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Mike_Stanbro's avatar
Mike_Stanbro
Explorer II
Jan 04, 2014

Replacing fuses with circuit breakers for motor circuits

I recently purchased a Lance 855S truck camper. When I went to pick up my camper from the dealer, one of the 30 Amp fuses for the jack motors popped when the jack hit its upper limit. We didn't know that had happened, all we knew was that one jack quit responding to the remote control. After 30 minutes of trying this and that, the technician at the dealer thought to check the fuses on the control board and found the problem. I made a mental note to replace those fuses with circuit breakers as soon as possible.

Motors generate huge current demand when at stall which occurs whenever the mechanisms they power hit end-of-travel. Most jack and slide-out mechanisms have clutches that release at end-of-travel but if it is really cold they can be rather stiff and cause excessive current demand.

There are different types of breakers, auto-reset (Type 1) are the best for jack and slide-out circuits. Type 1 breakers open when there is excess current and then quickly reset when the overload is removed.

I got four 30A blade-style circuit breakers for the jack motors, and one 5A breaker for the slide-out motor at NAPA. These have the same blades as mini fuses so they will plug into existing fuse holders. In other words, quick and easy to install.

If you have a camper or trailer with powered jacks or slide-outs, do yourself a favor and replace any fuses with auto-reset circuit breaker. I had a 30A fuse for the front landing gear of my 12,000 pound 5th-wheel pop while camping in Death Valley. After I got it hand cranked onto my truck, I made a promise to myself to get rid of all fuses from motor circuits.

Here are URLs to 5A and 30A blade-style breakers at the NAPA website if you are interested:

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx/Circuit-Breaker-5-Amp-Blade/_/R-BK_7823010_0006522027

http://www.napaonline.com/Catalog/CatalogItemDetail.aspx/Circuit-Breaker-30-Amp-Blade/_/R-BK_7823124_0361083750
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  • wnjj's avatar
    wnjj
    Explorer II
    Jan 05, 2014
    camperpaul wrote:
    Mike Stanbro wrote:

    -- snip -- 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.


    An even better choice is limit switches at both ends of the jacks travel.

    Instead of the motor stalling and blowing a fuse or tripping a breaker, the limit switches just turn off power to the motor.

    Anything else is SHODDY ENGINEERING.


    I disagree. Limit switches just add more places to fail and really aren't needed on a jack system where you can easily see the travel and simply avoid hitting the end. There's no reason to run the jacks even near the end. Limit switches means more wiring and connections to fail. Electronic controllers can monitor current flow automatically cut it off but that's pretty much what a self-reseting breaker does.
  • camperpaul's avatar
    camperpaul
    Explorer
    Jan 05, 2014
    Mike Stanbro wrote:

    -- snip -- 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.


    An even better choice is limit switches at both ends of the jacks travel.

    Instead of the motor stalling and blowing a fuse or tripping a breaker, the limit switches just turn off power to the motor.

    Anything else is SHODDY ENGINEERING.
  • Mike_Stanbro's avatar
    Mike_Stanbro
    Explorer II
    Jan 05, 2014
    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.
  • wnjj's avatar
    wnjj
    Explorer II
    Jan 05, 2014
    pianotuna wrote:
    Hi Mike,

    Your example applies to alternating current motors. My understanding is that DC motors do not behave in the same manner.


    DC motors produce a back EMF which effectively means they draw less current when spinning freely than loaded or stalled.
  • pianotuna's avatar
    pianotuna
    Nomad III
    Jan 05, 2014
    Hi Mike,

    Your example applies to alternating current motors. My understanding is that DC motors do not behave in the same manner.
  • Mike_Stanbro's avatar
    Mike_Stanbro
    Explorer II
    Jan 05, 2014
    Something else to keep in mind about motor currents...

