Parallel Breakers ?

[winnie]

I believe that in the situations where multiple poles of a breaker are used in parallel, the assemblies are designed so that _all_ poles trip at the same time. For example, there was a line of residential panel-boards where 4 '100A' poles were arranged in 2 parallel pairs to provide a '200A' '2 pole' main breaker.

You could contact the panel manufacturer to find out if there is a 'listed parallel assembly' of 2 100A breakers that fits your panel.

Musing here: what defines an OCPD connected in parallel? There are many situations where OCPD are somewhat parallel with each other; I'm thinking for example of a large 'dual head' service with a tie breaker or OCPD on transformer secondaries when there are multiple transformers. If you place two OCPD in a panel, run two _separate_ feeders, but both of these feeders go to the _same_ load, are those 'parallel' for the purpose of 240.8?

-Jon

 

[iwire]

But iWire.....both chains are connected to one Chain Block.

The chain block may have a 10,000 pound rating.

 

[winnie]

On the 'two chains holding the load' analogy, this works but requires some aspect of the system to permit the load to distribute reasonably evenly. Do it wrong, and each chain breaks in turn as it takes the entire load.

Consider ten chains, each rated for 1000 lb, with lengths of 10', 11', 12',...19' respectively. (Thus ten chains with ten different lengths.) Connect all of these chains together in 'parallel'. Now try to lift a 5000lb load. First the short chain snaps, then the next on, then the next one....

All breakers have some resistance, and different breakers will have different resistance. With two breakers in parallel, I would want some assurance by design and manufacture that the current will divide evenly enough. This might require matching for contact resistance that is much tighter than required for a 'solo' breaker. This might require a minimum amount of external 'ballast resistance' (in the form of external parallel wire length).

-Jon

 

[Jim W in Tampa]

Is it not part of engineering to specify what equipment is to be installed ? If what they designed is not available then its up to them to redesign.
As per Bob's chain idea, yes it will it work. That is taught in high school, does not require engineering degree.

 

[iwire]

On the 'two chains holding the load' analogy, this works but requires some aspect of the system to permit the load to distribute reasonably evenly. Do it wrong, and each chain breaks in turn as it takes the entire load.

Yes of course and I mentioned that when I first brought the analogy into the thread. :smile:

 

[Rick Christopherson]

If one conductor in the parallel set were to suffer a short circuit, then the breaker serving it would trip. The other parallel breaker would not. That would impose the full 200 amp load on one conductor and one breaker. Can the engineering company who is pushing this installation certify that the remaining breaker will trip in time to prevent damage to any part of the system? I would not sign such a statement.

So, no it wouldn?t ?work,? because it couldn?t do the ?other job,? the job of providing adequate protection.

I am not suggesting that it is the proper solution, but I disagree with your reasoning. If one of the circuit breakers could sustain a dead-short, where instantaneous current levels exceed thousands of amps without exploding, then surely the second breaker would perform similarly with only 2-times its rated trip. That is what they are supposed to do.

If one breaker trips for any reason, then the entire circuit becomes limited to 100 amps, which is the capacity of the remaining breaker. If the demand on the circuit exceeds the 100 amps, then the remaining breaker will do the job it was designed for and trip out as well. There will not be 200 amps flowing through the other half of the circuit, because the remaining breaker is there to prevent that from ever happening.

The NEC is written to be a generic catch-all, so it does not necessarily address every conceivable situation correctly.

Take motor operated appliances for example. Let's say you have two tablesaws which have identical motors from the same motor manufacturer (like Baldor). All other parts are identical from off-the-shelf suppliers. However, one manufacturer spent the extra money to get a UL sticker for their saw, but the other company didn't--not because it couldn't pass muster, but simply because that little sticker is very expensive.

So for identical motors, you are permitted to use the nameplate current rating on the product that has the little sticker, but for the very same motor on the other product, you are forced to use the inflated NEC table. Why? Because it was a catch-all section of code.

 

[steve66]

I am not suggesting that it is the proper solution, but I disagree with your reasoning. If one of the circuit breakers could sustain a dead-short, where instantaneous current levels exceed thousands of amps without exploding, then surely the second breaker would perform similarly with only 2-times its rated trip. That is what they are supposed to do.

If one breaker trips for any reason, then the entire circuit becomes limited to 100 amps, which is the capacity of the remaining breaker. If the demand on the circuit exceeds the 100 amps, then the remaining breaker will do the job it was designed for and trip out as well. There will not be 200 amps flowing through the other half of the circuit, because the remaining breaker is there to prevent that from ever happening.

