What size circuit breaker for windlass?

[Cliffhanger]

So, I bought a boat and found the anchor winch (Muir Cheetah) did not have a fuse or circuit breaker. So, I bought a 150A breaker based on 1000W x 12 V = 120A....without giving it much thought. Of course, it keeps tripping under heavy load. After investigation, the motor is 1200W so it will draw 144 A continuously - obviously more under load. I have about 4m of 4AWG (25mm) cable.


The question is, what size circuit breaker should I use?

[Wotname]

Originally Posted by Cliffhanger

So, I bought a boat and found the anchor winch (Muir Cheetah) did not have a fuse or circuit breaker. So, I bought a 150A breaker based on 1000W x 12 V = 120A....without giving it much thought. Of course, it keeps tripping under heavy load. After investigation, the motor is 1200W so it will draw 144 A continuously - obviously more under load. I have about 4m of 4AWG (25mm) cable.


The question is, what size circuit breaker should I use?

OK, a couple of points. You have made an error with the maths.
1000W /12V = 83.3 amps (P=VxI)

Muir suggests the working current draw is 120 A (for the 12V version) - see attachment.

If you are tripping your 150A breaker, you are either overloading the winch or have some other electrical fault.

[Wotname]

Also, remember the purpose of the windlass is to lift the anchor and the chain, it isn't designed to be used to break the anchor from the bottom.

The electric motor on the windlass is a DC series wound motor which means as it rpm slows with increasing load, the current draw increases rapidly - in theory, if it is stalled the current draw becomes infinite!

[Reefmagnet]

I think you'll find 135A is the recommended size for a 1200 watt winch on a 12VDC system.

[Cliffhanger]

Originally Posted by Wotname

OK, a couple of points. You have made an error with the maths.
1000W /12V = 83.3 amps (P=VxI)

HaHaHa...... what a careless mistake. Would actually calculate to be 12,000A the way i had it. Now that would be interesting!



I think you are right, there must be some other problem. A dodgy circuit breaker perhaps? Bad bearings in the gearbox? Any other ideas?

[SVTatia]

As far as I know, fuses or circuit breakers should be protecting the wiring not the equipment.
There was no mention of the cable size feeding the windlass, and if it keeps tripping it could mean it is wired improperly - wrong cable size given the distance from the battery.

[Sailmonkey]

Originally Posted by SVTatia

As far as I know, fuses or circuit breakers should be protecting the wiring not the equipment.

There was no mention of the cable size feeding the windlass, and if it keeps tripping it could mean it is wired improperly - wrong cable size given the distance from the battery.

The OP mentions 4 awg wire.

You are correct, the fuse is to protect the wire, not the device. Wire is sized for the device.

In this case, I suspect that the wire is grossly undersized, this chart suggests that 4awg @ 60C is good for 70 amps.



As the voltage drops from the load applied, the current goes up. The OP also states about 4m of wire. We don’t know if that’s round trip, or one way. Worst case is that is actually an 8 meter round trip. That means at 12 volts the motor is actually seeing a 2 bolt drop.

[rslifkin]

Marine wire is typically rated for 105*C insulation, not 60*C. So in a room temperature (not engine room) environment, 4 awg is good to 160A before a meltdown. So for a windlass that's rarely pulling full power for more than a second and is fed with a 120 or 135A breaker, it's safe. A 150A breaker is safe, but pushing it in my mind. However, voltage drop is likely an issue with 4 awg depending on the run length and the wiring should probably be up-sized.

[kenbo]

Originally Posted by SVTatia

As far as I know, fuses or circuit breakers should be protecting the wiring not the equipment.
There was no mention of the cable size feeding the windlass, and if it keeps tripping it could mean it is wired improperly - wrong cable size given the distance from the battery.

Agree, the cable should be 0awg. The fact the it was installed with 4 is risky business especially now that we know it's tripping a 150A breaker.

[David M]

You gauge a fuse to protect the wire....period. If the load demands more than the wire can supply then you replace the wire with wire of greater capacity. Or you reduce the load on the wire by changing out the load for a load that draws less current.

Refer to wire gauge tables if you are not certain.

[a64pilot]

Originally Posted by Cliffhanger

I have about 4m of 4AWG (25mm) cable.

This confuses me, even 4/0 is only 12mm

[Frankly]

The general rule is that the wire and circuit breaker should be selected at 125% of the rated full load current of the motor. On 12 volt circuits voltage drop considerations often dictate increasing the wire size (but not the circuit breaker). In a normal motor circuit we would have a circuit breaker protecting the circuit and a motor controller on the motor end protecting the motor. On our typical yacht set ups the circuit breaker does double duty. It protects the circuit and also the motor. Motors typically have a rating that based on thermal consideration, but short time can produce maybe 3 to 5 times that nameplate rating. To keep from overheating the motor we depend on the cicuit breaker.

The mfg data sheet on the windlass should have a recommended circuit breaker size.

As to wire ampacity/ temperature rating, 90 degree would be a reasonable column in the wire tables. Higher ratings insulation are suitable if the wire passes through hot space (engine compartment) but most devices are not rated for electrical connections at the higher rating in the wire tables.


Frankly

[SVTatia]

Originally Posted by Sailmonkey

The OP mentions 4 awg wire.....

I missed that... and it is undersized.

[KimSails]

Originally Posted by a64pilot

This confuses me, even 4/0 is only 12mm

I'm guessing he missed the "squared" on the mm^2.

[Seasic]

Are you running the engine while using the windless or running straight off the battery?

What was happening before you put the breaker in, did it perform normally?