    The power required to lift a camper/trailer or operate a slide-out mechanism is essentially constant while the voltage of the battery supplying the power is not. Electrical power = volts x amps. As the battery voltage goes down, the current demand increases in a linear fashion to deliver the same amount of power.

    In a previous post SkiPro3 said that he measured the current of each jack motor on his 855S to be 8 amps. If the voltage of the battery drops from 12.5 volts (charged) to 10 volts (discharged), the current to operate the jack motor will go from 8 amps to 10 amps.

    Here is the math:

    12.5 volts x 8 amps = 100 watts
    100 watts/10 volts = 10 amps

    So SkiPro3, if you don't want to blow those 10 amp fuses you installed, make sure your batteries are always well charged. Or, you could install 10 amp, auto-reset circuit breakers and not worry about it. Just a suggestion.
  • Mike_Stanbro's avatar
    Mike_Stanbro
    Explorer II
    Jan 05, 2014
    skipro3 wrote:
    I've measured current draw on my 2012 Lance 855S jacks. All 4 running the camper up will draw 32 amps at the battery. I checked each jack and the highest current draw was 8 amps. I replaced my fuses with 10 amp fuses. I want to KNOW when the current demand increases so I can fix why.

    If you are blowing a 30 amp fuse on a single jack, you got a BIG problem. I can guarantee that the wires at my jack are NOT rated for 30 amps. (with the length of wire, 30amps should be a 10 gauge wire at 12vdc) You were lucky the fuse went and not the wiring burn up.


    The current draw can easily exceed 30 amps, momentarily, when a jack reaches the end. A brushed electric motor is essentially a dead short when at stall. Only the resistance of the armature coil limits the current. Be sure to pack several extra 10 amp fuses, you will likely need them!

    The wires are not going to burn up in the length of time that it takes a fuse to blow or a circuit breaker to open. That's WHY they are fused.

    skipro3 wrote:
    Think about it; if these jacks are fused at 30 amps each, multiply that by 4 and you would potentially draw 120 amps just to run the jacks before a fuse would blow. WOW!!


    The factor installs 30 amp fuses and I hope I have explained why. Yes, 120 amps would be the total draw if all four motors were stalled. Not likely to occur and only for as long as it takes the fuses to blow or for the operator to take his/her finger off the button.

    Old-Biscuit wrote:
    Mark the jacks 1" from hitting full travel/upper & lower limits. Stop jacks when 1" mark reached and then you won't be blowing fuses/cbs from overload.


    Good idea and I plan to. Waiting for the cold and rain to stop here in the NW before attempting to mark the jack limits.
  • Old-Biscuit's avatar
    Old-Biscuit
    Explorer III
    Jan 04, 2014
    Mark the jacks 1" from hitting full travel/upper & lower limits. Stop jacks when 1" mark reached and then you won't be blowing fuses/cbs from overload.
    Not good for motor and not good for jack

    Also carry spare fuses/cbs.......good for IF a fuse blows or a cb fails
  • camperpaul's avatar
    camperpaul
    Explorer
    Jan 04, 2014
    The manufacturer of my tongue jack eliminated that problem by using 'limit switches'.

    The jack hits the limit switch before the motor would stall.
  • Ski_Pro_3's avatar
    Ski_Pro_3
    Explorer
    Jan 04, 2014
    I've measured current draw on my 2012 Lance 855S jacks. All 4 running the camper up will draw 32 amps at the battery. I checked each jack and the highest current draw was 8 amps. I replaced my fuses with 10 amp fuses. I want to KNOW when the current demand increases so I can fix why.

    If you are blowing a 30 amp fuse on a single jack, you got a BIG problem. I can guarantee that the wires at my jack are NOT rated for 30 amps. (with the length of wire, 30amps should be a 10 gauge wire at 12vdc) You were lucky the fuse went and not the wiring burn up.

    Think about it; if these jacks are fused at 30 amps each, multiply that by 4 and you would potentially draw 120 amps just to run the jacks before a fuse would blow. WOW!!

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