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I am confused something. I want to install a tankless water heater. Mounting the device, the plumbing, wiring, routing to the 200amp panel is all easy and straight forward.

Where I get stumped is the two breakers. 75A (2x40)A 240 8 AWG

The directions show to run 2 8/2 with ground. The Illustrations show to connect both black(hot) and white(N) to the two prongs on the breaker. I totally get 240v using a black red and white. but 240 connecting the Neutral to a hot I do not get.

I guessing something like a dryer really is has two components using 120v each sharing a nuetral and we call it a 240v. But true 240v is doubling up on the hot but what about the neutral?

If anyone can explain this I'd appreciate it.

UPDATE Thanks for all the comments but seriously guys, getting all hung up on the "White is just a color" is odd. I know a white wire is a wire.

So when you connect a wire to the hot and another wire to the neutral (in this case they are (pink and blue) on the panel. Then connect the other ends of these 2 wires to a 120v outlet. The current being AC travels in alternating directions on this pair of wires.

So if I instead connect both of these wires to hot and connect the other ends to lets say one of the heating chambers of a tankless water heater. I got lost on how the alternating current occurs the same on 2 hots rather that a hot and a neutral.

That my question. I am not asking how the current knows it's on a pink wire.

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  • For the answer to your edit, basically when one hot is +120V, the other hot is -120V, for a difference of 240V -- that's how we get the 240 in the first place. Which one is plus and which one is minus, and therefore which way the current flows, changes 60 times a second -- the alternating part of alternating current. Take a look at this question to see it in pictures: diy.stackexchange.com/questions/33602/…
    – Nate S.
    Jan 11 at 17:34
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Confusion from failure to use marking tape

Yeah, I know. Marking wires with electrical tape to indicate function seems like such a needless chore. "I only need to understand it right now. I will never be in here later troubleshooting!" This of course ends up not working 1/4 as well as expected. Some of the people who are marking-hostile write installation instructions.

Since UL approves installation instructions, I'm surprised they don't make the instructions follow Code. Because in this case, Code sides with me.

It is illegal to use white as a hot.

However, if you are buying pre-manufactured cable, what you are allowed to do is use paint, tape, shrink tubing, etc. to re-color a white wire to be an appropriate hot color.

But it's mandatory.

So the most straightforward answer is to take your #8 supply cables, and after you're done stripping off the cable sheath to fit, go ahead and wrap the white wire with red electrical tape.

enter image description here

You can also use black, since there's no particular need with a water heater to match up phases. A 240V circuit with black-black hot wires is perfectly fine.

If it were in conduit, that would be a different deal. Because individual wires are easy to swap, you're required to use the correct color in the first place - no remarking is allowed.

If you're absolutely militant about refusing to mark wires, then my only advice is to use /3+gnd cable (black red white) and just cap off neutral. That would allow you to use one of the cables for a subpanel if the tankless heater doesn't work out.

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  • Your pictures are always very helpful!
    – FreeMan
    Jan 11 at 16:41
  • @SteveSether Since it's easy to use the correct color wire, you must do so. Remarking white is a concession only granted in cables where there's no other option. And even then, electricians in the field don't do the cable marking they're required to do, and inspectors let them get away with it. Hang around here awhile and you'll see probably 50 unmarked hot whites for every one properly marked. Jan 11 at 19:08
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TL;DR: Your tankless heater is a 240V "only" circuit and the instructions assume cable.

There are 3 types of circuits and 2 kinds of wiring methods in typical US residential construction:

Circuit Types

120V

This generally includes lighting (though modern LED lighting can often handle 240V just fine), "regular" receptacles around the home and most hardwired devices - e.g., dishwasher, disposal, ceiling fans, exhaust fans, doorbell transformers, etc.

2 wires (hot + neutral) needed + ground.

240V

This generally includes appliances (either hardwired or dedicated receptacles) that need a lot of power. This includes electric hot water heaters (whether tank or tankless), electric resistance heat, EV chargers and sometimes shop receptacles - e.g., for welders or other big tools.

2 wires (hot + hot) needed + ground.

120V/240V

This typically includes clothes dryers, HVAC and ovens.

3 wires (hot + hot + neutral) needed + ground.

The neutral is used in order to provide 120V power to lights, controls, etc. that don't require 240V. In reality, anything made for 120V can be redesigned for 240V. But it saves a little money, and manufacturers know that in a residential installation combination 120V/240V circuits are available, so they make use of it.

Wiring Methods

Individual Wires in Conduit

This is more common in commercial/industrial construction, but can be used in residential and is required in a few places (e.g., Chicago). Individual wires are used of various colors:

  • Green or Green/Yellow or bare = Ground
  • White or Gray = Neutral
  • All other colors (black, red, blue, yellow, etc.) = Hot

Except for large wires (not used typically in residential construction past the main panel, except occasionally to connect to a subpanel), hot wires can't be white.

In some conduit installation, the conduit itself provides the ground. In others, a ground wire is required.

Cable

This is what most people are familiar with in US residential construction. Commonly called Romex after a very popular brand name, this includes individual wires inside an outer covering. The colors are standardized. The most common combinations are:

  • 2-wire = Black + White + bare ground
  • 3-wire = Black + Red + White + bare ground

In a perfect world, the colors used would be the same as with the conduit wiring method. However, there are two exceptions generally permitted:

  • Switch Loop - In a switch loop, both wires are hot - one is always hot, one switched hot. In this situation, the convention is to use the white wire as always hot and the black wire as switched hot and mark the white wire with colored tape on each end. But the tape is rarely used, and sometimes falls off, resulting in "why is this white wire hot?"

Note that new switch loops in many locations require neutral, in which case black/red/white, white being neutral, black always hot, red switched hot (though technically it could be red always hot, black switched hot.)

