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It's enough to power the heater, no problem. But the breaker current is determined by the section of the wire that's protected by it. The thicker the wire, the higher the breaker's rating can be, but you can't have a 30 amp breaker with thin wire behind it. I don't know the figures by heart, … Read more
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I don't know what your purpose is. If it's a boiler with a hot water reservoir, it should be on all the time. Otherwise you water will be lukewarm parts of the day and this poses a risk to get legionella bacteria in your water causing legionnaire's disease. Which can be deadly.
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Southern Spain: Very mild North of Sweden: Extremely cold And everything in between, you can't give a general answer for whole of Europe. In Western Europe consider that, the closer you get to the sea, the smaller the fluctuations because the sea water acts as a buffer.