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I’m thinking about putting a tankless water heater into a shop with a 100 A feeder. Based upon groundwater temperature where I am, the model I’ve selected runs on 240 V and requires:

a minimum 125 amp electrical panel for installation with (1) 60 amp double pole breaker connected to (1) set of #6 awg wire.

I suspect that the requirement/recommendation for a 125 A panel is simply so that you aren’t using more power than your service can supply in case the tankless heater is running at the same time as several other appliances that are using electricity, though I’m not an electrician. My questions are:

(1) Does the electrical code (I’m in Canada) specify the maximum breaker amperage for a given panel amperage.

(2) If the code doesn’t prohibit this setup, is it otherwise unwise to do so.

In case it matters, this tankless heater would only be used to supply a utility sink faucet primarily used for hand washing. There are some other relatively large current draws in the shop, but since I’m generally the only one working in it I don’t expect to be washing my hands and, say, welding at the same time.

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  • The magic word is "service". A "service" is the thing that happens at the electric meter. A "feeder" is what happens after the main breaker e.g. when power is sent to an outbuilding. Please edit to clarify in paragraph 1, and state your electric service size also. (this is not a spelling flame; the whole question turns on it!) – Harper - Reinstate Monica Jan 18 at 19:24
  • This sounds to me like a very poorly written installation manual. I mean, what if someone had a barn with 80A service and the only things on it being a 20A circuit for lights and a 20A circuit for convenience outlets and a 60A circuit for hot water. – manassehkatz-Moving 2 Codidact Jan 18 at 19:36
  • @Harper Thanks for the response. I edited the question to hopefully fix the terminology. The shop is fed from some kind 100 A overcurrent protection device near the pole and connected to the metre itself. I don’t know the service amperage off the top of my head but I’ll see if I can figure it out when I’m there. – canadianer Jan 18 at 19:50
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    I got snipped for posting this as an answer so I deleted it and creating a comment.I believe your solution is way over-kill for your needs. While product recommendations are frowned upon here, I just want to share what I have in my shop. It's a 4 gallon, "point of use" water heater. It only draws 12 amps at 120v. It's installed right under the sink in my shop so the hot water arrives almost instantly. It's been perfect for light use and hand washing. Even if it runs out of hot water, it recovers within a few minutes. Deleted the product recommendation. – George Anderson Jan 18 at 20:10
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    @canadianer Totally up to you of course. Just making a suggestion. I'm getting to be less and less of a fan of tankless water heaters, esp. electric due to high power requirements. My little 4 gal water heater under my shop's sink does the job. And the idea of "keeping it hot all the time" isn't that big an issue if well insulated. I can't do the math without more info, but I'd imagine it'd take many years to get cost recovery / ROI for a large, high power tankless and associated wiring needs, vs. a very small tank type that you can just plug into an outlet. – George Anderson Jan 18 at 21:27
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The instructions are CSA-approved (or UL-approved; UL is now licensed to approve in Canada thanks to trade deals like NAFTA and GATT, thanks Bill Clinton). They are requirements and must be followed, unless you obtain an AHJ waiver.

The reason the UL-approved instructions require at least a 125A service is because a 100A service would not leave enough headroom remaining to reasonably power a modern electric house with presumably at least one other large electric appliance along with normal lighting and small appliance loads.

Subpanels are irrelevant to this type of service calculation. The old joke "I can't be overdrawn, I still have checks left in my checkbook!" really nails the difference between breaker spaces (and how they are distributed in subpanels) -- versus -- ampacity drawn by those loads.

Your house's calculations are based on the ampacity of the various loads (as well as square footage and some other odd stuff that goes into a proper load calculation). The arrangement of panels and spaces doesn't enter into it.

That said, you have to do another load calculation per subpanel to make sure you are not overloading that subpanel's feeder. But the 125A admonishment in the instructions does not apply to that.

TLDR: you're fine if your house's service is >=125A and it won't bust load calculations for your house or sub.

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  • Thanks, your answer helps a lot. Just to make sure I understand correctly: when the product description calls for a “125 amp electrical panel” what they mean to say is it requires a 125 A service, right? – canadianer Jan 18 at 22:00
  • @canadianer It's like when you buy tires and the tire says "85 MPH rating". That doesn't mean you HAVE to drive 85 mph :) It's saying "if you live in Wyoming and actually drive 85-90 get a better tire". So you mustn't feed a 125A panel with a 150A feeder, but anything from 15A-125A is fine. – Harper - Reinstate Monica Jan 19 at 3:15
  • Approval requirements are found in Canadian Electrical Code, Article 2-024 – NoSparksPlease Jan 19 at 4:08

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