At what wattage does a pond pump pose a danger to human life?

Question

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?

score 6 · Answer 1 · answered Jul 01 '20 at 16:01

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.

Harper - Reinstate Monica · Answer 2 · 2020-07-06T20:05:08.737

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.

statueuphemism · Answer 3 · 2020-07-06T11:46:47.657

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

I_{_min_lethal} = 0.002 A (20 mA)
R_{_min_lethal} = 1,000 Ohms*

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:

Electrocution
Burn (caused directly by current running through tissue or by contact of the body with a short-circuited object)
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

score 0 · Answer 4 · answered Jul 06 '20 at 08:13

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.