Live Converter & Auto Electrical Calculators

Ohm's law and power

Enter any two values. The other two are worked out for you and shown highlighted.

Voltage

Current

Resistance

Power

Example: 12 V and 55 W gives about 4.6 A.

Typical figures for a 12 V car. Loads and fuse ratings vary between vehicles, so the car's own wiring diagram or fuse chart always takes priority.

Circuit	Typical load	Typical current	Common fuse
Headlamp bulb (55/60 W halogen)	55–60 W each	4.5–5 A each	10–15 A per side
Front fog lamp (55 W)	55 W each	about 4.6 A each	15–20 A
Brake, indicator or rear fog bulb (21 W)	21 W each	about 1.75 A each	10–15 A
Side or tail lamp (5 W)	5 W each	about 0.4 A each	5–10 A
Horn	40–70 W	3–6 A	15 A
Radio or head unit	10–60 W	1–5 A	10–15 A
12 V socket or cigarette lighter	up to 120–180 W	up to 10–15 A	15–20 A
Fuel pump	50–100 W	4–8 A	15–20 A
Wiper motor	40–100 W	3–8 A (more if stalled)	15–25 A
Heated seat	50–85 W each	4–7 A each	15–20 A
Electric window motor	60–120 W	5–10 A (15–25 A stalled)	20–30 A
Heated rear window	120–240 W	10–20 A	20–30 A
Heater blower (full speed)	150–250 W	12–20 A	25–40 A
Radiator cooling fan	180–400 W	15–30 A (more at start-up)	30–50 A
ABS pump	250–500 W	20–40 A	30–60 A
Diesel glow plugs (all)	500–1,000 W	40–80 A at first	60–80 A
Electric power steering	up to about 1 kW	40–80 A peak	60–100 A
Alternator output	–	70–180 A	100–200 A link
Starter motor	1–2.5 kW	150–300 A petrol, 300–600 A diesel	Usually unfused or a very large link

Standard blade fuse colours

1 ABlack

2 AGrey

3 AViolet

4 APink

5 ATan

7.5 ABrown

10 ARed

15 ABlue

20 AYellow

25 AClear

30 AGreen

40 AOrange

Choosing a fuse

Work out the normal current: watts ÷ volts. A 120 W load at 12 V draws 10 A.
Pick a fuse a step above that, so the normal load is about 75–80% of the fuse rating. A 10 A load would take a 15 A fuse.
The fuse rating must not exceed what the cable can safely carry. The fuse is there to protect the cable.
Motors draw several times their running current for a moment at start-up, which is why motor circuits have larger fuses.
Never fit a bigger fuse to stop one blowing. A fuse that keeps blowing means there's a fault to find.

Cable voltage drop

Check whether a cable is big enough for the job. Enter the total cable length, and include any return cable: a circuit with a 5 m feed and a 5 m return is 10 m. Where the circuit returns through the body, enter the feed length only. Copper cable at 20 °C.

System

Current

Total cable length

Cable size

Voltage drop—

Of supply—

At the load—

Heat in cable—

— A common target is under 3% drop for lighting and critical circuits. This checks voltage drop only; always size the cable for its current rating too.

UK and European cable is sized by cross-section in mm². American cable and many wiring diagrams use AWG (American Wire Gauge), where a bigger number means a thinner cable.

AWG	Exact size	Metric cable to use
22 AWG	0.33 mm²	0.35 mm²
20 AWG	0.52 mm²	0.5 mm²
18 AWG	0.82 mm²	1.0 mm²
16 AWG	1.31 mm²	1.5 mm²
14 AWG	2.08 mm²	2.5 mm²
12 AWG	3.31 mm²	4 mm²
10 AWG	5.26 mm²	6 mm²
8 AWG	8.37 mm²	10 mm²
6 AWG	13.30 mm²	16 mm²
4 AWG	21.15 mm²	25 mm²
2 AWG	33.63 mm²	35 mm²
1 AWG	42.41 mm²	50 mm²
1/0 AWG	53.48 mm²	70 mm²
2/0 AWG	67.43 mm²	70 mm²

AWG sizes don't line up exactly with metric sizes, so the last column gives the matching metric size (within about 5%) or the next one up.

This table compares copper area only. A cable's current rating also depends on its insulation, temperature and how it's bundled, so check the cable maker's data. For other sizes, use the mm² ⇄ AWG option in the unit converter below.

Parasitic drain

Work out how long a battery will last with a drain on it while the car is parked. Measure the drain with everything off, doors shut and the car locked, after the modules have gone to sleep. This usually takes 20 to 45 minutes, and up to an hour on some prestige models.

Measured drain

Battery size

Starting charge

Won't start below

Car won't start after—

Charge lost per day—

Charge lost per week—

Usable charge—

— Most modern cars settle at 20–50 mA once asleep. Check the manufacturer's figure for the specific vehicle where you can. Cold weather and an older battery shorten the time.

Approximate resting voltages by battery type. Use the percentage as the starting charge in the parasitic drain calculator.

State of charge	Flooded lead-acid	EFB	AGM
100%	12.6–12.7 V	12.6–12.7 V	12.8–12.9 V
75%	about 12.4 V	about 12.4 V	about 12.6 V
50%	about 12.2 V	about 12.2 V	about 12.3 V
25%	about 12.0 V	about 12.0 V	about 12.0 V
Discharged	11.9 V or below	11.9 V or below	11.8 V or below

Getting a true reading

Let the battery rest with no load or charge for at least 4 hours before measuring. A reading taken straight after driving or charging will be too high.
The figures assume about 25 °C. In freezing conditions a good battery reads roughly 0.1–0.2 V lower.
Below 12.4 V the battery needs recharging. Below 12.0 V it is deeply discharged and needs a proper charger, not just a drive.
An EFB is a strengthened flooded battery, so it reads much the same as a standard flooded one.
On EFB and AGM stop-start batteries, voltage alone can mislead. A full test also checks remaining capacity and how well the battery accepts charge.

Once all the modules have gone to sleep, most modern cars settle at 20–50 mA. Older cars with fewer modules usually draw less, often under 25 mA.

Drain after sleep	What it usually means
20–50 mA	Normal for most cars
50–75 mA	Borderline Can be normal on prestige cars with lots of modules
75–100 mA	Investigate Will flatten a battery in a week or so
100–300 mA	Fault Can flatten a battery over a weekend
300–500 mA +	Serious fault Can flatten a battery overnight or within a day or two

Taking the reading

Wait for the modules to sleep: usually 20–45 minutes, and up to an hour on some prestige models.
Opening a door, the bonnet or the fuse box cover can wake the body control module and restart the wait.
Some cars wake briefly every so often and draw an amp or two for a moment. That's normal if the reading drops back afterwards.
Use a bypass lead across the meter while the car goes to sleep, so the meter's fuse doesn't blow when it's connected.

How long a battery lasts

At 50 mA, a 60 Ah battery would take about 50 days to go completely flat, and would stop starting the car well before that. Use the calculator for your own figures. These are general guides; the manufacturer's figure for a specific vehicle always takes priority.

Unit converter

Pick a type, then type in either box.

Convert

miles

Live Converter and Calculators

Ohm's law and power

Cable voltage drop

Parasitic drain

Unit converter

Need an auto electrician?