The NEC is written to be a generic catch-all, so it does not necessarily address every conceivable situation correctly.

Take motor operated appliances for example. Let's say you have two tablesaws which have identical motors from the same motor manufacturer (like Baldor). All other parts are identical from off-the-shelf suppliers. However, one manufacturer spent the extra money to get a UL sticker for their saw, but the other company didn't--not because it couldn't pass muster, but simply because that little sticker is very expensive.

So for identical motors, you are permitted to use the nameplate current rating on the product that has the little sticker, but for the very same motor on the other product, you are forced to use the inflated NEC table. Why? Because it was a catch-all section of code.

A 100 amp breaker will not trip at 100 amps.

The NEC is very clear that you have to do load calculations before adding loads to a breaker. You don't just assume you can load up a circuit and count on the breaker to trip if the circuit is overloaded.

 

[iwire]

The NEC is very clear that you have to do load calculations before adding loads to a breaker. You don't just assume you can load up a circuit and count on the breaker to trip if the circuit is overloaded.

So a branch circuit with multiple receptacles is in violation and a safety hazard?

 

[Rick Christopherson]

A 100 amp breaker will not trip at 100 amps.

The NEC is very clear that you have to do load calculations before adding loads to a breaker. You don't just assume you can load up a circuit and count on the breaker to trip if the circuit is overloaded.

No, those calculations are there so you don't deliberately, intentionally, and continuously load a circuit breaker to 100% of its rating. That doesn't mean that the circuit cannot handle 100% of its rating, just that you shouldn't intentionally load it to 100%. (However, if the breaker is rated for 100%, then you are permitted to load it to 100%.) This derating is to protect the circuit breaker, not the circuit.

Even though a breaker doesn't trip at its exact trip point, neither does a wire melt down at its rated amperage. The trip curve of a thermal magnetic breaker is intended to follow the heating capacity of the conductors it is protecting. The more the current exceeds the capacity of the conductor, the faster the breaker trips.

 

[steve66]

So a branch circuit with multiple receptacles is in violation and a safety hazard?

Of course, there isn't any way to know exactly how much load will be plugged into a receptacle. So they have to be treated a little different.

But I think the idea is still to limit the load on a recepacle circuit to the branch circuit rating. I think that's why there are so many rules for receptacles - two SA circuits, one bath circuit, one laundry ckt, 180 Va per outlet, max 80% for one device on a multiple receptacle circuit. They all seem to be making sure there are enough circuits that its unlikely one circuit will be overloaded.

Steve

 

[barbeer]

They engineered the job themself.
What would you do ?

Conflict of interest? Maybe not.

 

[steve66]

No, those calculations are there so you don't deliberately, intentionally, and continuously load a circuit breaker to 100% of its rating. That doesn't mean that the circuit cannot handle 100% of its rating, just that you shouldn't intentionally load it to 100%. (However, if the breaker is rated for 100%, then you are permitted to load it to 100%.) This derating is to protect the circuit breaker, not the circuit.

Even though a breaker doesn't trip at its exact trip point, neither does a wire melt down at its rated amperage. The trip curve of a thermal magnetic breaker is intended to follow the heating capacity of the conductors it is protecting. The more the current exceeds the capacity of the conductor, the faster the breaker trips.

It still sounds like you're still saying its OK to overload a circuit. I realize a 105 amps on a 100 amp circuit isn't going to start a fire, but it's still a code violation.

Steve

 

[Rick Christopherson]

It still sounds like you're still saying its OK to overload a circuit. I realize a 105 amps on a 100 amp circuit isn't going to start a fire, but it's still a code violation.

Steve

So it's a code violation when your daughter plugs in her hairdryer, curling iron, straightener, and godknows whatever else they bring into the bathroom every morning; and trips the circuit breaker? You must have a lot of red slips stapled to the bathroom door.

The condition exists because a fault has occurred which was caused by an over current situation in the first place, and the system would respond exactly as intended. If the fault had not occurred, then there would be no over current situation. You've kind of skipped one of the steps in your logic.

 

[steve66]

So it's a code violation when your daughter plugs in her hairdryer, curling iron, straightener, and godknows whatever else they bring into the bathroom every morning; and trips the circuit breaker? You must have a lot of red slips stapled to the bathroom door.

The condition exists because a fault has occurred which was caused by an over current situation in the first place, and the system would respond exactly as intended. If the fault had not occurred, then there would be no over current situation. You've kind of skipped one of the steps in your logic.

