Chris Collins

Tools · cars

12 V and 24 V vehicle wiring

Wire a car, van, truck or motorbike: cable size in mm² and AWG from the current, the round-trip length and the voltage drop you allow, with insulation rating, under-bonnet heat and bundling; the fuse that protects it; relay circuits drawn as schematics with the 30/85/86/87/87a pins and the flyback diode; crimp terminals and connectors; parked battery drain and the alternator's load; a second battery on a DC-DC charger; and the DIN 72552 terminal numbers.

Added

Works with

How to use it

Most low-voltage wiring faults come from cable that is too thin for its length, a fuse that does not protect the cable, or a switch carrying a current it was never meant to. This page works out the cable, the fuse and the relay for a circuit on a 12 V or 24 V vehicle.

How to use it

  • Cable & fuse — give the load current, the one-way length from the source (battery, fuse box or busbar) to the load, the voltage drop you can accept, the cable’s insulation and where it runs. You get the smallest cable that meets both the voltage drop and the current it may carry, the fuse that protects it, the terminals to crimp on, and every size with its drop.

  • Relays — pick a circuit (driving lights, horn, cooling fan, fuel pump, a dip/main headlamp changeover, an ignition-switched accessory socket) and the lengths of each leg. The schematic shows how it goes together; the cable, fuse and relay rating come from the load.

  • Terminals & connectors — crimp terminal colours and the sealed connector families that take a cable size, with the current per contact.

  • Battery & charging — how long a parked battery lasts against a parasitic drain, whether the alternator keeps up with the loads you switch on, and a second battery charged through a DC-DC charger.

  • Terminal numbers & colours — the DIN 72552 terminal numbers printed on relays, switches and diagrams, and what the common wire-colour habits are worth.

Lengths can be in metres or feet; sizes in metric mm² or AWG.

How it is worked out

Voltage drop. Current goes out to the load and comes back, so the cable length is twice the one-way run (or, for a relay circuit, the whole loop: battery to relay, relay to load, load to earth). The drop is current × length × resistivity ÷ area, with copper’s resistivity taken as 0.0179 Ω·mm²/m. That is the figure behind the ABYC’s K of 10.75 Ω·circular mil per foot, about copper at 25–30 °C; a hot cable reads a little more. The two usual limits come from boat-wiring practice (the ABYC E-11 standard), and suit vehicles just as well:

  • 3 % for anything that minds a low voltage — lamps (a halogen bulb gives about 30 % less light at 10 % less voltage), fuel pumps, electronics, chargers, and the main feeds.

  • 10 % for things that only run a bit slower or cooler — fans, wipers, heaters, a winch for its short bursts.

At 24 V the same percentage allows twice the volts, so a cable can be half the area, and the same power draws half the current: a quarter of the area overall.

Current a cable may carry. The figure is the allowable current for a single conductor by its insulation’s temperature rating, from the ABYC E-11 table for systems under 50 V, the table that RV and boat DC power uses. Metric sizes are interpolated between the AWG rows by area. Under the bonnet, the engine-space factor for that rating applies (0.75 for 75 °C insulation up to 0.89 for 125 °C); in a bundle of current-carrying cables, the bundling factor (0.7 for three, 0.6 for four to six, 0.5 for seven to 24). The insulation choices follow the names you will find on cable:

  • SAE J1128 (North American automotive cable): GPT and HDT are PVC rated 80 °C, general and heavy wall; TWP is thin-wall PVC rated 105 °C; SXL, GXL and TXL are cross-linked polyethylene rated 125 °C, in standard, thin and extra-thin walls. Thinner walls save weight and space but resist chafing less.

  • ISO 6722 (the international equivalent, used across Europe and Asia): classes by temperature — A 85 °C (FLRY thin-wall PVC is the common one), B 100 °C, C 125 °C, D 150 °C.

SAE "gauge" automotive cable is often a little smaller in copper area than the AWG size it is named after. If a cable’s area is marked in mm², go by that; if a result is marginal, take the next size up.

