Thought I'd ask the brains trust; I get a bit confused about this...
===== BACKGROUND INFO =====
The past 15 years, we have been using 24v DC plastic bilge pumps (usually RULE 3700GPH).
On one installation, we have a 24v DC solar pump; 2 x 200watt panels, and power-lead out to a floating pump in the middle of a dam.
Roughly 25 metres to the pump from the panels(when the dam is low).
We had a local fabricator make us up a pump housing box, made a couple of floats fashioned from PVC pipe, and the pump fills a 5,000 gallon tank on the dam bank. Overflow from the tank runs back into the dam, which has been helpful in keeping the dam water quality a bit better.
The tank feeds 2 troughs for watering livestock, so we figured out the summer daily consumption can easily be 35,000 litres or so.
But given the pump is pumping about 4 times the required a day, the system has more than enough 'fat' in it to work well.
The pumps last about 2-3 years, and in that time, they work perfectly.
===== END BACKGROUND INFO =====
So, the time has come for me to start setting up replacements for two of these pumps; one at the aforementioned dam, and another at a little bore-drain dam at another site.
Both pumps have died after about 3 years each of hard labour.
In both cases, the pump/tank/solar panel setup is about identical.
But the bore-drain install had a little problem the other day; the pump burned, the fire burned back along the polypipe, and burned the empty 5000 gallon tank!
That bore-drain dam install hasnt been running for over 18 months, and the bullrushes in the dam had all died, and left a whacking big pile of kindling..
So we missed the fact that the power was still directly connected to the pump from the panel-set.
[In retrospect, a really bad idea to omit circuit protection, and I now realise we've been running a big risk all this time at both sites.]
After looking over the remnants of the torched pump/tank, it's starting to look like the cause might have been an enthusiastic rat, and I know the bush rats were in plague proportions around that time.
I know the pump itself has a run-dry/overheat shutoff function built-in, so either Mr Ratty, or a really bored cow decided to chew a cable(not impossible, either).
Anyhoo; end result is I'm replacing both pumps, and this time installing a suitable circuit breaker under the panel-set.
So, my question to you all is about circuit breaker size, and whether cable length will affect the choice...
My pump draws 7.8 amps at full noise, and breaker recommendations are to go around 25-30% above expected load, so I'm working on a 10 amp breaker.
I've considered a 15 amp breaker, but it's getting very close to the max output of the panels, and it might never trip before the burning begins(again)..
I also note that DC motors dont tend to have the power surge of AC motors on startup, and so the breaker being this close to the expected load shouldnt be a problem.
But I get confused about what effect the length of the cable run might have on the power demands at the breaker end of the cable.
I've read the explanations that use the analogy of power in a cable being like a garden hose (Volts==Pressure & Power==Pipe Diameter), but I'm still missing something.
I get that I'll have some voltage loss, and for my cable size, I'm seeing the expected loss will pull the solar panel output back down to very close to the 24V the pump expects, by the time it reaches the pump.
Also; these pumps have previously demonstrated they are fine with the voltage from these panels with this size of cable.
But what happens to the current load/demand over distance?
From what I have read, the current should remain the same, but is that possible?
If I have an 8-amp load at 1 metre, and running happy through a 10 amp breaker, is that same load going to still be 8 amps at 25 metres of cable?
Or is the voltage loss in the cable going to also affect the current load/demand, and therefore ramp it up above my trigger current for the breaker?
The one piece of info I could scrounge on the net about this suggests the current stays the same while the voltage drops, but I'm unsure how that is possible.
If anyone can unravel what I'm talking about, I'd welcome any useful input.