How do I measure the amps that a 12V motor pulls, so that I use the correct fuse size?

Question

I'm interested in the process.
The motor in question is for the sink on a motor home.
I'm not interested in using the manufacture's recommendation.

Answer 1

You say you are interested in the process but not the manufacturers recommendation. This is a contradiction, since that is how you correctly size the fuse.

Measuring the current doesn't generally work because you are interested in the maximum current the motor can pull not its running current. If you size the fuse to running current, the load may increase later and the fuse blow. You also need to consider the start current, which you will not measure with an ammeter because it happens too quick. The fuse needs to be at least 2-3 times the full load current.

At the end of the day a fuse is a very crude and basic protection and you will likely cause yourself more problems with spurious blowing than you want during your motor home vacation. Consider over-current devices and leave the fuse at the manufacturers recommendation if you want to improve things.

Answer 2

You can use an Ammeter. Put this in series with the motor and switch on. Make sure you put it on the 10A setting or you will blow the meter fuse.

Or much safer, put a low value power resistor in series (say 0.1Ω) and switch on. Measure the voltage dropped across the resistor and use Ohm's law to calculate the current.

Then add maybe 50% for a suitable fuse value.

Answer 3

Really a 20 amps fuse would be fine. I would need to know the horse power of the motor. HP = Power * 750 watts (approx). Power = volts * current.

Know the power, know the volts and the current falls out. The fuse needs to be big enough to handle the little inrush current.

Answer 4

You would need an ammeter to be put in series with the motor. Turn it on and read the answer. Most people believe the manufacturer and do not bother to test this.

Answer 5

if the voltage rating is 12, i think the power rating will be also less ( see the power rating from the specification of the motor)
current i = power / 12

fuse rating ~ i + 0.25 * i
usually, for lower power rating fuse rating will be ~ 1 to 2 amp

Answer 6

First use a DMM to measure the out put of the motor, then I would use Ohms law (amps= voltage/ resistance) then select a fuse that is rated appx. 30% higher

Answer 7

Start with a 10A fuse and work your way up by 5, you shouldn't pass 30.