Sadly they don't explain WHY not to use it with an extension cord. I wonder if it's to do with the risk of having such connections outside in the elements?

The cord it comes with is 7m long, and where we want to site it is 6m from our outhouse. I've drilled a 1ft long hole through the stone wall of the outhouse to thread the spa's cord through... But once in the OH the plug still ends up 3m away from the socket. Would it be considered ok to run a standard 13A indoors extension cord to bridge the gap?

The OH socket is grounded (I'm in the UK) and on an RCD-protected circuit, and the spa has an RCD in its cord (and another in the pump/heating unit, I think).

Maybe their concern is voltage drop in the thinner conductors of a typical extension cord. Maybe I can buy a "heavy duty" one which would be largely equivalent to wiring a new socket in?

Or do I need to just admit defeat and wire a new socket in?


  • 1
    What risk are you referring to? Commented Apr 7, 2020 at 12:34
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    The average homeowner doesn't have waterproof extension leads. They have indoor extension leads. So I wondered if the manufacturer's concerns centre around the use of indoor leads outside, and them becoming wet
    – Codemonkey
    Commented Apr 7, 2020 at 12:45
  • 14
    My guess is it is because of the lawyers. If you try to make a list of conditions that you could use an extension cord and somebody gets hurt some lawyer is going to find some weakness and drive explosives through it. That's not to say I think it would be safe or unsafe, I don't know the detailed criteria that they would include if it wasn't for the lawyers.. Commented Apr 7, 2020 at 15:12
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    Well, if it's because of the lawyers, that'd make it alright then. Come to think of it half of NEC is "because of the lawyers" so we can throw that in the bonfire too. Commented Apr 8, 2020 at 16:23
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    @Harper-ReinstateMonica There are very few outdoor rated extension cords available, and anyone who doesn't know about them clearly doesn't know about them so safer for the manufacturer just to say "no extensions" than to say "only use a suitable extension" and get sued for not explaining it properly.
    – MikeB
    Commented Apr 9, 2020 at 10:40

10 Answers 10


I think you have likely answered your own question. The use of an extension cord could cause a voltage drop, especially since the factory cord is long already and the spa probably has an electric motor that would be damaged by voltage drop. Also, cord/plug connections are not weatherproof.

This is a CYA (cover-your-arse) by the manufacturer. Your plan of using a short heavy duty cord (same wire gauge as the factory cord) with the connections indoor should be fine.

  • 2
    Note- your plan will work; however, it may void the warranty if there were a problem of some sort and the maker became aware of your setup... Commented Apr 7, 2020 at 14:09
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    Yes - Short and 10 or 12 gauge - and rated for outdoor. The danger is the extension cord over-heating. Each plug and receptacle adds a small amount of resistance. "Heavy" is also applied to 14 & 16 gauge extension cords. Commented Apr 7, 2020 at 14:46
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    To be clear - I am not suggesting to use 14 or 16 gauge, just that 14 & 16 gauge extensions are also called "Heavy". - Use 10 or 12 gauge. Commented Apr 7, 2020 at 15:02
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    In catastrophic scenarios, improper use of extension cords can even nullify home insurance. Glad you did extra research before messing with electricity.
    – Nelson
    Commented Apr 8, 2020 at 2:32
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    Because the cables proper (proper rating/gauge provided) are equivalent the point of problem must be the connector. With household connectors it's probably impossible to guarantee large enough contact surfaces, creating resistors with corresponding heat which with long-running, high-current consumers can be a hazard. Even if it does not start a fire it compromises insulation and plastic case, resulting in potential short-circuits or unintended current paths through carbon. Commented Apr 8, 2020 at 12:01

The basic reason is that if they said, "Feel free to use an extension cord", someone would connect up a 50m, 3A, two-core cable with no Earth and a loose socket. They would then sue the company when it didn't work (too much voltage drop across long thin extension cables) or it caught fire (current drawn exceeds capability of thin extension cables) or they got electrocuted (no Earth).

In reality, if you use a short (2-3m), heavy-duty, 3-core (with Earth), outdoor extension with a good stiff socket and shield it from spillage, you should be OK. However, remember that, as Abraham Lincoln once said, "don't believe everything you read on the Internet".

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    This. "Extension cord" in any "don't use an extension cord" warning means the flimsy things sold at dollar stores, Kmart, Roses, etc. that are rated for 7 amps or less. It doesn't mean an actual proper extension cord with ground and sufficient wire gauge and insulation to safely use with power tools, equipment, etc. Commented Apr 9, 2020 at 2:20
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    Considering Lincoln died in 1865, he was truly ahead of his time with quotes like that.
    – Mast
    Commented Apr 9, 2020 at 19:50

I have always heard that the reason for the "no extension chords" is because they are not supposed to be used for permanent installations, only temporary, i.e., Christmas lights. Extension chords are pretty cheaply made and the ones I have had fail on me always fail at either the male or female end. If you're going to keep your tub inflated year-round, think about adding a new socket. Enjoy it,


The manufacturer will be guarding against both voltage drop on the supply (which could affect the operation) and creating too high a fault loop impedance (which could affect the disconnect time of the circuit breaker in case of a fault).

