why would someone do this

Status
Not open for further replies.

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
maybe not a NEC question, but on a pretty regular basis I get drawings (sometimes just sketches) for systems showing a main CB and another slightly smaller CB protecting a motor. In the latest case, It was a 125HP motor that had a 250A main CB with a 225A CB feeding a soft start. There are some misc small loads the main CB is also feeding, but I am having a hard time figuring out why anyone would spend the money on the 225A CB, as it appears to be redundant to me.

Is there some reason that I might be missing that would make a second CB useful in some way. It is not like they would ever need to shut off the 225A breaker and leave the other loads on.

Just curious if I am missing something obvious or it is just something they are doing for no good reason.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The lack of responses suggests to me that I am not alone in thinking the second CB is completely redundant and serves no useful purpose.

I sometimes think that people cut and paste stuff without really thinking about what they are actually doing. But an extra CB at this size is maybe $500 by the time you add in the CB, the lugs, and the labor. It also adds to the panel space required and complicates the internal wiring.

I think without the 2nd CB one could run the wires straight into the soft starter by mounting the SS directly below the main CB. Shortens the run of big wire substantially.
 

kenaslan

Senior Member
Location
Billings MT
I am going to assume the MCB is for a panel. Is it fed from an upstream transformer? The main breaker is probablly an MCCB and the smaller breaker is probablly a MCP. Different functions. Sounds correct to me. Done all the time. Have a 480V MCC use a 480 delta wye transformer feeding a DP somewhere. Main CB at panel (MCCB) install a MCP to feed a soft start, install branch circuit MCCB's to feed other stuff. maybe a motor space heater, or light above.
 

kwired

Electron manager
Location
NE Nebraska
maybe not a NEC question, but on a pretty regular basis I get drawings (sometimes just sketches) for systems showing a main CB and another slightly smaller CB protecting a motor. In the latest case, It was a 125HP motor that had a 250A main CB with a 225A CB feeding a soft start. There are some misc small loads the main CB is also feeding, but I am having a hard time figuring out why anyone would spend the money on the 225A CB, as it appears to be redundant to me.

Is there some reason that I might be missing that would make a second CB useful in some way. It is not like they would ever need to shut off the 225A breaker and leave the other loads on.

Just curious if I am missing something obvious or it is just something they are doing for no good reason.

If the motor is the only load why not just one breaker.

If there is one large motor and minimal small loads I might just install one service disconnect for the motor and one for the smaller loads. We do this for irrigation equipment all the time. Service usually consists of two disconnects - one is part of the pump panel which is service rated, the other is usually a 30 amp disconnect to supply the remaining equipment which is usually just the center pivot machine.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It did occurr to me that one consideration would be that pesky "in sight" rule. one could not use the main CB as the required disconnecting means if it was more than 50" away from the motor.
 

kwired

Electron manager
Location
NE Nebraska
It did occurr to me that one consideration would be that pesky "in sight" rule. one could not use the main CB as the required disconnecting means if it was more than 50" away from the motor.

But the disconnect that is within sight of the motor does not necessarily require overcurrent protection if it has already been provided upstream, it is there for disconnecting not for overcurrent protection.
 

Smart $

Esteemed Member
Location
Ohio
I am going to assume the MCB is for a panel. Is it fed from an upstream transformer? The main breaker is probablly an MCCB and the smaller breaker is probablly a MCP. Different functions. Sounds correct to me. Done all the time. Have a 480V MCC use a 480 delta wye transformer feeding a DP somewhere. Main CB at panel (MCCB) install a MCP to feed a soft start, install branch circuit MCCB's to feed other stuff. maybe a motor space heater, or light above.
MCP = ?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
But the disconnect that is within sight of the motor does not necessarily require overcurrent protection if it has already been provided upstream, it is there for disconnecting not for overcurrent protection.

a lot of our customers are dead set against having local disconnect switches as they get in the way of the equipment, and sometimes the equipment itself is located in a classified area so having a local disconnect is considered undesirable.
 

Smart $

Esteemed Member
Location
Ohio
...

Is there some reason that I might be missing that would make a second CB useful in some way. It is not like they would ever need to shut off the 225A breaker and leave the other loads on.

...
Don't know your particular conditions, but there are times when adjusting or troubleshooting control circuits where you want the control circuits to function normally... without running the associated motor.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I am going to assume the MCB is for a panel. Is it fed from an upstream transformer? The main breaker is probablly an MCCB and the smaller breaker is probablly a MCP. Different functions. Sounds correct to me. Done all the time. Have a 480V MCC use a 480 delta wye transformer feeding a DP somewhere. Main CB at panel (MCCB) install a MCP to feed a soft start, install branch circuit MCCB's to feed other stuff. maybe a motor space heater, or light above.
You can't use an MCP (motor circuit protector, magnetic only) for the application in the original post. MCPs can only be used as part of a listed combination starter.
 

dicklaxt

Senior Member
The MCP is referring to a "Motor Circuit Protector"( I kinda think anyway) in this case an integral CB within the reduced voltage/torque starter enclosure/package.


