I'm installing a water heater when I opened the junction from the the house there was a black wire, a white wire and a black and a white wire nutted together. What is the purpose of this and what should I do?enter image description here

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    Can you post a photo of the junction box please, including its innards? – ThreePhaseEel Jun 30 '19 at 23:14
  • Likely, the black & white already connected go to the heater on one side and an overheat control (thermal fuse or thermostat) on the other. Check the manufacturer's site for a schematic, if it wasn't included with the papers attached to the device. – DrMoishe Pippik Jun 30 '19 at 23:27
  • The pic is the box from my house the two wires not connected are from the new water heater. I'm asking about the black and white wire that are connected together – Matthew Curry Jul 1 '19 at 1:06
  • How large a water heater is it? Is it on a switch? If so, that's a switch loop... regardless, this is a matter of idle curiosity. It goes without saying that you'll be leaving it alone. – Harper - Reinstate Monica Jul 1 '19 at 1:30
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    Can you post a photo that looks down into the box so that the back of the box is visible? – ThreePhaseEel Jul 1 '19 at 23:57

It's fairly likely that the power comes first to the heater, and then has a "spur line" going to the switch or thermostat. That "spur line" is wired as a switch loop, which by the way means you will not be using 240V smart switches.

Leave the black-white nutted wires together. This is less important here, but vitally important on 120V switch loops. Generally when doing swaps like this, take careful note (mark) which wires the old device was connected to, and don't disturb any wires except those.

The two capped-off wires are surely the ones that went to the old heater, and should go to the new heater.

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