pianotuna wrote:
Hi Mike,
Your example applies to alternating current motors. My understanding is that DC motors do not behave in the same manner.
When a brushed DC motor is stalled, the only thing limiting the current flow is the DC resistance of the armature winding. For a motor the size of a jack motor, that will be less an one ohm. The winding resistance of the Atwood jack motor of my camper is approximately 0.25 ohms (measured).
Ohms law: volts/resistance = amps
Therefore: 12.5 volts/0.25 ohms = 50 amps
Based on the above, it is not unreasonable to install a 30 amp fuse for the jack motor. But, as I have been somewhat preaching, a Type 1 circuit breaker is a much better protection choice as it is possible to exceed 30 amps when the jack hits the top or bottom end and stalls.
