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How can i wire directly an Asian single 240 volt line with neutral instant hot water heater to my 240 twin line tap in the United States? Can i tie the two legs on the breaker into a single wire? I am not sure how the neutral would feed then. What is the best way?

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Asian and US power work in a very different way. Both transformers are 240V. However Asia applies safety earthing on one end of the transformer's 240V winding. America pegs it in the middle. Philippines is a weird compromise because they have both systems and are in the middle of trying to harmonize on one.

The concept is that American power is safer since voltage is only 120V from earth.

Neutral is not earth/ground. Neutral is a normal current-flowing conductor like the hot(s), it is simply the one that uses the same transformer lug as earth. That makes it the "less dangerous" conductor. It is not earth and should not be connected to earth anywhere except one bonding location specified by Code.

Some appliances are built with weaker insulation on the conductor planned for neutral. Code doesn't allow them to do that, but they do it anyway. Not mentioning any subcontinents here.

Your water heater wants 240V across its two supply terminals. in Asia, one of them will be neutral, in the US neither will be. This use needs to be permitted in its labeling or instructions as approved by UL, CSA or other competent listing agency. CE is not a listing agency.

In a US installation of a five-continent 240V device, you will have no use for US neutral. You will need two hots, and this must come off a 2-pole 240V breaker.

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    Odds would be excellent that his device is not listed and therefore connecting it is a good way to void your household insurance, at the least. – Ecnerwal Feb 21 '18 at 19:41
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Absolutely, if it's a traditional immersion type, you don't need neutral just use only the 2 hots (1 from each leg). Suggest anyway to bring a 4-pole (L1, L2, N, G) cable, just in case.

But if your boiler is heat-pump it's a problem as it's (almost surely) 50hz while in us 60 hz is used.

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  • I'm not sure I see an "in case" where neutral would be useful on a water heater line. No 240V water heaters use neutral. – Harper - Reinstate Monica Feb 21 '18 at 18:10
  • In case you'll replace it with a heat-pump one, where neutral may be needed. – DDS Feb 21 '18 at 18:12
  • I hear you, but I would be surprised to see one of those. It would prevent them from bolting up to existing water heater circuits, forcing people to rewire. The appliance industry is hyper-sensitive to that, hence their constant lobbying to retain dangerous NEMA 10. – Harper - Reinstate Monica Feb 21 '18 at 18:18
  • @Harper Doesn't that go against the DIY.SE mantra to "go bigger than you need right now, cause cable / wire / panels / what-have-you is cheap" :) – mmathis Feb 21 '18 at 18:21
  • @mmathis yes it does. But as we're seeing in another question, confusion about what to do with an excess neutral has issues of its own. – Harper - Reinstate Monica Feb 21 '18 at 18:22

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