I have a basic 12v pump that runs the boat water pressure. When you open the tap full blast in will keep pumping no problem with decent pressure. But if you open the tap just a little the pump turns on and off repeatedly, which is fine however after 20-30 seconds the circuit breaker trips.

Anyone know if this is a feature or a problem? Maybe i just need a slightly bigger fuse? Any suggestions on how to stop the circuit from tripping?

It shouldn't be a problem on and offing if the fuse & wiring is correct. But a pump cycling like that is annoying, an accumulator smooths things out considerably

All electric motors draw much higher current when starting. So by starting and shutting down, you are increasing current draw. I would hesitate increasing the fuse size without checking to see if the wire could handle the current but Wokos idea is much better. Your pump will last much longer too with an accumulator

So, the questions are:

Is this a new problem?
When did this problem start?
Was anything done to the system that might have caused this problem?
Is an accumulator already fitted?
If fitted, are there any apparent problems with the accumulator (diaphragm, air valve fittings, corrosion )?
Could there be a problem with the water tank pick up tube that lets air into the water line?




www.amazon.com/rv-water-pump-accumulator/s?k=rv+water+pump+accumulator

If you are a pressure controlled system, the at low flows you are throttling the outlet valve to increase pressure and reduce flow. This is called dead heading a pump (or it is when you completely stop the pump). On a centrifugal pump (which I assume yours is) you can dead head a pump for a short while but eventually you will cause damage by either over current or over pressure in the system.

The fact that your breaker is tripping is good as it is protecting the wiring from over current.

Solution.

Don't throttle the flow so much
Or if you must
By a lower flow rated pump, if you do this make sure that the zero flow pressure is not more than the rating on your pipework and fittings (shouldn't be a problem for a 12v pump)

You could also try to get a pressure switch that you can wire into the pump supply circuit so that the pump switches off at a certain pressure.

Your pump is probably a swash-plate diaphragm (ie positive displacement) type with a built-in pressure switch. A centrifugal pump would generally draw less current as it was throttled and hence be less likely to trip the breaker, and unlikely to develop enough pressure to damage the piping.
The higher current draw at start up of your pump is of very short duration and hence unlikely to cause thermal damage to the wiring or windings. It's just annoying. The accumulator is the easiest fix. You might still get intermittent but less frequent pump cycling at low flow, but it will be much less annoying and less likely to trip the breaker, which sounds like it is undersized.

Cheers, Graeme

Thanks everyone for the great replies.

I noticed the problem when the pressure sensor on the old pump died and it would continually switch the pump on and off with all outlets closed, eventually tripping the breaker. I replaced the pump, it's just a cheap seaflo diaphram pump I believe.

So now its building pressure fine and running like it used to. I started using the deck shower at low speeds and then the breaker tripped again. The accumulator would probably slow down the cycling enough to not trip the breaker.. I hope, and would actually improve the flow.

But i wouldn't mind fixing it quickly before I get around to buying/installing an accumulator. So I may check the size of the fuse and put a multimeter on it, see if I can just bump it up slightly to resolve it.

Unless your multimeter has a peak-hold function you won't learn much, as the initial current peak will be too fast for you to see. Just replace the breaker, and/or use more water!

There should be a pressure or cut off adjustment screw. Google the pump model number and pressure adjustment.
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So now its building pressure fine and running like it used to. I started using the deck shower at low speeds and then the breaker tripped again.

I am with Wongaga. use more water!

Tarquin1 that looks like it might be the answer, i did check and the pump max amps is 17 amps, and i only have a 15a breaker so that explains the trips.

I'll try find that adjustment screw, see if it makes a difference

And incidentally the little pressure switch is the only part on those type pumps that usually needs replacement, & they can cost anything from $30 to $3 I found electrical wholesalers the cheapest. The switch has multiple applications including neutral fail safe for engine starting