If two 120V circuits shock you, is that 240V?
December 30, 2021 9:26 PM   Subscribe

Today I touched the live wires of two lightswitches at once. One thumb on each, each one a different circuit. I didn't trip any breakers or catch on fire, but is it possible I got a double dose at 240 volts?

As mentioned, they were different circuits/breakers and they switch lights in two different rooms. The wall they are on is mostly a third circuit, which is why that's the only one I turned off. Obviously, the house was wired by a maniac in the 70s, so there may be additional bad idea variables like shared neutrals.

My personal experience was mostly just a numbing of both hands and instantly moving them away. I've been shocked with house current a few times in my life, but I have no recent or vivid memories to compare. I wouldn't necessarily describe any shock as painful, but I don't feel pain normally. Really, the only notable part of the shock was that it hit two places at once.

I found a hilarious example of someone trying to have my experience, but I'm really only slightly smarter than this guy. I know enough to question if 120+120 still equals 120, but not enough to search or confirm anything.
posted by Snijglau to Home & Garden (7 answers total) 1 user marked this as a favorite
Best answer: Assuming this is in the US and the house is conventionally wired with split-phase 120/240V, it would depend on whether the breakers from each circuit were on the same or opposite 120V "legs" of the incoming feed. There's no way to tell this except by using an AC voltmeter, or looking at the panel box and the breakers in question.

Most modern panel boxes in the US are set up so that alternating breakers are on different "legs". In other words, breaker positions 1, 3, 5, etc. will be on one "hot" leg, while breakers 2, 4, 6, etc. will be on the opposite "leg", 180-degrees out of phase. In this sort of box, a regular 115V breaker will take up a single position and have a single-pole switch, while a 240V breaker like for an AC or stove will take up two vertical positions and have a double-pole switch, generally with the two switch poles linked together mechanically so they trip at the same time.

If that's the kind of panel you have, and the breakers feeding the two circuits you touched are adjacent (like 1 and 2, 3 and 4, etc.) or if one is even and the other is odd, there's a chance you got zapped with the whole 240V. If—and only if—you touched the "hot" side on both circuits. If you touched the hot side on one, and the neutral or ground on the other, you'd get 120V.

If you still have access to the circuits in question, a multimeter in AC V mode will quickly tell you. If they're different legs, you'll get ~220ish V when you measure between the two hots.

I've been told by old salts that it's possible to discern 120V from 240V by feel when it zaps you, but this is not a skill I've ever picked up. (And if you think that's interesting, check out this guy.)
posted by Kadin2048 at 9:47 PM on December 30, 2021 [9 favorites]

Voltage only makes sense as a difference between *two* points - each of the wires you touched had a potential difference from the neutral, varying at any given moment since we're talking about alternating current.

If both of the wires are ultimately connected to the same phase, both your thumbs were at the same potential relative to neutral. If they are on different phases, you'd have a potential difference across your thumbs depending on how out of phase they are, which could be more or less than 120V.
posted by each day we work at 9:48 PM on December 30, 2021

Entirely possible; in fact, there's a device on the market designed to plug into one outlet of each 120V leg of a typical US 120/240V system and deliver 240V at a third outlet on the device.
posted by jamjam at 10:28 PM on December 30, 2021

Best answer: (You didn't ask for electrical safety advice, but if you happen to be in the mood -- it's often advised to work with one hand where possible, since setting up a circuit across your chest cavity is how you get dead from the lowest current.)
posted by away for regrooving at 10:33 PM on December 30, 2021 [17 favorites]

If you're in an area which uses conduit, the two circuits are far more likely to be on separate phases and therefore it's more likely you "caught" 240V. (Conduit being a code requirement for residential building codes is pretty much limited to Chicago and its environs but I think some cities also require it in high-rise apts.) We have the option of using a "networked neutral" where the neutral wire is shared between two phases, allowing you to have two independent 120V circuits using only 3 wires rather than the usual four. (You did mention shared neutrals in your question- they work fine if properly implemented and are building code approved here.)
posted by Larry David Syndrome at 8:18 AM on December 31, 2021

And to clarify, shared neutrals between 2 different circuits on the same phase are indeed a bad idea, as the neutral conductor will potentially have double the rated current flowing through it.
posted by Larry David Syndrome at 8:25 AM on December 31, 2021

Response by poster: So, to check back in the next morning, yes. I touched 240 volts. Swapping thumbs for probes reveals ~242-245V.

Kadin2048's suggestion of a voltmeter is so obvious, it reveals how little experience I have with house current. I literally grabbed my NCV multimeter and outlet tester after getting shocked to check the rest of the room for power. But didn't think to go back and answer my own question by turning the knob to V~ and plugging in the probes.

Looking at the breaker box, I would describe the two circuits as #13 on the left and #8 on the right. I had presumed the two phases (which I only vaguely know about) were separated left/right, so alternating 1/2 was news to me. So thanks for that, too. 1+2, 3+4 is totally how my 240V Range/Dryer/etc are setup, I just never thought about it.

This kitchen has three walls and nine(!) circuits. Ultimately, I'm not even trying to do electrical work. I was tiling in a backsplash. So the whole situation was, well, shocking.
posted by Snijglau at 10:14 AM on December 31, 2021 [3 favorites]

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