12V DC Power question

[Jorlcrin]

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.

[scoot]

Power to run the pumps is a product of voltage and current. 24v @ 10amps creates 240watts of power ( as an example) Longer cable run means voltage drop so more amperage is required to produce the same "power". If the system has been running faultlessly and using max 8A until external sources chewed the wire and shorted it out then a 10A breaker should be fine.

[straightshooter]

It's not that simple.
If the short circuit current of the solar panel is not significantly greater than 10 amps and there is some resistance in the wiring then a 10 amp circuit breaker will either take too long to trip to be effective or will never trip. As with all things electrical circuit breakers have performance specifications as distinct from their nominal rating.
Please take a picture of the rating plate on the solar panels and post it in this thread in order to get some sensible advice.

[Die Judicii]

As most qualified electricians would say, and, especially so in 12v, BIGGER is BETTER, regarding voltage drop and distance.

[yoshie]

I'd put a ceramic fuse carried on it instead of a CB and leave a selection of spare fuses of different ratings. That way if you get nuisance tripping at 10A you can put in a 15A.

[Larry]

Personally I think you are looking at the wrong solution to the problem. Sure a cow would love to chew on the wires its a dangerous situation for a cow. To stop yourseld from loosing stock or your life you really need to fit a RCD a residual current device that looks for small leakage currents to ground rather than the current from the active to neutral. Good luck and stay safe.

[Jorlcrin]

Personally I think you are looking at the wrong solution to the problem. Sure a cow would love to chew on the wires its a dangerous situation for a cow. To stop yourseld from loosing stock or your life you really need to fit a RCD a residual current device that looks for small leakage currents to ground rather than the current from the active to neutral. Good luck and stay safe.

???

Where am I finding Active, Neutral and Ground on a DC circuit??

DC Solar Panel => Circuit Breaker => DC Pump

Very low chance that I'm going to lose any bored livestock, or myself, from electrocution on the wiring setup as described..
(Unless the rats/cows start a fire, but thats why I'm installing a circuit breaker..)

I do appreciate the advice I've received so far, but from a number of replies, I think my question has been misunderstood.
[Likely badly worded by myself, and perhaps too much info to confuse the question..]

I was asking that for a given DC voltage and given power load, does the length of cable run change the current load at the source(ie the solar panel, where the breaker is installed).
I know the length of cable run does change the voltage over distance(due to resistance of the wiring), but I was wondering if same occurs with the current.

Anyhoo; I shall test out my 10amp install, and see if a dead short will trip it as I expect.
If the 10amp fails when the pump is installed, I'll put a 15amp breaker in, create a dead short, and see what transpires.
But first, I'll see what the 10amp does.

Thanks again for the advice from everyone.

[yoshie]

Personally I think you are looking at the wrong solution to the problem. Sure a cow would love to chew on the wires its a dangerous situation for a cow. To stop yourseld from loosing stock or your life you really need to fit a RCD a residual current device that looks for small leakage currents to ground rather than the current from the active to neutral. Good luck and stay safe.

???

Where am I finding Active, Neutral and Ground on a DC circuit??

DC Solar Panel => Circuit Breaker => DC Pump

Very low chance that I'm going to lose any bored livestock, or myself, from electrocution on the wiring setup as described..
(Unless the rats/cows start a fire, but thats why I'm installing a circuit breaker..)

I do appreciate the advice I've received so far, but from a number of replies, I think my question has been misunderstood.
[Likely badly worded by myself, and perhaps too much info to confuse the question..]

I was asking that for a given DC voltage and given power load, does the length of cable run change the current load at the source(ie the solar panel, where the breaker is installed).
I know the length of cable run does change the voltage over distance(due to resistance of the wiring), but I was wondering if same occurs with the current.

Anyhoo; I shall test out my 10amp install, and see if a dead short will trip it as I expect.
If the 10amp fails when the pump is installed, I'll put a 15amp breaker in, create a dead short, and see what transpires.
But first, I'll see what the 10amp does.

Thanks again for the advice from everyone.

In answer to you original question, no current doesn't drop with distance, voltage dose (still the same amount of electrons going down the cable). The power is a product of current multiplied by volts. When the pump works it uses power, the power is uses is constant, if the voltage is low it will consume more current to do the work (P over V x I) . (This is why 110V appliances burn out quickly and you shouldn't use travel adapters). You will find that the pump is using more current the longer the cable so nuisance tripping is more common, a thicker gauge cable will minimise voltage drop. The reason why transmission lines are super high voltage is because voltage drop is proportional to distance, up the volts and loose less of a percentage of total generated power then transform it down in the neighbourhood it going to do the work in.

[Larry]

As Yoshie says. Sorry with my post not sure where my mind switch tracks from what was read to the answer to the problem in my head. I think there must have been some stepping stones in there somewhere.

[Jorlcrin]

In answer to you original question, no current doesn't drop with distance, voltage dose (still the same amount of electrons going down the cable). The power is a product of current multiplied by volts. When the pump works it uses power, the power is uses is constant, if the voltage is low it will consume more current to do the work (P over V x I) . (This is why 110V appliances burn out quickly and you shouldn't use travel adapters). You will find that the pump is using more current the longer the cable so nuisance tripping is more common, a thicker gauge cable will minimise voltage drop. The reason why transmission lines are super high voltage is because voltage drop is proportional to distance, up the volts and loose less of a percentage of total generated power then transform it down in the neighbourhood it going to do the work in.

Yoshie,

Many thanks; you might have answered my question about the volts==current relationship.
I think I'll trial the 10amp, and I have heavier cable ordered in the event it keeps tripping out.
I'll also order a couple of spare 15amp breakers, as a backup plan..

Someone suggested a fuse, but I wanted something that can be re-set manually, and doesnt require the person (checking the site) to have any spares.
[It's about 22kms from the homestead, so a long run home to retrieve spares..]
Anything left there(like spare fuses) run the risk of being lost, or stolen by Bower Birds..
At least the thieving little feathered Nigerian Scammers cant steal the breaker I've bolted to the back of the panels...

Thanks for that(and to all who offered advice/ideas.)

[Jorlcrin]

UPDATE..

FINALLY got around to installing the first of the two replacement DC solar pumps.
While doing the install, I plugged a load meter into the power cable, to see what draw I was getting at the panel.

The cable we used for this setup was around 27 metres, and it seems the draw at the panels is around 8.9 amps.
The cable for this install wasnt quite as heavy as I'd like, but it was the heaviest I could get at the time.
The second pump will be installed with a much heavier cable run, so I'll take note how much this affects the current draw at the panel.
The pump is rated at 7.8 amps, so it looks like I'm losing roughly 1.1 amps in the ~29 metres of cable and connections, though no connections feel to be heating up.
Not perfect, but I can live with that.
Pump has now been running for 3 days through night/day cycles, and overseer reports the tank is always full whenever he checks late afternoons.
I'm interested to see that the 10 amp circuit breaker has not tripped so far, though it's early days.

Thanks to all for ideas/advice; all good.