From what I've seen and understood, electric water heating elements are usually copper tubes/pipes with electricity running through them to heat up the surrounding water. I'm not sure if the electricity directly goes through the tubes/pipes though. How do the heating elements make sure electricity only heats up the water and not run through the water, causing unwanted currents through the water instead?


The element is not made of copper (the resistance would way too low). Normally it'd be nichrome (nickel+chromium) and then enclosed.

This is a straight quote from wikipedia

Tubular (sheathed) elements normally comprise a fine coil of nichrome (NiCr) resistance heating alloy wire, that is located in a metallic tube (of stainless steel alloys, such as Incoloy, or copper) and insulated by magnesium oxide powder.




It is used extensively as an electrical insulator in tubular construction heating elements. There are several mesh sizes available and most commonly used ones are 40 and 80 mesh per the American Foundry Society. The extensive use is due to its high dielectric strength and average thermal conductivity. MgO is usually crushed and compacted with minimal airgaps or voids. The electrical heating industry also experimented with aluminium oxide, but it is not used anymore. It is also used as an insulator in heat-resistant electrical cable.

The expansion CTE ought to match or in between the metals used. Cu and NiCr.

please report back with all values.


The conducting element is isolated from the exterior tube by a ceramic powder spacer layer that serves as an insulator. There is no electrical contact between the tube and the element (or at least, there shouldn't be.)

More here: https://en.wikipedia.org/wiki/Heating_element


enter image description here

Figure 1. Heater construction looking from the heater towards the terminal. Source: Omega - Electric tubular heaters.

The magnesium oxide provides electrical insulation while giving reasonably good thermal conduction. The helically coiled wire helps when bending elements and avoids buckling due to thermal expansion.

This low-tech construction method YouTube video may help. Note in the video the annealing of the tube using very high current at low voltage to make it very hot.

anneal /əˈniːl/ verb

gerund or present participle: annealing

1. heat (metal or glass) and allow it to cool slowly, in order to remove internal stresses and toughen it. "copper tubes must be annealed after bending or they will be brittle"


More modern heaters like tankless water heaters use unisolated wire heating elements that have direct contact with the water. In other words, the tap water itself is the only isolator. There is no danger since tap water has a high resistance. Advantage is faster heat transfer, less losses and less lime stone coating since the thin heating wire is expanding and contracting with temperature changes. So lime stone coatings are automatically blown off. But they need sophisticated electronics to quickly detect low/no water flow and possible air bubbles, to avoid overheating.

See also https://physics.stackexchange.com/questions/211847/why-dont-get-i-shocked-in-an-electric-shower-if-the-resistance-makes-contact-wi/268356#268356

  • 1
    \$\begingroup\$ I'm not sure this is correct. Mains voltage electricity in contact with tap water does not sound like it would even be legal in the US or Europe at the very least, and it's certainly not safe. Especially if the heating element breaks. \$\endgroup\$
    – Hearth
    Aug 24 '19 at 11:52
  • \$\begingroup\$ You can be very sure this is correct. See de.wikipedia.org/wiki/Durchlauferhitzer Seems to be missing in the english and french wikipedia pages. Citation of the original text in German: "Das Wasser wird bei vielen modernen Geräten mittels eines Blankdrahtheizelements erhitzt, bei dem die Heizdrähte direkt vom Wasser umspült werden. Das Leitungswasser wirkt isolierend gegenüber den leitenden Teilen, ..." \$\endgroup\$
    – xeeka
    Aug 24 '19 at 12:18
  • \$\begingroup\$ It is safe, and those direct inline water heaters are sold and installed since many years all over this planet. But for marketing reasons, the manufacturers are not very much interested to spread the information about using heating wires with direct water contact. The reason can be seen in this very question plus reactions, since even many technicians do not have enough knowledge to judge the (non-) danger. \$\endgroup\$
    – xeeka
    Aug 24 '19 at 12:33
  • \$\begingroup\$ Maybe it's safe while working as intended, but the possible fault conditions seem too dangerous to even consider using. \$\endgroup\$
    – Hearth
    Aug 24 '19 at 14:49
  • 2
    \$\begingroup\$ Be cool, man! All of us get occasional downvotes and they're not necessarily from those who wrote critical comments. Instead expand your answer to explain how electrocution is avoided in, for example, a shower. I had never heard of bare wire water heating until today. \$\endgroup\$
    – Transistor
    Aug 25 '19 at 14:35

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