  • 240V circuit - In a 240V circuit (no 120V needed), you only need two wires carrying current (the ground wire is different). Since the standard 2-wire cable only comes in Black/White, code allows using it - with white as a hot wire - for 240V circuits.

Back to Your Circuit

Instructions should really not assume any colors. If your area allows cable and you feel like using standard cable then "black = hot, white = neutral" is, in fact, correct. However, if your area only allows conduit, or if you choose to use conduit for other reasons, then you may not use white for neutral. You can then use black + black or black + red or blue + yellow or any other 2 colors not designated for neutral or ground.

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Soo.. maybe the question, or at least the remaining portion of it, is this: "how does current flow between two hots, ie when there's no neutral connection?"

To understand the reason, consider the source of the power. It comes from a transformer, which is basically just a coil of wire. There's something special about how utility transformers are constructed and connected in North America and some other places around the world.

The "special" part about the construction is that these transformers have three connections rather than two. The extra connection is called a "center tap." It literally comes off the center of the coil of wire inside the transformer. The special part about the connection is that we drive a stake into the dirt, wire it to that center tap connection, and call this our "neutral" point.

The remaining two connections on the transformer both are called "hot." But note that they're at opposite ends of the transformer coil. This means that power will flow when we connect a load from hot-A to neutral, or if we connect a load from hot-B to neutral... or, if we connect a load from hot-A to hot-B and ignore neutral entirely!

For historical reasons involving perception of safety and I-don't-know-what-else our North American convention is to use the hot-to-neutral 120 volt connection for nearly all of our loads. We generally use the hot-to-hot 240 volt connection only for high-power loads, which in a home are almost always resistance-based heaters of some kind - like your tankless water heater. These appliances are sometimes designed so that they operate exclusively from a 240 volt supply (as all appliances do in many other parts of the world). In that case there's no need for neutral.

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  • Wow. Thank you. That is a really good illustration with words. Thanks again.
    – jtpntx
    Jan 12 at 12:36
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You are confusing color with function.

White is not Neutral. White is a color found in cables; the function of the wire can vary, in a cable.

At least in the US Market, /2 cables have black, white and ground (bare or green)

/3 cables have black, white, red and ground.

For a true 240V load (no 120V components) there is no need for neutral. For those loads, a /2 cable with hot white and hot black is the norm. A polite gesture (which I believe is now also required by code, but did not used to be) is to mark the white wire with red or black tape at each end, but "the function is (or should be) obvious."

In a conduit rather than cable situation, white or gray is (or should be) Neutral, since you are free to pull two blacks, or any two hot colors, since wires in conduit are individually supplied as needed - but in cables, you get what's made, and code permits not buying a /3 when you only need a /2, so white can be hot in that case.

In short - beware of assuming that color implies function, since it sometimes does not.

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  • Thanks guys and yeah get that it's just a color. In my mind completing a circuit requires the neutral as current always travels back to the source. I don't get the physics of it and have yet to find an explanation I know the basic difference in AC/DC but I seem to be stuck in 120v mode
    – jtpntx
    Jan 8 at 15:40
  • 2
    Current does flow back to the source. The source, in this case, is a 240V transformer secondary (outside your house), which is "center-tapped" for the neutral. The neutral is tied to ground, so each side of the 240V secondary is 120V to ground (and Neutral, since Neutral is bonded to ground at a single point), while "hot to hot" is 240V. "Hot to hot" is a complete circuit.
    – Ecnerwal
    Jan 8 at 15:53
  • The confusion comes about (as Harper often talks about) because people think "neutral is ground" (because they are bonded) + "in DC electronics, everything goes back to ground" + "for a 120V circuit, everything goes back to neutral". The catch is that ground & neutral don't work quite that way in mains AC and a circuit can simply be two hots with ground as safety valve only. Jan 8 at 18:09
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White is a color and on 240 you have no neutral so the white is used as a hot this is code compliant fro 240v only loads if there was 120v in the heater it would require a x-3 with ground wire. Switch loops also allow the hot to be on the white. White is a color and usually used for neutral but not always and almost never marked or reidentified in this case. There are ways if 120 was needed to make it from 240v today switching power supplies are common, in the past transformers were used but most of the time everything in the box is 240v so no neutral is needed.

If Just to clarify 240 is leg to leg, 120 is leg to neutral either leg can be used. It is not doubling up each leg of the 120 are out of phase by 180 degrees so it takes both to get 240. this lower voltage to ground reduces the shock hazard by lowering the voltage the voltage to ground is 120. 240 volts to ground is common in other countries.

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The white is allowed to be used as a hot in specific conditions such as shown in the diagram they supplied. Here is the code reference that allows it:

NEC 200.7 Use of insulation of a white or gray color...(C) Circuits of 50 volts or more. The use of insulation that is white or gray.. for other than grounded circuit conductor..only ...(1) if part of a cable assembly that has insulation permanently reidentified...where the conductor is visible... If used for...switch loops..shall only for the supply to the switch...

"Grounded circuit conductor" is NEC terminology for the neutral.

You could use a 8/3 cable and use the red as the second hot conductor, and insulate the unused white, but that would increase cost and not add any electrical benefit.

Your explanation of a 240v circuit seems cloudy, a typical residential 240v service is a 240v supply from a single transformer winding. The center of the winding is tapped and grounded. Both ends and the center are supplied to the service panel in your house. 120v loads are connected to either end of the 240v winding and to the grounded middle point of the winding. 240v loads are connected to both ends. https://external-content.duckduckgo.com/iu/?u=https%3A%2F%2Ftse1.mm.bing.net%2Fth%3Fid%3DOIP.tdsOyR0oKQ1zCYi0oUC0QAHaDd%26pid%3DApi&f=1

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