Well, first of all, I have boys - so its hard enough to get them to pick up a hairbrush, much less a hairdryer

But I still feel the intent of the NEC is to prevent circuits from being overloaded except in a fault condition.

And I don't agree that a fault has to occur first before an overload occurs. There is no guarentee the current will divide equally between the two breakers. If you have 200 amps, and one breaker has .02 ohms, and the other has .03 ohms, then you will get 120 amps through one 100 amp breaker, and 80 through the other.

Or, what happens if someone turns one breaker off?? You have still created an overload.

I think its pretty obvious why the NEC doesn't allow parallel breakers. Its just not a rule that I would ignore even if I could.

 

[iwire]

But I think the idea is still to limit the load on a recepacle circuit to the branch circuit rating. I think that's why there are so many rules for receptacles - two SA circuits, one bath circuit, one laundry ckt, 180 Va per outlet, max 80% for one device on a multiple receptacle circuit. They all seem to be making sure there are enough circuits that its unlikely one circuit will be overloaded.

In a dwelling unit I can install an unlimited number of receptacle outlets on a circuit.

I think its pretty obvious why the NEC doesn't allow parallel breakers. Its just not a rule that I would ignore even if I could.

I agree with that 100%. :smile:

 

[Rick Christopherson]

But I still feel the intent of the NEC is to prevent circuits from being overloaded except in a fault condition.

You state this, yet you still use 200 amps (100%) for your example.

There is no guarentee the current will divide equally between the two breakers. If you have 200 amps, and one breaker has .02 ohms, and the other has .03 ohms, then you will get 120 amps through one 100 amp breaker, and 80 through the other.

First off, voltage division includes the resistance of the whole parallel circuit, not just the breakers, so the ratio between the two will never be this extreme so long as the conductors are the same size (i.e. you don't mix 1/0 and 2/0).

More importantly, you don't think twice about paralleling conductors from the same breaker, when that breaker ampacity is several times larger than the ampacity of any single conductor, do you? When is the last time you saw a 1200 amp I-line panel fed with a single conductor?

If you lost one of those parallel lines, the circuit breaker is not going to know that the total ampacity of the remaining lines is well below the trip point, and it is not going to trip until you have a puddle of plastic and copper laying on the floor (figuratively).

At least in the parallel breaker scenario, each conductor is fully protected by its own OCPD, which is rated for the size of the conductor. So in this case, it makes no difference whether you loose one conductor or one breaker, the system is still going to protect itself in its entirety, and shut down.

Or, what happens if someone turns one breaker off?? You have still created an overload.

That's a fault...it is a human fault, but still a fault. Nevertheless, the OCPD--Over Current Protection Device will still protect the circuit, which is the reason why they exist.

I think its pretty obvious why the NEC doesn't allow parallel breakers. Its just not a rule that I would ignore even if I could.

I never said the OP should parallel breakers. That's not my call to make and I don't need to put my name behind the decision. However, as an engineer, I would actually feel far safer paralleling breakers which match the ampacity of the conductors than I do paralleling feeders that individually do not match the breaker.

 

[ptrip]

Most of the discourse on this topic has been if one breaker trips or if human error only flips off one breaker instead of both. Wouldn't this be solved with a handle-tie of some sort?

Now, if NEC says no ... then NEC says no ... and the engineer needs to get his/her head wrapped around the problem at hand and all of our discussion is moot.

 

[benaround]

We are in the process of installing a generator for a large Engineering firm in town.
They engineered the job themself. In the process of running the normal power from the transfer switch we discovered that we could not install a 200 amp, 480 volt breaker in the existing panel as they had specified. The largest breaker rated for this panel was 150 amps.
So now thier engineers are telling me to install two 100 amp breakers in parallel and pull a parallel feeder. This seems to be in direct violation of NEC 240.8 and I have pointed this out to them. They still insist that I do this, and say they will install signage to indicate these breakers are installed this way. I am still not comfortable doing this and have provided letters stating ths fact. I have also stated I am not sure this will even work much less be with-in code..I have already started this job and can't pull out now.

What would you do ?

What would be wrong with a 200a fusible disconnect before the panel?

 

[tkb]

I have seen what looks like a 4 pole 200 amp breaker as a main in a panel.
It was a two pole 400 amp breaker.
Two of each pole were paralled together.
It must of had an internal common trip.
This was in a panel from the factory.

 

[Cow]

So after 4 pages it comes down to:

1. Make them live with the 150 amp feed.
2. Change the panel to one that'll accept a 200 amp breaker.
3. Bolt lugs to the buss, apply the tap rule and .......???