In my boat I have a 2.2 kW starter motor, last week the starter motor shorted and melted one of the cable lugs. The starter motor is connected directly to the battery without fuse, so now I want to prevent this from happening again.

I have changed the wiring (Sqr:50mm², Len:~1,5M) and starter motor now, but I want to install a fuse for the starter motor. The starter motor is marked with 2.2 kW and it runs at 12VDC. How do I calculate the startup current, and how big fuse should I install?

  • I've heard of using a fusible link on a starter circuit, but never a fuse.
    – user28910
    Dec 8, 2016 at 21:30

3 Answers 3


To make a reasonable recommendation we'd need to know something about the engine that the starter is servicing. The starter on a diesel engine on a cold day is going to draw a lot of current – I wouldn't be surprised to see the starter for a 2 liter diesel pulling 300+ Amps starting a cold engine. A stalled motor will draw considerably more than its rated current. The "inrush" current will drop quickly as the starter comes up to speed (or it will stay high if the connected engine is hard to turn over).

Because of the magnitude of the currents in starter circuits they are often unfused. If you do decide to fuse it, the conservative thing to do would be to use a slow blow fuse sized for the lower of the wire ampacity or the CCA rating of the battery. A slow blow fuse will allow the current to significantly exceed the wire capacity for brief period but will blow at a lower current if the load persists. Here is the curve for an ANL type fuse from the Blue Sea Systems website. The ANL is a ignition protected (up to 500 A) fuse that would be appropriate for use on a boat with a gasoline/petrol engine (and of course it would be fine with a diesel engine as well).

ANL Fuse Trip Time Curve

Reasonable cranking times will probably be in the flat are of the curve where the fuse is designed to blow at between 125% and 150% of its rated load (e.g. between 375 and 450 A for a 300 A fuse).

The fuse itself looks like this:

ANL Fuse

and goes in a carrier that looks like this:

enter image description here


Fuses are there to protect cabling from melting/catching fire, thus they are rated for the max safe current, which specifies what fuse to install. There are many tables for this on the internet.

While you want to install a slow reacting fuse to allow for some inrush current, you do not want to size the fuse for the current of the motor, so measuring that is in so far irrelevant as you already have the cabling.

The correct way would be to check the current, add some safety margin, use wire for that current, then a fuse for the max wire current. The currents for the motor you can usually find in the datasheet/documentation, sometimes even on a label on it.

  • I found sevaral tables that recommended 300 - 325A fuse for 50mm² wiring, is it enough if i use 300A fuse for 183.33A (2.2kW at 12V) starter motor, when we consider the startup current?
    – BufferOverflow
    Dec 8, 2016 at 21:28
  • @sure, the fuse can always be lower amps than the cable rating, and using a slow one allows for temporary overload in startup conditions
    – PlasmaHH
    Dec 9, 2016 at 7:42

That's pretty massive wire. Usual method for automotive applications is a fusible link which is something like 4 AWG notches below the wire size, which would put it at about AWG 4, and perhaps 150A 'rating', but of course higher currents could be sustained briefly.

You may be able to measure the starting current if you can put a few amperes from a bench supply through the motor and measure the voltage drop in the Kelvin (4-wire) fashion with a multimeter on mV range. If the starter is the automotive type with the solenoid on the motor you would have to find a way to actuate it. Measure the battery voltage during cranking too, preferably with an oscilloscope. Or a hall-effect clamp-on probe with an oscilloscope would measure it directly. Aside from the raw rating there should be data on how quickly a given protective device will open given a certain amount of overload.

In a boat you may have an additional issue that gasoline/petrol fumes can accumulate and the sparking resulting from an unsealed fuse or link blowing may lead to additional and very undesirable drama. You may wish to seek out a device that is specifically rated for marine use.

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