The fuse. A fuse protects the cable, not the load, so it goes where the cable starts, as close to the battery or busbar as possible. It must be at least the load current (125 % of it for a load that runs for hours, such as lights, a fridge or a pump), and no more than the current the cable may carry. When the fuse a load needs is above what the voltage-drop size can carry, the cable goes up a size until it fits. ABYC allows a fuse of up to 150 % of the cable’s rating; this page keeps to the cable’s rating, the usual vehicle habit. Blade fuses share a colour code: 5 A tan, 7.5 A brown, 10 A red, 15 A blue, 20 A yellow, 25 A natural, 30 A green, 40 A orange.

Relays. A switch, horn button, thermostat or ECU carries only the relay coil’s current (about 0.15 A at 12 V). The relay’s contacts switch the load. The cable from the battery to terminal 30, from 87 to the load and from the load to earth is sized for the whole loop’s voltage drop, and the fuse sits on the feed to 30. Mini ISO relays are made in 30 A and 40 A contact ratings.

Batteries. Hours until flat is the usable capacity (from the charge now to the lowest you accept) divided by the parked current. A lead-acid starter battery cranks unreliably below about half charge and is damaged by sitting flat. For the alternator, the loads you tick are added to the vehicle’s own running load and compared with the alternator at idle (about half its rating when hot) and at cruising revs (about 90 %). A DC-DC charger draws its output × charging voltage ÷ (input voltage × efficiency) from the starter battery, so a 25 A charger takes about 30 A. Its input cable is sized for a 3 % drop, because a charger that sees low input voltage cuts its output.

Limits

  • The allowable currents are for copper cable in free air, with the ABYC derates. A cable inside a sealed, tightly wrapped loom near the exhaust runs hotter still. If in doubt, go a size up.

  • Connector and terminal ratings are typical figures for the largest cable each contact takes; check the data sheet for your parts.

  • Inrush is not modelled. Motors, compressors and halogen lamps draw several times their running current for a moment, so a slow-blow fuse or a size up avoids nuisance blowing.

  • The DC-DC recharge time is the bulk stage only; the last 10–20 % takes longer.

  • For vehicles still under warranty, or anything safety-related (lights, brakes, airbags), follow the maker’s wiring diagram, and do not tap into CAN-bus or airbag wiring.

Linking to an example

Share link copies a link with every setting. A hand-written link can set any of these (lengths in the lenu unit):

Parameter Meaning Example

sysv

System voltage: 12 or 24

sysv=24

a

Load current for the cable (A)

a=20

len

One-way run (m or ft)

len=4

lenu

Length unit: m or ft

lenu=ft

drop

Allowed voltage drop (%)

drop=10

ins

Insulation: pvc70, gpt, isoA, twp, isoB, xl, isoC, silicone

ins=xl

bay

Under the bonnet: true or false

bay=true

bun

Cables bundled: 1, 3, 4, 7, 25

bun=4

cont

Runs for hours (fuse at 125 %): true or false

cont=false

sys

Sizes: metric or awg

sys=awg

rc

Relay circuit: lights, horn, fan, pump, headlamps, aux, other

rc=fan

ra, feed, loadm, earthm

Relay load current and leg lengths

ra=20&loadm=1

bah, ma, from, stop

Parked battery: Ah, drain (mA), charge now and lowest (%)

bah=60&ma=250

alt, base

Alternator rating and the vehicle’s own load (A)

alt=150

dout, din, drun, dah, dfrom, dto

DC-DC: output A, input volts, run length, second battery Ah, from and to (%)

dout=40

example

A preset (below)

example=cooling-fan

Presets:

example= Shows

light-bar

A 10 A light bar on a dash switch through a relay

cooling-fan

A 20 A radiator fan switched by a thermo-switch to earth, under the bonnet

fuel-pump

An 8 A fuel pump driven by the ECU, with a 4 m run to the tank

horn

A horn relay with the button switching to earth

fridge-rear

A 5 A fridge 5 m from the battery at a 3 % drop

winch

A 300 A winch 3 m from the battery, 10 % drop, short bursts

trailer-24v

An 8 A trailer circuit on a 24 V truck in a bundle of seven

dcdc-25

A 25 A DC-DC charger with a 4.5 m run to a 100 Ah second battery

dashcam

A dash camera on parking mode draining a 60 Ah battery

For example ?example=cooling-fan or ?a=20&len=4&drop=3&sysv=12#cable.

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