ECN Forum Forums General Topics of the Trade General Discussion Area Parallel Breakers?

From an engineering standpoint, it doesn't seem to be much of a power risk- if one of a paralleled breaker pair trips from an overload, the other will trip in turn, regardless of whether they're yoked together or not. And it's impossible to overload the cabling. Really, I think this makes more sense than paralleling transformers, as there is less risk of a damaging overcurrent fault.

As to whether they come like that at the factory or not, that seems kinda BS. So long as you can retrofit a yoke, I see no problem- that was the only way we ever did it on naval ships, we'd buy the individual breakers and then a 2x or 3x yoke depending if it was a "1-phase" or 3-phase circuit. I really fail to see how it could make a difference being factory or field assembled. I wonder if there were problems in the past with improper field assemblies that precipitated this?

I would never dream of doing such a thing unless you were locking the handles together. Otherwise, unless the breaker characteristics were exactly the same, one might trip first. If the O/L cleared before the 2nd breaker tripped, you would have a 200A feed until someone noticed or the next O/L. If it was for a split feed, they wouldn't be parallel. Also, could you be absolutely sure that each breaker was flowing 50% of the load?
Joe

Joe is absolutely right! there is no way to guarantee that the load will always equally divide itself between the two fuses or breakers.

Of course he is right the current will never divide perfectly evenly.

However it will divide evenly enough not to matter in the least assuming the circuit is reasonably identical.


Again I am just pointing out it works and many manufactures take advantage of this.

I do agree in regards to the opening post it is wrong and he needs a single 400 amp device.


Without a doubt one will trip first.

I would like to know how the other would not open fairly quickly at over 200% it's rating and if it did not open the safety hazard that would create.

Iwire:
Of course he is right the current will never divide perfectly evenly.

I would like to know how the other would not open fairly quickly at over 200% it's rating and if it did not open the safety hazard that would create.


So if we consider the possibility of uneven loading, even 2 breakers with identical curves, would be operating at different points on their curves. So the O/L condition might be trailing off as the first breaker is tripping. The 2nd breaker wouldn't trip. Now you have a 200A service instead of 400A and wouldn't know it. I don't see it as a safety hazard as much as a nuisance. But why wouldn't you tie them together if you were going to use them in place of 1 breaker? Can you imagine how tightly individual breakers would have to be inspected to ensure a match along the entire curve? Considering I just got an Agastat that didn't have its mounting holes tapped, I don't think I have that much confidence. User adjustments on the breakers would make things really interesting.
Joe



[This message has been edited by JoeTestingEngr (edited 07-01-2006).]

Joe I think your grasping at straws.

That is as likely as me hitting the lottery.

Bob

Just for fun, most thermal mag breakers will begin to open @ approx. 1.1 x handle rating in 1000 seconds. @ 2 x handle rating in 20-40 seconds.
That would be continuous load for the time period. Would a residential service see 400A load for >20 seconds to activate the thermal trip? It's got to be a big house.

Joe I think your grasping at straws.
That is as likely as me hitting the lottery.
Bob

Bob, I think that you are either:
1.) Overly optimistic of installation variables.
2.) Overly optimistic of manufacturing tolerances.
or
3.) Overly optimistic of your chances of winning the lottery.

The only advice I can offer is that you should either:
A.) Tie the breaker handles together
or
B.) Fill all of the balls that you don't want with a liquid to weigh them down.

Both of these methods have proven highly effective in the past. I suggest the former because a famous bowling and movie host in Pa. went to jail for the latter.

What I guess I don't understand is this: You claim that you are saving a thousand bucks using two breakers as one. Why then not spend a small fraction of that to guarantee that they function as one. A simple handle would eliminate a large number of variables.

I've never dealt with a case of parallel breakers. I can't pretend to know all of the possible conditions and variables. Part of my job is to deal with as many of the "what ifs" as i can discern. What if, for instance, these breakers have ground fault settings. Again, I don't know. One could trip on a non-overload. You wouldn't know.

It has been mentioned before that I might view things differently as a Testing Engineer. This is true but I think that it is more because I have been trained as a pilot and skydiver. We are drilled over and over and over to respond to emergencies that have a whole number zero percent chance of happening to us. But yet, every emergency we train for has taken human lives so we train. We carry a reserve on our backs, worry about best glide and balanced field lengths, tie our breaker handles together, and spend our lottery money on lift tickets.<G>
Joe

Fair enough Joe we can once again disagree.

This for me has been strictly something for discussion.

I will now remove my GE service panel and send it to GE with a note that says "Joe says it won't work".


I am not optimistic about either of those, I am counting on the fact the breakers will have different characteristics.

2 - 200 amp breakers in parallel supplying one feeder.

The load on the feeder some how goes above 400 amps.

The first breaker trips leaving the remaining breaker to hold the load which would still be over 400 amps.

This second breaker no matter how far out of whack from the first is going to trip quickly with 200% or more of it's rating passing through it.

Now lets say the planets where perfectly aliened and for some reason just as the first one tripped the current dropped from more than 400 amps to less than 200 amps just at the right moment leaving us with a '200 amp feeder'

All that has happened is the power stayed on perhaps to trip out on another day.

The level of inconvenience seems the same.

Do you work with building wiring systems?

I ask because you seem to look at it like building a fine watch when in reality it is a fairly crude process when dealing with typical inexpensive molded case circuit breakers.



Handle ties do not ensure both breakers will open on a 'trip'. Handle ties only ensure manual operation is simultaneous.

This is a result of breakers 'trip free' design.

I will see if I can post a picture of my GE panel.

I also never said I would do this, it is an NEC violation to do this in the field.

Bob

Bob, I think I'll just agree to agree with you. You wouldn't do it, I wouldn't do it, and the NEC doesn't condone it. I see your point about the trip free aspect. I was just thinking of how I have never had a two pole breaker trip one half of the assembly. My building wiring system experience has been bus maintenance and transportation facilities, rail maintenance and yard facilities, passenger stations and traction power substations. There's lots of misc. stuff thrown in there too. In all of these locations, I have not seen parallel breakers. This is a concept that I had never given any great consideration to until this thread.


"I will now remove my GE service panel and send it to GE with a note that says "Joe says it won't work"."

How about just sending them an Email inquiring as to reports of only one breaker tripping?
Joe

Let me just add that I may have seen panels constructed this way and not realized it. I don't typically ask electricians to take panel covers off to examine the bus work. I do deal with many split panels with lighting contactors and the like.

[This message has been edited by JoeTestingEngr (edited 07-02-2006).]