At a guess, a minimum length, heavy duty extension lead will likely be fine.

However, for peace of mind, if you can readily add an extra socket in the right place, I’d suggest that is the correct thing to do.

  • I tried to buy the requisite stuff for a new socket while I was food shopping this evening... I failed; they had sockets but no cable. Instead I bought a heavier-looking duty single socket extension cord, and when I got home I shortened it to the correct length to minimise voltage drop.
    – Codemonkey
    Commented Apr 7, 2020 at 22:22
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    Just been out to feel it after it's been running a few hours and the cable didn't feel even vaguely warm to the touch so let's hope all is ok!
    – Codemonkey
    Commented Apr 7, 2020 at 22:23
  • Hello @Codemonkey, I always read, that you shorten the premanufactured cords. I dont think that this is a good idea. Since it is easily done, it still is a flexible and moving cord, that may break when moved around a lot. I could be safer if you installed a wall socket. there are no moving wires in it and the cords plugged into it are still new... Commented Apr 8, 2020 at 10:06
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    This is now stapled to the wall and not moving. It'll certainly do until I get some cable to add a dedicated socket.
    – Codemonkey
    Commented Apr 8, 2020 at 13:14
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    Four answers down, only here is the correct one: The circuit breaker might not activate in case of a short circuit due to the increased resistance in the connection once a (unspecified!) extension cord is connected. That is indeed the real reason. Commented Apr 8, 2020 at 18:07

As with all Q&A on this site, answers are in the context of the question.

The issue is electrical drownings

One thing people really don't understand about electricity and water: a shock that would be trivial anywhere else is fatal when water is around.

You're hit with a triple whammy. First, ordinary water and wet skin conduct electricity well, meaning a shock you barely felt before, now has much worse effect.

"Water doesn't conduct electricity" only true for artificially demineralized water; water does not like to be demineralized. Demin is corrosive and will attack anything it can leach minerals out of. The city's water source is not demin, and they don't entirely demin it; it would eat the pipes if they did.

Second, when water is around, a stun is as good as a kill. People in electrical drownings weren't electrocuted to death. They just lost control of their muscles and fell down into the water. They did not raise their nose out of the water because they were unable; either their muscles would not do what their brain commanded, or they were laid unconscious by the fall (or the shock). See how this becomes a domino effect: your legs malfunction, your head cracks the ground, which knocks you unconscious, and then, you drown.

And third, the same thing happens to the people who try to rescue you. In one famous drowning in a fountain, one girl fell, another girl went in to rescue her, the other two went in to help them, and all four died.

That's how it goes; the third or fourth cohort of rescuers recognizes they're looking at a victim and a rescuer who has become a victim, and does not repeat the mistake.

Of course it's made worse by the first factor; people who have been bit in the past and live to tell the tale, start thinking they're immune to electricity, and don't realize they've just been lucky so far due to being in the dry.

There is no way to insulate an extension cord connection. If it gets wet, it will leak current.

"Oh, but my house is protected by the bog-standard Euro 30ma RCD! I'm fine!"

Nope. Personnel protection is not the purpose of Euro style 30ma RCDs. 30ma will stun pretty much everybody, and will directly kill a more vulnerable person. That is why North America requires 6ma GFCIs where the objective is personnel protection (e.g. kitchens/bathrooms/pool areas).

Now, your hot tub's electrical cord may have a lumpy 6ma GFCI/RCD on its electrical plug. Obviously, the extension cord connection will be before that, so, not protected.

  • "There is no way to insulate an extension cord connection" google water proof electric cord extension box. You will find that you were wrong.
    – Questor
    Commented May 1 at 18:28
  • @Questor Sorry, we give answers here that comply with the Electrical Code, not folk knowledge. Challenges to answers need a code citation. Also, this is a Q&A format, all statements are in the context of a question, not blanket statements. Your claim doesn't work in the context of the question. Commented May 3 at 19:11
  • Seeing as water proof junction boxes for extension cords are IP54 Water Resistant certified for water proofing extension cord junctions, it is not folk knowledge.
    – Questor
    Commented May 3 at 19:33
  • If you don't know what that means it means that a certifying agency has looked at the box and said "yes this makes a power extension junction water proof".
    – Questor
    Commented May 3 at 19:34
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    But that doesn't make it permissible in the application. Commented May 3 at 19:58

I have seen the same rules on devices for a bathroom. In my opinion its all about exposing bare wires. If the two (the extension and the power cord) are not a waterproof connection and resting on a floor (very common), then there is risk of shock. That is far more likely in a bathroom (or near a hottub) where water is common.

