find where it is being lost.
• What size inverter do I need for my RV? — the watts question this page picks up from.
• Best RV inverters — seven units compared, including the two whose cable figures this page uses.
• RV battery calculator — bank size and inverter size from your own appliance list.
• RV amp draw calculator — the same arithmetic on the shore power side of the transfer switch.
• The electrical section collects everything published so far.

For how products on this site are selected and what we do not claim about them, see the editorial policy.

Questions worth answering