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I am building a pond, and notice that pond pumps are available in incremental wattages and GPH. I'd like to choose a wattage that is safe, so in the event their is some electrical problem, no children are hurt touching electrocuted water. Is there a point where the wattage level becomes dangerous? If I keep below a certain wattage amount, does it make for no risk of death?

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    the pump is irrelevant ... what if you remove the pump and just leave the bare wires in the water? ... do you think that would be safe?
    – jsotola
    Commented Jul 1, 2020 at 16:36
  • That's not how any of this works. If you have the wiring, and the pump, installed per code, preferably by a licensed electrician, then GFCIs, circuit breakers, fully insulated connectors,etc. will guarantee no human risk exists. Commented Jul 2, 2020 at 12:19
  • I wouldn’t size the pump for safety. Just be safe, dangerous voltages depend on the person as well. There are credible accounts of heard defibrillation with < 50V and 750mA. Many people wouldn’t even notice that voltage at their fingertips.
    – mreff555
    Commented Jul 6, 2020 at 12:30

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The rated wattage of the pump won't really make a difference. If there is some sort of fault, it will likely allow whatever the supply has to give to flow. The solution you are looking for would be to make sure the supply you are attaching the pump to is ground-fault protected, by either having:

  • A GFCI-protected receptacle (GFCI built into the receptacle, or into a down-stream one connected through the LOAD terminals) or
  • a GFCI-protected circuit breaker

This will make it so that if there is a failure in the pump, the GFCI device will detect the failure and shut the power off.

If you are really worried about it, you may also look into low-voltage pumps. I am not sure if these exist or not, but I think this would be inherently less dangerous. However, GFCI protection is generally accepted to be perfectly safe for these types of uses.

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  • Indeed, a GFCI is almost certainly required by the electric code for this use case.
    – Nate S.
    Commented Jul 1, 2020 at 16:47
  • Low voltage pumps are widely available, though mostly as "connected to low voltage solar panels" rather than cord and plug to low voltage adapter (at least as sold.)
    – Ecnerwal
    Commented Jul 1, 2020 at 22:04
  • Any electrical pump has the potential for being dangerous, but the size of the pump has little to no effect on the danger level. If the pump fails, it will be with the mains electricity "leaking" out... I think that this is the best answer from a safety perspective, but getting back to the pond... You'll need a pump that is properly sized for your pond. That includes the overall size of the pond and allowances for fountains, waterfalls, and "lifting" water above the surface of the pond (if necessary) or, otherwise, you're not going to be successful with your pond.
    – gnicko
    Commented Jul 6, 2020 at 19:45
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TLDR

0.004 W is a theoretical safe limit for the power supply to the pump.

You’re not going to find a pump at this wattage and as long as everything is installed per manufacturer recommendations with appropriate circuit protections, you won’t be doing much to make things safer anyway.


Electrical current is what is fundamentally dangerous.

Depending on the path it takes through the human body, as little as 20 mA can be fatal.

As additional supporting evidence, the de-facto safety standard in medical device development (IEC 60601-1) specifies and tests only electrical currents with respect to patient safety (refer to section 8 of IEC 60601-1 Edition 3.1).

As an example of why voltage itself is not inherently dangerous without capacity for current, think about electrostatic discharge (e.g. rubbing your socks on the carpet and then touching a door knob). This discharge can be a very high voltage (10 kV is used as an approximation in at least one model). However, discharge's capacity for current is very low and is therefore not dangerous.

However, current does not exist without voltage.

You don't get one without the other because of Ohm's law:

  • Voltage = Current * Resistance (V=IR)

Therefore, higher voltages make it easier to supply current through circuit paths with higher resistance and increase the likelihood of electrocution.

Per NIOSH 98-131: "Under dry conditions, the resistance offered by the human body may be as high as 100,000 Ohms. Wet or broken skin may drop the body’s resistance to 1,000 Ohms. ... High-voltage electrical energy quickly breaks down human skin, reducing the human body’s resistance to 500 Ohms."

Putting the theoretical pieces together...

  • V_min_lethal = I_min_lethal * R_min_lethal

with

Therefore:

  • V_min_lethal = 1,000 Ohms * 0.002 A
  • V_min_lethal = 2 V

Theoretically, a voltage as low as 2V is potentially lethal!

And since you asked about wattage (i.e power) and P = IV, the lowest safe theoretical wattage is therefore 0.004 W. If you can find a pond pump that runs with those kinds of numbers... well good luck.

And to boot, it is really about the ability of the power supply (e.g. wall socket, marine battery, solar panels) to provide power. If a pond pump is working correctly, it should pose no risk to anyone. It is when it is in a fault condition (e.g. frayed wire, weathered insulation) that it becomes hazardous and at that point, the worst case assumption is the full power capable of being supplied by the power supply (with any independent safeties such as GFCI, circuit breakers, and fuses).

