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.
Best Answer
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.