Ref to not liking the disconnects,,,,the owners can do as they like when they are paying the freight on the property and life damages,,,,,kinda an antiquated view point IMO by the owner tho.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
The MCP is referring to a "Motor Circuit Protector"( I kinda think anyway) in this case an integral CB within the reduced voltage/torque starter enclosure/package.


Ref to not liking the disconnects,,,,the owners can do as they like when they are paying the freight on the property and life damages,,,,,kinda an antiquated view point IMO by the owner tho.

You get three or four motors on a skid and putting disconnect switches there can be painful. If they can be included in the control panel 100 feet away instead, it is a huge deal to a lot of end users. In general, I would prefer to have a local disconnect switch but they do clutter things up a lot and add a lot of unnecessary cost in many cases. They also eat up a lot of real estate that most end users would rather see used for other things.

A lot of owners expect package equipment not to require very much in the way of labor/materials to get them up and running in the field. They buy package equipment for that reason. If they wanted to roll their own they could buy all the parts and assemble it themselves.
 

kwired

Electron manager
Location
NE Nebraska
You get three or four motors on a skid and putting disconnect switches there can be painful. If they can be included in the control panel 100 feet away instead, it is a huge deal to a lot of end users. In general, I would prefer to have a local disconnect switch but they do clutter things up a lot and add a lot of unnecessary cost in many cases. They also eat up a lot of real estate that most end users would rather see used for other things.

A lot of owners expect package equipment not to require very much in the way of labor/materials to get them up and running in the field. They buy package equipment for that reason. If they wanted to roll their own they could buy all the parts and assemble it themselves.

Which package equipment do you suppose they are going to purchase, the one with a disconnect included in the package, or the one that cost less, primarily because it did not include a disconnect?
 

pfalcon

Senior Member
Location
Indiana
Which package equipment do you suppose they are going to purchase, the one with a disconnect included in the package, or the one that cost less, primarily because it did not include a disconnect?

The second the first time around. The first thereafter when they discover it costs more, time and money, in the field to add after the fact.
 

Cmdr_Suds

Member
Short answer - Ignorance. As an control panel builder, I often see specifications that require CBs or fuses in control panels that are being protected upstream. As a bidding contractor, if you don't put it in your price and design and the consulting"engineer" or owner calls you on it, your hosed. Some of those guys will listen and some will not. Unless you know the guy making the call and you know he will listen to reason, you had better just put it in or possibly face the consequences. I just recently built a pump panel that was being feed with a fused buss head and thinking that the owner would understand that putting in a second set of fuses was silly, I built it with a non-fused disconnect. I was required to go out in the field and add a fuse block to the panel along with having to relocate numerous components under threat of not getting paid. Without regards as to why this is a waste, I was told that the maintenance personnel did not like to get a lift or ladder out and change the fuses in the buss heads. Even after explaining that when you have two sets of fuses that are close in size, it is crap shoot as to which will blow first.
 

pfalcon

Senior Member
Location
Indiana
Short answer - Ignorance. As an control panel builder, I often see specifications that require CBs or fuses in control panels that are being protected upstream. ... I just recently built a pump panel that was being fed with a fused buss head and thinking that the owner would understand that putting in a second set of fuses was silly, I built it with a non-fused disconnect. ..., I was told that the maintenance personnel did not like to get a lift or ladder out and change the fuses in the buss heads. Even after explaining that when you have two sets of fuses that are close in size, it is crap shoot as to which will blow first.

409.21(B) allows the buss head to cover but ... our buss heads are high enough a lift is required. Therefore two men, harnesses, tie-offs, and 15+ minutes for a lift to arrive. That versus one man who can walk up to the panel. One fuse blown at the buss and we've paid for having fuses added to several panels. But you get to submit prints to us and we'll tell you up front it's what we want. Then we coordinate the buss.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
409.21(B) allows the buss head to cover but ... our buss heads are high enough a lift is required. Therefore two men, harnesses, tie-offs, and 15+ minutes for a lift to arrive. That versus one man who can walk up to the panel. One fuse blown at the buss and we've paid for having fuses added to several panels. But you get to submit prints to us and we'll tell you up front it's what we want. Then we coordinate the buss.

A few years ago I was at a plant to help startup a machine. I think the machine control panel had 200A fuses in the main. They fed it off 90 Amp fuses from an overhead bus. The electrician who wired it up made a point of telling me that, and was quite proud of himself for doing so. I think the real load was about 80A , but I don't like to be real close as you never know what someone will add to a panel down the road.
 
Status
Not open for further replies.
Top