Electrical wiring question for instant water heater

Question

I am installing a point of use Chronomite SR-20L/240 HTR 240-Volt 20-Amp SR Series Instant-Flow Low Flow Tankless Water Heater. https://www.amazon.com/Chronomite-SR-20L-240-HTR-Instant-Flow/dp/B001B1KGUQ

I have called the company and they tell me that I need two pole 240v, 20 or 30 amp breaker. They said that it is wired with two hot and one ground 10 gauge wire. This is a new install for a bathroom I am building in my garage. I am having a problems finding romex type wire that has two black (hot) wires or a black and a red (hot) wire and a ground wire. They told me if I used 10/3 wire with a ground to cap the neutral wire. I need about 35 feet of wire.

My concern is with my inspection and having the proper wiring colors for my hot wires. If I get 10/2 with a ground, I will have a hot,neutral and a ground, but the neutral will be white. Please advise what the name of the particular wiring I need is called.... that has a black,red and a ground wire. Thanks!

Answer 1

Interesting. In Canada the cable you need exists (2 conductor 12 or 10 AWG, red and black jackets, with bare ground), is commonly used to wire baseboard heaters at 240V and 20A, and can be found at any HomeDepot. The cable is even made in USA! See here for 12 AWG: https://www.homedepot.ca/en/home/p.electrical-cable--copper-electrical-wire-gauge-122---romex-simpull-nmd90-122-red---10m.1000108220.html; and here for 10AWG:https://www.homedepot.ca/en/home/p.electrical-cable--copper-electrical-wire-gauge-102---romex-simpull-nmd90-102-red---75m.1000108211.html

If you can't find it in the USA, then see Harper's taping solution in another answer.

Answer 2

You are allowed to use a white wire for a hot wire, but you need to mark it with tape at both ends. The tape can be any color besides white, gray, green or copper, as those denote other things.

If you want to be fancy and use wire natively colored the official colors, use 10/3. Wire-nut or tape the unused neutral.

If your heater is 16 amps or less actual, 12 AWG wire will suffice unless the wiring is a very long run. However, you may be disappointed by the hot water output. I would go for a larger heater that requires 10 AWG.

Just as an aside... marking wires is not allowed in the other direction. You can't mark a colored wire to be neutral, and you can't mark any wire to be a ground (though you can strip all the insulation off.)