Years ago I worked for a major commercial aircraft manufacturer. As a test tech I saw a guy with four circuit breakers all wired in parallel. One was a 2 amp breaker, the second was 3A and the third was 5A and fourth, 10A. He claimed he could select a variety of amperages all based on which breakers he had switched in. "Suppose you need 7A" he said. " Push in the 5A and 2A and you had 7 amps of capability." In theory he could select 2, 3, 5, 7, 8, 10, 12, 13, 15, 17, 18 and 20 amps, depending on the circuit he needed to test and protect while testing. I don't know why this suddenly came to mind, but is this actually possible? Suppose you're set for 7A and you run a 7A circuit. Wouldn't the 2A breaker pop?

If I had some breakers I could test to see for myself, but I thought this might be an interesting topic to discuss.

 

Something smells fishy, in order to work each breaker would have to carry exactly the amount of current it's rated for, that's not likely.

 

There is more to this than amps. There is also trip time. How fast is the breaker.

I worked in a lab once with its own panel. When I had a serious short the 100A main breaker popped and the small 20A breaker would not have enough time to trip. (breakers in series) Under a 1000+ amp short, the response time is most critical.

Under a 200% or 150% over current case, response time is still a factor.

In the case of parallel breakers and fuses, we don't really know if the current shares correctly. In the case where all breakers are the same model number and same current, I think two 5A breakers will share current. A 1A and a 10A in parallel, there is a hope that most of the current will pass through the 10A and only 9% will pass in the 1A but that is not certain. If the breakers are from different manufactures then correct sharing is not certain!

 

I didn't think it would work the way he was saying. The breakers were out of an aircraft, so I'm assuming they were bought in bulk and all the breakers would have been the same manufacture. They were the typical breakers you see in a cockpit - push in to energize, pull out to de-energize. Or they pop when too much current - - - you get it.

Anyway, this was a hypothetical question. I don't think I would have ever trusted parallel breakers.

Thanks for the feedback y'all.

 

I worked in a lab once with its own panel. When I had a serious short the 100A main breaker popped and the small 20A breaker would not have enough time to trip. (breakers in series) Under a 1000+ amp short, the response time is most critical.

That is indicative of what is called unbalanced feed, in other words an overloaded panel. I had a brand new 480V VFD installed on a project that failed and the shift electrician called me out in the middle of the night to the plant about it. Our electricians had very little experience with VFDs back then. Checked it over and could find nothing glaringly wrong with it so told him to re-energize it. He did and the substation's 400A 9600V main tripped taking out 2 operating areas. Substation was running near its limit. Later added a new substation and put each operating area on its own substation. Also had to replace the obviously failed brand new VFD that had less than 100 hours runtime on it.

 

One would have to query as to what design component would be required to ensure a division of current between such paralleled breakers. You would then somehow justify why a breaker would be designed that way if they wasn't intended to operate that way.
I would suggest it being the classic example of knowing enough to be dangerous. However, if a manufacture had a certified device intended for such, there you go.

 

240.8 Fuses or Circuit Breakers in Parallel. Fuses and circuit
breakers shall be permitted to be connected in parallel where
they are factory assembled in parallel and listed as a unit. Individual
fuses, circuit breakers, or combinations thereof shall not
otherwise be connected in parallel.

 

Many breakers are designed as "ganged" breakers. However, they are all equal amperages. If it was only amperage... But then thermals, electromagnetics, etc. come into play with unmatched amperages. Beyond my ken. Although parallel resistors do come to mind...

 

That is indicative of what is called unbalanced feed, in other words an overloaded panel.

no
I had a 100A panel running just my room. The total load ran about 20A most of the time. (Scope, iron, signal generator on 20A) and (some light on 20A) and (the power supply I am testing on 20A but drawing 5A) With every short I tripped the 100A and not any of the 20A. I had a electrician look it over and he put in a slower 100A breaker.

 

That is indicative of what is called unbalanced feed, in other words an overloaded panel. I had a brand new 480V VFD installed on a project that failed and the shift electrician called me out in the middle of the night to the plant about it. Our electricians had very little experience with VFDs back then. Checked it over and could find nothing glaringly wrong with it so told him to re-energize it. He did and the substation's 400A 9600V main tripped taking out 2 operating areas. Substation was running near its limit. Later added a new substation and put each operating area on its own substation. Also had to replace the obviously failed brand new VFD that had less than 100 hours runtime on it.

That's what I'd call an 'uncoordinated' protection scheme.

 

Years ago I worked for a major commercial aircraft manufacturer. As a test tech I saw a guy with four circuit breakers all wired in parallel. One was a 2 amp breaker, the second was 3A and the third was 5A and fourth, 10A. He claimed he could select a variety of amperages all based on which breakers he had switched in. "Suppose you need 7A" he said. " Push in the 5A and 2A and you had 7 amps of capability." In theory he could select 2, 3, 5, 7, 8, 10, 12, 13, 15, 17, 18 and 20 amps, depending on the circuit he needed to test and protect while testing. I don't know why this suddenly came to mind, but is this actually possible? Suppose you're set for 7A and you run a 7A circuit. Wouldn't the 2A breaker pop?

If I had some breakers I could test to see for myself, but I thought this might be an interesting topic to discuss.

@ElectricSpidey makes a good point about current sharing. So would it work better if the 2A breaker were attached to a length of 2A cable, the 3A breaker to the same length of 3A cable etc. with the cables commoned at the output socket, so that the cables would act as ballast resistors to make the current through each breaker proportional to the breaker ratings?

