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I bought an electric heater such as this one:

Electric heater

The manual mentions this part:

The heater should not be connected to adapters or an outlet multiplier.

The heating device should not be placed below an outlet power socket.

I translated it from Portuguese:

A escalfeta não deve ser ligada a adaptadores ou fichas triplas.

O aparelho de aquecimento não deve ser colocado por baixo de uma tomada de corrente de posto fixo.

This was probably automatically translated itself since two expressions have no definite meaning ("adaptador" and "tomada de corrente de posto fixo").

I imagine that the first recommendation refers to the practice of "daisy-chaining" outlet multipliers to one another. I have been using it on an extension cord with a multiplier connected to a power outlet. It has been working fine. I'm not sure about the second recommendation.

What do the recommendations mean, and why?

Update with details on volts, amperes, and watts

The power strip (made in Germany) says 3680 W, 16 A, 250 V~. Appliances currently connected are:

  • the electric heater mentioned above: 230 V, 50 Hz, 70 W
  • an upright heater: 220-240V, 50 Hz, 2000 W
  • a laptop: Input 100-240 V~, 1A, Output 14.85V, 3.05A
  • a USB charger: Input 100-240V, 50-60 Hz, Output 5V, 0.5A
  • a monitor: Input 100-240V, 0.7A, 50-60Hz, Output 14V, 2.14A
  • a desktop with a C13 connector: 10A, 250 V~

It seems to me that, for the first recommendation, I should be fine to use all of these at the same time, except for the desktop and the upright heater which both take over 2000 W. Is that correct?

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The first recommendation would be related to the amount of current such a device draws. If one uses an outlet multiplier or a power strip, one can add other devices which would increase the power draw, placing the chain in danger of overheating, fire or tripping a circuit breaker. One could argue that the operator can take the responsibility of not connecting other devices into the multiplier. This can be valid, but legal statements such as these "protect" the manufacturer.

My experience with outlet multipliers is that the quality can be suspect. A poorly made one may work fine with today's lower power electronics but have failures when subjected to high currents such as those drawn by heaters.

The second caution follows that heat generated by the heater should be be applied to the power outlet and power cord.

I have a different stand-alone heater which is connected to a power strip. The power strip power cord does not get warm, but the heater cord does get warm. If both levels of heating are engaged on the heater, providing maximum heat, the power strip pops/opens the breaker circuit, terminating power.

This places the responsibility of safe operation on me and I never activate both levels of heat simultaneously.

One could expect that if you observe caution, you can be successful operating your heater in a manner inconsistent with the safety instructions. If you can trust that others in the household will also use appropriate caution, all the better.

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    Adapters include any device to extend the length of the designed power cord to an outlet. Extension cords shouldn't be used for the same safety reasons stated above.+ – JACK Feb 16 at 15:14
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This answer brought to you by the phrase "An inconvenient truth".

Those aren't recommendations, those are requirements. I suspect something is lost in translation, because we say "should" for advisory and "must" for mandatory.

Your belief that "surely they mean outlet multipliers fed off other outlet multipliers" is a case of you reading what you want to read, not what it says. It clearly outlaws use of even one outlet multiplier.

Now, if you need more outlets, you can extend off your existing outlets using whatever permanent wiring methods are approved in your jurisdiction. In America you might extend from a flush outlet using a surface conduit system such as Legrand Wiremold. Given how much European housing is ancient and stone/brick-based, I'm sure that's quite common there.

It sounds like you have other high-current-draw appliances and you should review very carefully how they are connected. The PC doesn't necessarily draw 10 amps actually; that depends on what it is doing.

I have been using it on an extension cord with a multiplier connected to a power outlet. It has been working fine. I'm not sure about the second recommendation.

Well, that's exactly what they told you not to do. Their intention is clear (and perfectly reasonable given the high current pull of a heater): Plug this only into a wall receptacle directly. Simple enough! Do that.

The reason outlet multipliers can work is that 99% of what people plug into them are small loads - lights (0.1 amps these days), TV (0.2 amps), cable TV box (0.2 amps), PC (2 amps), various USB chargers (0.05 amps) or other wall warts (0.1 amps) etc. So this doesn't add up to much, compared to circuit capacity (~13 amps).

A heater uses the lion's share of circuit capacity, so that crazy chain of multipliers (typically cheap ones from China) end up seeing far, far more current than they typically do. If you read the fine print on your extension cord, it will tell you it's not a heavy duty cord and not to use it with big loads like heaters.

They do make heavy duty "air conditioner grade" extension cords, of typically 12 AWG; if you have no other option, use that cord and nothing else between the receptacle and the heater.

  • The numbers are quite informative and I updated my question with the information from the labels. – miguelmorin Feb 17 at 14:40

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