Breaker Plug Replacement

[adamscb]

Forum,

In one of the projects I'm working on, I'm pretty sure the breaker we have currently in the field will trip when I feed a new MCC from it. The current plug on the breaker is for 800A, and I'm convinced it will go over that limit. Therefore I plan on buying a new breaker plug (1200A) and adjust the settings. Is there anything else I need to do in order to "upsize" this circuit breaker? I've already picked out the right kind of plug, do I need to ugprade anything else? Thanks

 

[iwire]

To me the first thing you have to do is a load calculation.

Does this 800 amp overcurrent device supply 1,200 amp rated conductors and equipment?

 

[adamscb]

To me the first thing you have to do is a load calculation.

Does this 800 amp overcurrent device supply 1,200 amp rated conductors and equipment?

Ok let me explain. We currently have an MCC that feeds about 600A of total load. This includes two 200hp motors that use wye-delta starters. The starting torque on these is tremendous, and it takes a while to get these up to speed. As well as physically moving this MCC, I'm also plan on using new solid-state soft starters, which change the starting characteristics slightly. We use these soft starts on identical motors in the plant, so I'm confident with the starting characteristics. They usually pull 850A for 1.5-2 seconds to get the motor rolling, and then draw ~550A for 12 minutes to get fully up to speed. It's this starting characteristic that worries me, because while one is starting and one is at nearly full load, that's above 800A, and I haven't seen a breaker that has a long time delay of 12 minutes. Therefore I'm wanting to upgrade to a 1200A breaker.

 

[iwire]

Maybe it is my lack of formal training but this sure sounds like WAG and not engineering but be that as it may, with the breakers I have switched rating plugs there was nothing more to it. Remove one, install the other. I have always done this with the breaker in the open position.

 

[wbdvt]

A couple of things:

1. Is the breaker frame size 1200A or above? If it is only a 800A or 1000A frame breaker, then you can't use a 1200A plug.
2. Since you most likely are replacing the conductors to handle the increase load, are the existing breaker lugs sized for the new conductors?
3. How does upgrading to a 1200A plug affect the coordination with upstream protective devices?
4. How does upgrading to a 1200A plug affect the incident energy (arc flash) on the mcc?

 

[adamscb]

Maybe it is my lack of formal training but this sure sounds like WAG and not engineering but be that as it may, with the breakers I have switched rating plugs there was nothing more to it. Remove one, install the other. I have always done this with the breaker in the open position.

It's not a WAG, it's called using math. When the motor is going up to speed (~550A for 12 minutes) and the other motors are running full load, then the total amp draw will be > 800A, and it will be in this condition for 12 minutes. I don't know of any breakers that have a LTD of over 700 seconds, do you?

A couple of things:

1. Is the breaker frame size 1200A or above? If it is only a 800A or 1000A frame breaker, then you can't use a 1200A plug.
2. Since you most likely are replacing the conductors to handle the increase load, are the existing breaker lugs sized for the new conductors?
3. How does upgrading to a 1200A plug affect the coordination with upstream protective devices?
4. How does upgrading to a 1200A plug affect the incident energy (arc flash) on the mcc?

1) Yes, breaker frame size is 1200A
2) Existing breaker lugs can fit the new cabling
3) Upstream coordination will hardly be affected. The next upstream device has a 3200A plug, and it's set to trip at slightly below that.
4) Arc flash will hardly change, according to SKM.

And when I asked if I have to upgrade anything else, I meant from the circuit breaker standpoint, aside from the plug. I was aware that I had to upsize wires, update arc flash, coordination, etc. Sorry I worded that really poorly.

 

[Ingenieur]

850 A on a 800 A thermal setting should > 12 minutes
what specific breaker? Mfg/model/pn

 

[zog]

And when I asked if I have to upgrade anything else, I meant from the circuit breaker standpoint, aside from the plug. I was aware that I had to upsize wires, update arc flash, coordination, etc. Sorry I worded that really poorly.

Can't answer that without specifics about the existing breaker and plug