Using a 3-phase wire for 240v on-demand water heater

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Old 04-09-12, 10:35 AM
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Using a 3-phase wire for 240v on-demand water heater

Hey folks,

A friend of mine purchased a used on-demand electric water heater. The person he bought it from gave him some #8awg wire that he used. It's got a black, red and blue which means 3-phase (right?).

Would it be against code to use this wire with the blue being unused? I assume you would de-rate the cable because of the number of conductors?

Thanks
 
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Old 04-09-12, 11:12 AM
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No reason to derate but if there is no ground you can't use it. I presume when you write wire you mean a flex conduit containing wires (whip)? If cable what type of cable? Individual wires not in a raceway (conduit or sheath) can not be used.

Is the water heater 3-phase or single-phase? What are its electrical requrements in amps?
 
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Old 04-09-12, 11:53 AM
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The water heater is single-phase. 7.9kW, breaker recommended 40A 240v could go with #8/2.

The wire he was given is #8/3 (black, red, blue, ground) AWG THHN Stranded. Is it frowned upon to use #8/3 with unused blue vs using proper #8/2.


edit: Accidentally said the heater was three-phase when its indeed single phase. Apologies
 

Last edited by bakrr; 04-09-12 at 02:51 PM.
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Old 04-09-12, 12:05 PM
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The water heater is 3-phase.
Then the water heater requires 3-phase power. Three separate hot phases. That's why the cable or whip has three wires designated for ungrounded power. Does your friend have that?

breaker recommended 40A 240v
3-phase 120V power is 208/120. 240 volts is only available in residential single-phase - two-wire - services.

Is it frowned upon to use #8/3 with unused blue vs using proper #8/2
If the water heater requires three hot wires the blue is not an "unused" wire. How would you wire it with only two hots? Is there a way to change its requirements by changing some of the internal wiring?

A picture of the nameplate on the heater might help (How To Include Pictures). The make and model of the water heater might help. Or a link to the specs or manual for the heater.
 

Last edited by Nashkat1; 04-09-12 at 12:25 PM.
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Old 04-09-12, 02:50 PM
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-edit- sorry I mis-typed in my previous post saying that it was a 3-phase heater. I meant single phase heater but he received a wire typically used for 3-phase systems (black red blue).

The on-demand water heater is single phase.

He was given a wire which is for 3-phase systems being a black, red, blue and ground. The on-demand water heater only requires #8/2 being single phase 240v.

Can I use a #8/3 with the blue being unused because I only require 240v single phase power.
 
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Old 04-09-12, 03:40 PM
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Please read my first reply.
No reason to derate but if there is no ground you can't use it. I presume when you write wire you mean a flex conduit containing wires (whip)? If cable what type of cable? Individual wires not in a raceway (conduit or sheath) can not be used....What are its electrical requirements in amps?
40 amps is low for an on demand WH. Often they require two separate 60 amp feeds.
 
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Old 04-09-12, 04:00 PM
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The on-demand water heater is single phase.
OK, that makes more sense.

Can I use a #8/3 with the blue being unused because I only require 240v single phase power.
I think you could. Others may differ.
 
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Old 04-09-12, 06:52 PM
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Would it be against code to use this wire with the blue being unused?
It wouldn't violate the NEC, but as far as Canadian codes, I don't know. What is the application for this heater because ray brings up a valid point. Is this heater just for a single kitchen sink or dishwasher?
 
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