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Mike Lamb

Tied Circuit Breakers

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It looks like someone was trying to tie together the halves of multi-wire circuits. 

They missed a few and, of course, it would be better to use real handle ties. 

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I'd write it up.  Someone might come along and not understand why they're there and take them off. At some point later, the breakers of a single MWC end up on the same pole, overloading the neutral and perhaps causing a fire.

Edited by Marc

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12 hours ago, Marc said:

Someone might come along and not understand why they're there ...

Like me. My grasp of electricity is not the greatest.  Many questions here.

So the tied breakers in question are sharing the same neutral? And if one breaker trips you want both to trip so the neutral cannot be overloaded?  And the ties should not be removed?  And tying the two breakers together with something other than electrical wire should be done?

I also had to look up the acronym MWC. Minor Work Certificate?  That would mean someone other than a licensed electrician did the work?

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17 hours ago, Mike Lamb said:

But why?  And is it a bad idea?

With a multi-wire circuit, if you were to cut power to one breaker and not the other (or others in the case of 3-phase systems) and then attempt to work on the circuit, the live neutral could bite you. That's why the 2008 NEC began requiring a simultaneous disconnect at the circuit's breakers. (Before that you only needed a simultaneous disconnect when different parts of the multi-wire circuit fed two receptacles on the same yoke.) 

My guess is that this installation pre-dated the 2008 requirements but someone decided to try to make it safer anyway. (Maybe a home inspector told them to do it.)

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1 hour ago, Mike Lamb said:

So the tied breakers in question are sharing the same neutral? And if one breaker trips you want both to trip so the neutral cannot be overloaded?  And the ties should not be removed?  And tying the two breakers together with something other than electrical wire should be done?

In your picture, each set of tied breakers (and some sets that aren't tied) form "mult-wire circuits" (MWCs). 

These save wire by using 3 wires (instead of 4) to serve 2 circuits. To do this, the 2 breakers must originate on different poles of the system, so their power is 180-degrees apart (for a single-phase system). In this way, the shared neutral only carries the difference in current between the two circuits. Long ago, there was no rule about having a simultaneous disconnect between the two breakers (or fuses). The danger of a system like this is that if you cut power to one half of the circuit you could still be shocked by the current on the neutral from the other half of the circuit. In 1981, MWC breakers had to be tied together only for those circuits where both halves of the circuit delivered power to two receptacles on the same yoke. (Often the dishwasher and disposer.). Then, in 2008, all MWC breakers had to be tied together. The ties are there to ensure a *simultaneous disconnect*, not a *common trip*; its still possible for the breakers to trip independently. 

The danger that Marc referred to is that someone might re-arrange the breakers in the panel, unaware of the presence of MWCs. If you were to move the breakers or wires such that two halves of a MWC originated on the *same* pole, then the shared neutral would not carry the difference in current, but rather the sum of the current. This could overload the neutral and cause it to burn. By installing formal handle ties, you also signal to future amateurs that these two breakers "go together." 

 

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Hi, newbie here, just trying to learn. Could they also be a 240 circuit with a cheezy way of using 2 singles as a double? How could you tell from the photo that they were all MWCs? Thanks, John

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Good question. You can't always tell. It's just a guess. In most cases, however, a regular 240-volt circuit won't require a neutral and the installer won't waste money installing a cable with an unnecessary neutral wire. So most 240-volt circuits will re-identify the white wire and use it as one of the "hot" wires.

They could be 120v/240v circuits - where an appliance uses both voltages. Those would be indistinguishable from a multi-wire circuit when viewed from the panel. However, there aren't too many 15-amp or 20-amp 120v/240v appliances. (Electric ranges come to mind - they often need both 240-volts for the elements and 120 volts for the controls, but they're almost always 40 or 50 amps.) 

If you were inspecting the house, you'd be able to tell for sure simply by observing what electrical appliances were installed. 

The overwhelming likelihood, though, is that they're MWCs, which are very common and which, until relatively recently, required no handle ties. 

 

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20 hours ago, Jim Katen said:

Good question. You can't always tell. It's just a guess. In most cases, however, a regular 240-volt circuit won't require a neutral and the installer won't waste money installing a cable with an unnecessary neutral wire. So most 240-volt circuits will re-identify the white wire and use it as one of the "hot" wires.

They could be 120v/240v circuits - where an appliance uses both voltages. Those would be indistinguishable from a multi-wire circuit when viewed from the panel. However, there aren't too many 15-amp or 20-amp 120v/240v appliances. (Electric ranges come to mind - they often need both 240-volts for the elements and 120 volts for the controls, but they're almost always 40 or 50 amps.) 

If you were inspecting the house, you'd be able to tell for sure simply by observing what electrical appliances were installed. 

The overwhelming likelihood, though, is that they're MWCs, which are very common and which, until relatively recently, required no handle ties. 

 

Hi Jim. Got to get picky here. Technically, any circuit that has two hots and share a neutral is a multi-wire circuit....even range and dryer circuits.....according to NEC definition. The main difference in requirement would be that if supplying a 240 volt appliance, a common trip would be required. That being said, I think what Johnny was asking was about multi-wire circuits that supply two 120 volt loads and how to identify. Your explanation totally covered his question. You know how nerdy I can get.

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Thanks for the clarification, Mike. Yes, a 120v/240v circuit is, by definition, a multiwire circuit as well. So that slightly simplifies the answer to Johnny-A's question. The difference between a MWC and a 240-volt circuit is that one will have a neutral and one won't. (Well, that and the circuit sizes.) 

 

 

 

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12 hours ago, mtwitty said:

Hi Jim. Got to get picky here. Technically, any circuit that has two hots and share a neutral is a multi-wire circuit....even range and dryer circuits.....according to NEC definition. The main difference in requirement would be that if supplying a 240 volt appliance, a common trip would be required. That being said, I think what Johnny was asking was about multi-wire circuits that supply two 120 volt loads and how to identify. Your explanation totally covered his question. You know how nerdy I can get.

When someone knows as much as you do, nerdy is good.  Keep it up.

BTW, which edition has the 'multiwire' definition?  All I have is the 2008 and it doesn't have it.

Edited by Marc

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