I believe a better way is to replace the current power cable with a longer, properly rated, power cable. That cable would connect to the hot tub (You will likely have to open up a cover where the current power cable is screwed into the electrical of the hottub) and plug directly into the outlet without any breaks.

Obvious disclaimer: Its better to pay an electrician than trust a random guy on stackexchange.

  • 2
    This really is the best advice here, not sure why it has not received more votes. Commented Apr 10, 2020 at 17:29
  • If I could easily have bought some external fat-gauge armoured cable in this time of lockdown, I might have gone that route. But I think wiring in a new socket is preferable anyway, once I get around to it...
    – Codemonkey
    Commented Apr 14, 2020 at 8:30

To Avoid Fire

As noted in other answers and comments, there are issues related to wire size and length that can cause overheating if not appropriately dealt with, and the issue of waterproofing the connection, both that can lead to shorts and fires.

So You and Your Family and Your Friends Don't Die

A person, just out of the hot tub, is covered with water, walking around dripping it on the ground, it's pooling in to puddles, their barefoot (who wears shoes in a hot tub?), they're in the UK where power is 240v and likes to really mess things up if shocked, AND then let's effectively put an outlet and plug on the ground nearby in the form of a cord to extension cord connection. It's as if someone just got out of the shower and then throws water at the wall socket and then puts both hands all over the wall. Sooner or later, it simply a matter of time, someone is going to get a major shock, or worse, electrocuted.

Related, recall how smart and vigil we are, and think how much is the same for the average person, then recall that half the people in the world are below average, and that the instructions are written for everyone, not just us.

They make those statements as a CYA yes, but also a COA (Our in place of Your) because we really do need protecting from ourselves, at least some of us do. It's very likely a government mandated item that the instructions note not to use the extension cord.

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    um, the OP states in the question that the plug and cord connection will be inside of a building like 20' away from the spa... Commented Apr 7, 2020 at 20:50
  • @JimmyFix-it yet, the OP asked this as well: "WHY not to use it with an extension cord. I wonder if it's to do with the risk of having such connections outside in the elements?"
    – Ack
    Commented Apr 7, 2020 at 22:01
  • If there's any damage to the cable, you and your family are still going to be toasty critters, with or without an extension cable. That's why we have RCDs, and RCDs make this answer irrelevant The instructions are simply wrong - they should say to use an RCD instead, and then the whole extension cable thing doesn't matter.
    – Graham
    Commented Apr 8, 2020 at 11:52
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    ... In the UK, wiring regs for some time have required all sockets to go through an RCD for new installs. With or without that though, plug-in RCDs have been cheaply available for over 30 years, and all advice for that time has been to use one on every outdoors electrical appliance. Anyone using a hot tub without an RCD is frankly too stupid to deserve to live.
    – Graham
    Commented Apr 8, 2020 at 11:53
  • 1
    @Codemonkey Fair enough - if there's one in the cable to the tub, that explains why they don't want extra extension cables. As you say though, if you do have an extension cable then you want something to cover that. For myself, plug RCDs are cheap enough that it's a no-brainer to put them on anything which connects to outdoors, just on general principle. Just by the way, we do have a hot tub, and we do have an extension cable, but that extension has an IP67-rated enclosure round the soocket and it's hung off the ground with drip loops on the cables. And an RCD indoors, of course.
    – Graham
    Commented Apr 8, 2020 at 13:37

Depends on the current requirement of the tub

Your tub will likely have a label that specifies the rated current draw and/or rated power. If only power is stated, divide Watts by RMS voltage (220V) to get amps required. You mention using a 13A cord - comparing to the rated draw of the tub is necessary to determine if this cord is up for it.

The adequacy of the 13A cord will probably turn on whether your tub is electrically heated. Some tubs have gas or propane heating, and others heat electrically. An electrically heated tub would undoubtedly require much more amperage.

This is in addition to the other very valid considerations already mentioned.

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. Commented Apr 8, 2020 at 13:13

Earthing protection is an important consideration. You cannot have a situation where you may not be earthed...even temporarily.

  • 2
    UK power plugs (and many others) are designed in a way that when you plug them you get the earth first and then the power. When unplugged, you lose first the power and then the earth. It is the same on the extension cord.
    – fraxinus
    Commented Apr 8, 2020 at 9:49

In the EU-style consumer power grids (220-240v, separate ground) these marks "don't use extension cords" are put on all appliances that draw 1kW or more. AFAICRecall, every space heater has it.

I don't know the reason, but it has to do something with the risk of overloading a single outlet (~3.5kW) by plugging few heavy loads into it. The most probable outcome of overloading is usually a tripped breaker, but sometimes the breaker allows for some tolerance and contacts or cables overheat.

IMHO, if you use a good extension cord and promise not to plug a total of more than 3.5kW, you are OK. Avoid using a cord longer than you need, avoid covering the cable w/ carpets or likes.

And for sure, avoid water near the plugs.

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