Parallel Breakers ?

Status
Not open for further replies.
W

wired1

Member
Location
Louisiana
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 ?
 
wired1 said:
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 ?
Click to expand...

I'm not sure it wouldn't work. I have seen a similar setup up in manufactured custom products.

You're in a tough spot. Is the job going to be inspected?
 
Twoskinsoneman said:
I'm not sure it wouldn't work. I have seen a similar setup up in manufactured custom products.

You're in a tough spot. Is the job going to be inspected?
Click to expand...


Small differences in the breakers and wiring will cause unequal currents in the breakers.

You're right, it might work, but it also might not. And I wouldn't consider it safe.

If one breaker trips, the other breaker (and wire) would have to handle the entire current.
 
To me, this just doesn't make sense. As you know, the chain is only as strong as the weakest link. In this case, it's a 100A breaker. Unless I'm not understanding what you're saying, this doesn't add any more ampacity to the circuit.

Am I not seeing something here?

Andrew
 
Technically it would work, but it wouldn't be right. Some manufactures do it with main breakers, but they have are UL listed for that purpose. If something happened the lawyers would throw it back on you because you knew it wasn't right even though the engineers who supposedly have a higher education told you to do it. I don't know of too many inspectors that would allow it to slide if they knew about it.
 
buzzbar said:
To me, this just doesn't make sense. As you know, the chain is only as strong as the weakest link. In this case, it's a 100A breaker. Unless I'm not understanding what you're saying, this doesn't add any more ampacity to the circuit.

Am I not seeing something here?

Andrew
Click to expand...

It would work, the current will divide fairly equally across the two breakers.

Parallel fuses are pretty common in factory made equipment.
 
wired1 said:
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.
Click to expand...

You sure about that? Lets get some specs here and find a solution. What is the panel type and breaker type?
 
buzzbar maybe this will help clear it up. :smile:

buzzbar said:
As you know, the chain is only as strong as the weakest link. In this case, it's a 100A breaker.
Click to expand...

Lets say you have two 10' chains each with a 1000 pound capacity.

If you link them end to end (series) to make one 20' chain the capacity is still limited to 1000 pounds.

Now if you use the two chains side by side (parallel) you would have a total lifting capacity of 2000 pounds assuming you dived the load evenly between the two chains
 
I actually demoed a piece of Square D gear once that had 2 400 amp fuses per phase bolted side by side for an 800 amp capacity main. One of the oddest things I have come across. But back to the OP's question . I would never tie the 2 100 amp breakers in parallel
 
Good analogy, Bob. And it helps me address those who suggest that this installation would "work." What does it mean to say that something works? If current actually does flow to the downstream panel, and if the loads actually do get power, can you say that this has worked? No. If Bob?s set of lifting chains is able to lift a 2000 pound load without either chain breaking, can you say that this has worked? No. If you put on blinders before backing your car out of your driveway, and you manage to make it into the street without hitting anything or anybody, can you say that this has worked? No.

The function of a power distribution system involves more than simply getting power to the loads. It also has the function of providing adequate protection, in the event of equipment failure. If one of Bob?s two parallel chains were to fail, then the entire 2000 pound load would be imposed upon the other chain. Yes, it would likely fail too. But it could fail more explosively than was the design intent. More than simply dropping the load onto the ground, it might cause elements of the chain to break apart, creating a second opportunity for causing injury (in this example, by having someone hit by flying metal).

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.

My suggestion is that you offer to install a 150 amp breaker and conductors rated for 200 amps. Then look for another way to give them a 200 amp breaker. Not knowing the nature of the load, I don?t know if this will be of any value.

My other suggestion is one I can offer because I don?t have any money, any business, on the line. If a registered professional electrical engineer has signed a design document that includes a code violation, and if that person, having been informed of the violation, continues to insist that it be installed anyway, I would report that person to the state licensing department. To begin with, I would inform the engineer that that was my intent, in the hopes that this would talk him or her out of this installation.
 
iwire said:
But Charlie, it does work every day with rigging. :smile:
Click to expand...
Then I infer that the situation has been studied sufficiently to provide adequate assurance that the parallel chain method is safe. I further infer that this method is approved by whatever codes, standards, or rules are used by professional riggers.

On the other hand, we have a rule that explicitly forbids it. So I infer that it has been demonstrated to be unsafe, or at least that nobody has bothered to undertake whatever testing might be needed to prove that is is safe.
 
I ran into an old piece of 800A gear that had fuse holders for 2 400A fuses in parrallel. The owner of the building did not want to pay for a service upgrade. Foretunately, the load only required a 400A service, so I, with the Electrical Contractor, and the Inspector met at the site, and the inspector approved the continued use of the gear, but only if the fuse holder for the parralled fuse holder was hacksawed off.

Engineer, EC's, and Inspectors need to work to solve installation issues within the bounds of the safety standards, not based on COST.

More to the point of this thread, don't do it. Contact the inspecting authority and present your documentation (email, letter, redlined drawings), and let them handle the engineer.
 
iwire said:
buzzbar maybe this will help clear it up. :smile:



Lets say you have two 10' chains each with a 1000 pound capacity.

If you link them end to end (series) to make one 20' chain the capacity is still limited to 1000 pounds.

Now if you use the two chains side by side (parallel) you would have a total lifting capacity of 2000 pounds assuming you dived the load evenly between the two chains
Click to expand...



But iWire.....both chains are connected to one Chain Block. I've never seen a Crane with 2 chain blocks hanging from it.

Same with CB's, 2,3,4,5,6, cables.....1 CB.
 
markstg said:
I ran into an old piece of 800A gear that had fuse holders for 2 400A fuses in parrallel. The owner of the building did not want to pay for a service upgrade. Foretunately, the load only required a 400A service, so I, with the Electrical Contractor, and the Inspector met at the site, and the inspector approved the continued use of the gear, but only if the fuse holder for the parralled fuse holder was hacksawed off.
Click to expand...

But 240.8 expressly allows the paralleling of fuses by the factory so I do not see why anything was 'hacksawed off'.
 
Status
Not open for further replies.
Top