*The lowest resistance for the low-voltage model is used from the publication because the publication considers "high-voltage electrical energy" to be 600 V or greater and we are well below that threshold in this theoretical exercise.

Practical Perspective

From a practical perspective, is this worst-case scenario likely (or even probable)? No. People handle 12 V batteries on a daily basis and both 12 V and 24 V are very common in marine applications because of the significantly reduced probability of harm.

If safety is your primary concern, then the lower the voltage the better. However, proper and functioning GFCI circuit protections should make any solution about as safe as another.


An aside about the word "Dangerous"

There are a couple of potential harms resulting directly from direct contact with electricity:

  1. Electrocution
  2. Burn (caused directly by current running through tissue or by contact of the body with a short-circuited object)
  3. Fall (e.g. locked muscles while working on a ladder and you fall off)

I would consider each of these to be potentially dangerous (though of differing severity) even though electrocution is the primary concern voiced in the question. Luckily, the same mitigations more or less work well for all of them. :-D

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Welcome network visitors. We’re not talking pure science here; here on diy.se we deal in the practical world of Code compliant mains electrical installations. Code is very clear that voltage is the deciding factor as to intrinsic safety, which is what OP is asking about. If you have a theory question, by all means ask it on ee.se or physics.se; but that is not this question here now, and I am not answering a theory question. Here on DIY stack, we are aware of physics, but the laws we follow are passed by Legislatures.

Wattage doesn’t make it dangerous. Voltage does.

Therefore, reducing wattage is not a safety strategy.

The reason voltage is dangerous is that it is the motive force that pushes electrical current through the human body. If voltage is kept low enough, there simply isn’t enough force to harm a human.

This is not a barrier to success, because what does the actual work is power (wattage), and that is voltage x current. If you need to lower voltage, just raise current likewise, and you have the same wattage.

NEC (the model Electrical Code adopted by most US Legislatures) declares that up to 15 volts is generally recognized as safe in exposed outdoor conditions, so you can run any current you want to at 15V (or more likely, you would choose the very common 12V).

AC mains is 120V and up, and it’s not sized to be safe - it’s sized to be powerful. They have GFCIs which are pretty good, but not quite perfect.

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    hey.. to be more precise.. Voltage is not the danger. its all about the Amperage -- to elaborate, static electricity can be around 20000v .. -- then also conductivity is the other factor..
    – Hightower
    Commented Jul 2, 2020 at 6:24
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    @Hightower well, of course, technically speaking. But in a "supplying power to a pump" scenario, this is utterly irrelevant. Voltage is very much the danger in this case, in that it's the voltage that drives the current that kills you.
    – SiHa
    Commented Jul 6, 2020 at 8:48
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    A 12 V car battery is plenty dangerous and that is all about it's capacity for supplying current. Example: Accidentally arc-welding your wedding band to the car while handling a car battery. Also, voltage and current are both technically relevant though the current is what kills you (voltage makes it easier to deliver current through your body, but if a low enough resistance path is found through your heart, then a lower voltage could still potentially do it if it has enough capacity for current). Refer to IEC 60601-1. Commented Jul 6, 2020 at 9:08
  • @statueuphemism I think you mean “destructive”. If you want to claim “dangerous” you need to show a body count. Darwin awards where a person intentionally implanted electrodes specifically to bypass skin resistance don’t count :) Commented Jul 6, 2020 at 19:28
  • “Able or likely to cause harm or injury” (Oxford Dictionary). I think the potential for third degree burns and lost appendages count as dangerous by that definition. Commented Jul 6, 2020 at 21:23
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As few as 100 mA can stop a human heart. At standard US/NA 120 V, that comes out to 120V × 0.1A =12 watts.

Your not likely to find a useful pond pump at less than 12 watts. Furthermore, the power supply will certainly provide more than 12 watts of available power.

The only answer to be certain your pond is safe is to have everything is installed properly. A competent, licensed electrician is your best chance of completing the job properly.

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  • It’s not enough to say “the pump draws <100ma under normal conditions, there would need to be a device entirely outside the pool which limited current to non-dangerous amperage. Otherwise someone could be killed during abnormal conditions. Commented Jul 6, 2020 at 19:31
  • @Harper-ReinstateMonica that was the point of my second paragraph. If you can edit it to more clearly express that I would appreciate it.
    – psaxton
    Commented Jul 6, 2020 at 19:35
  • statueuphemism expressed the idea I was trying to get across much more effectively. I will remove this answer tomorrow.
    – psaxton
    Commented Jul 6, 2020 at 19:37

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