 

knowing enough to be dangerous.

So true. Couldn't agree more.

But things are coming to mind concerning a project I AM working on: I have a double 20A breaker like this one and a single duplex outlet installed in a weather proof box with weather proof cover. Each outlet is isolated from the other so that 20A goes from L1 to one of the two plugs and L2 goes to the other. It's mostly out there for convenience and to run the pond pump and filter; and some low amperage lighting under the pavilion. I am uncertain whether an overload on just one of the two legs would trip the double breaker. But should I need 240V I don't want to have a dedicated plug for such. It's easy enough to construct a double headed power plug to connect to the available 240V from the outlets.

 

@ElectricSpidey makes a good point about current sharing. So would it work better if the 2A breaker were attached to a length of 2A cable, the 3A breaker to the same length of 3A cable etc. with the cables commoned at the output socket, so that the cables would act as ballast resistors to make the current through each breaker proportional to the breaker ratings?

Good idea, but I am not planning such a device. I don't even know why this topic came to mind but I've been thinking about it for about two weeks now; just wondering if it was a sound proposition.

 

Good idea, but I am not planning such a device. I don't even know why this topic came to mind but I've been thinking about it for about two weeks now; just wondering if it was a sound proposition.

I'm glad you put "Theory Question" in the title! I'm not planning on making one either.
However, when I did my year as a student at Marconi High Power in Lincoln, the last department I worked it before setting off to university was "fusing". Just how difficult could fusing be? It was a bit of a rude awakening to find out just what a technical subject it was. We were dealing with diodes and thyristors up to several kA, and kV. I think some of the solutions involved combinations of different values and time characteristics of fuse.

 

Just for clarification:

If either side experiences a fault in circuit - will both sides trip? Or should I change this to a double 20A breaker with independent switching?

 

Both sides will trip as long as you don't remove the handle connecting bar.

Also be very sure to remove the common bar at the screw terminals.

Sorry the terminology is probably wrong.

 

be very sure to remove the common bar at the screw terminals.

Already has been done. I've known about that jumper bar for many years. There have been many instances where it was necessary to isolate one outlet from the other. Most often I've had to switch one side while the other side remained hot.

 

Paralelled breakers and fuses are only permitted if they are installed as a listed combination.

I.E. the manufacturer has to design and then have them independently tested by UL, ETL, CSA, etc. for use in that configuration.

Usually this is only done where multiple smaller devices will perform substantially better than one big one. Providing ultra-fast protection for large semiconductors, certain medium voltage applications, etc.

EDIT:

Misinformation above. Handle ties alone will not assure common-trip operation. Modern toggle-style circuit breakers are designed to trip free of the operating handle. That is to say, they will trip even if the handle is locked in the 'on' position. This means that there is only a very modest spring pressure available to drive the handle to the center 'tripped' position if applicable. Handle ties are only intended to assure that all tied breakers are opened simultaneously when they are being used as a disconnecting means. This is to help prevent 'neutral shock' during electrical work on multi-wire branch circuits and has nothing to do with the breakers' operation as overcurrent protective devices. In order to obtain common-trip operation, you need to use a common-trip breaker. Common trip breakers differ in that the trip mechanisms for all poles are deliberately interlinked internally. When one trips, they all trip.

Just for clarification:
View attachment 249402
If either side experiences a fault in circuit - will both sides trip? Or should I change this to a double 20A breaker with independent switching?

What you have pictured is known as a multi-wire branch circuit. Common tripping is not required, but all poles must operate in unison when the breakers are switched manually. That is to say, either a 2-pole common-trip breaker OR two single pole breakers linked with a listed handle tie will work for the above application. In the case of a fuse panel, you would have to use a 2-gang fuse pull-out. Two single pole breakers or fuses WITHOUT a handle tie or ganged pull-out would be a code violation, and arguably a particularly heinous one at that.

If someone were to switch off only a single-pole breaker on L1 without realizing that a second breaker supplying L2 is also part of the same circuit, they'd be liable to get shocked upon breaking the neutral conductor if there is any load across L2-N. If for instance L2 is pulled-through in a junction box where N and L1 are spliced, then it is a very easy mistake to make and a condition that is impossible to test for short of checking for neutral current with a clamp-on ammeter. The only warning you'd have is the presence of two 'hots' and one neutral in a given box - which is not even a dead certainty since people often use combinations of red and black or brown and orange to denote 'hot' and 'switch leg'.

Hence handle tie rules are adhered to religiously by anyone in the trade worth a damn.

 

Just Another Sparky thank you. That's extremely useful information. Much appreciated.

In my application on a 20 amp circuit breaker I used 10 gauge (because that was what I had handy, and it's "More than" as opposed to "Less than" adequate for the current rating). Plus, the wire length is less than three feet. To energize or de-energize that breaker I'm right there at the main disconnect breaker. Black wire is one of the two L's and the white is the other, with a black piece of shrink sleeve at the plug end and the breaker end. An additional white wire connects the neutral to the neutral bar and the ground is connected to the ground bar inside the panel.

I'm not a licensed electrician but I know a little. A VERY little. But the one thing I know best of all is to get sound advice from someone who knows. If I can't get that advice then the next best thing is to hire someone. And I have a "Someone" in my contacts list for just such purposes. And he's been arcing and sparking since the 60's.