Using a 30-amp tandem circuit breaker for a 120/240v circuit?

Question

I inherited a main panel box from a homeowner that wasn't always fond of meeting the required NEC codes. As such, there are some oddities in the panel. One such oddity is the fact that he installed a couple of tandem circuit breakers for use in 120/240 circuits.

Specifically, there are two Murray MH-T 30-amp breakers. These are tandem breakers and not "slimline" or "thin" breakers -- the two breaker switches on each are not connected in any way. One of these breakers is used for the clothes dryer and the other for an electric water heater.

In the case of the dryer circuit, there's one 10/3 NM wire, with the hot black attached to one of the sides of the tandem breaker and the hot red attached to the other side. The neutral is connected to the neutral bar. In the water heater case, there's a 10/2, with both the white and black attached to their respective sides of the tandem breaker.

I want to stress that this absolutely is working. Both the dryer and water heater were wired this way well before I moved in and it's only now, years later, that I noticed what was going on. It's worked fine all along.

This seems all sorts of wrong, though. It's still a single pole breaker, even if there are two 120v circuits coming off it. It seems like if this was okay, that I'd hear about using a tandem breaker as a space-saving slim 220v breaker... but I've seem nothing.

So, what's wrong with doing this? What's the ramifications if I don't change these two out with traditional double-pole breakers?

EDIT: Added photo of the breakers in question:

The very top is a double-pole 20-amp breaker that's fine. The first 30-amp tandem is the dryer breaker and the second is the water heater.

EDIT 2: Added photo of the dryer breaker popped off the panel:

You can clearly see the two buses and that there doesn't appear to be any shenanigans connecting the two of them. I removed the top double-pole breaker for clarity. The popped dryer breaker is clearly a single pole, as you can see the one entry slot. It's apparently connected to the left bus and only that bus.

Answer 1

It sounds like you may be mistaken as to how this is wired, or that perhaps I'm just not understanding your explanation. As others have mentioned, it's not possible to get 240 volts from a single pole in a 120/240V split phase system. Each tandem breaker provides 2 120 V circuits, this is true. However, if you measure between the terminals on a single tandem breaker, you'll get 0 volts. This is because the terminals are both powered from the same leg, and so are at the same voltage potential. If you measure from a terminal on the top tandem breaker to a terminal on the bottom one, then you'll measure 240 volts. This is because each breaker is connected to a different leg, which are each one half of a 240 volt circuit.

With all that said. For this setup to work, one appliance would have to be connected to both breaker. Something like this...

Notice that each appliance circuit has one wire connected to each of the tandem breakers. In this situation, you'd need a device like Speedy Petey shows.

Which ties the breaker handles together, to provide common trip characteristics.

Notice how the inner handles are tied together, and that the outer handles are also tied to each other. This way if either trip (or are turned off by the user), the entire circuit is shut off.

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If this is wired the way you've explained, where the dryer is connected to the top tandem and the heater is connected to the bottom. Then there's some magic going on in those breakers.

Answer 2

What you have will electrically work fine, but it is not code and not safe for anyone working on these units.

You need to replace those two twins with one of these:

Answer 3

Modified to match to new provided information. Speedy Petey's answer contains the circuit breaker you need to install to correct the situation. As you can see from the breaker inter-ties, they basically take two of these breakers and manually finish off what the installer was attempting to do in a safe manner.

Murray MH-T MP3030 (this unit gets its power from one leg to produce two 120V circuits)

The setup is working because each breaker pair feeds off the proper panel bus leg (One attaches to L1, the other to L2). From your added information, each 240V circuit is split across the two breakers to do this.

What's Wrong -- The major safety concern here is that with a circuit fault, only one of the breakers in the set may trip, leaving the 240V circuit partially energized. For instance if the dryer heating coil burns through and one end touches ground, that side will trip, leaving the other leg hot. You definitely want a circuit fault to totally power down both legs.

Also related is the fact that anyone not paying attention to the panel setup may power off the circuit to work on it and not realize they switched off adjacent pairs instead of alternate pairs of each 240V circuit, leaving a hot leg in the circuit they think is powered down. You want things to be fail-safe. This isn't fail-safe, but is a booby trap waiting for the inattentive or hurried. NEC frowns heavily on things like this.

Murray bus backplane with interleaved fingers so adjacent full size breakers are on opposite legs. Not true for the MH-T MP3030 breaker pair, each breaker in the set is half-width and the combination is fed off only one leg blade.

Answer 4

I have read that tandem breakers can be mounted to either feed power from one leg of the bus bar or from two different legs. If it's the latter in your case, then you'd be getting 240V after all. Check the terminals of the breakers in question (or check at the outlets where your appliances are plugged in) with a voltmeter, if you haven't already, and confirm what is actually being fed to your appliances.

Answer 5

In US household electrical service, you CAN'T get 240V from a single pole. If you have 240V across them, the breaker is straddling both busses. You just can't get 240v from the one bus.

The only issue is connecting the two switch handles. Obviously a nail or piece of wire will work but it looks shoddy. There actually is a real product for this, called a circuit breaker handle tie. I just bought some myself at the local big orange home improvement store to replace a piece of wire stuck through one of mine.

http://www.platt.com/platt-electric-supply/Circuit-Breaker-Accessories-Handle-Ties/Eaton/THS1/product.aspx?zpid=6847

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Pop the two breakers out, and slide the pins into the two handles.

There are also other ones that are metal clips that go over and around the two handles.

Answer 6

I ran across a panel today that has a water heater and a dryer hooked up using tandem breakers. They are not split either. This panel is configurable to half a slot, meaning a regular 110VAC breaker would be clipped into 2 prongs of a live bus, and there is a small plastic wall separating the B side from the A side keeping you from shorting the two 110VAC circuits together. However, the tandem breakers have a slot cut out allowing you to plug into both sides. Each side of the tandem breaker gets its own feed from its own prong on the buses making 220VAC.