Sizing Mains

[fifty60]

I am designing an industrial machine. NEC 430 gives me the minimum Feeder OCPD for a mixed motor load. Since this is a minimum, my question is how to correctly select the Mains feeder service so that it is correctly coordinated with the machine. In some instances, the OCPD of a motor on my machine is only 5 AMPS away from the mains service. Please keep in mind that this is a mixed load of heaters and motors. If I am running at full capacity and have a short at my motor, how do I prevent my Mains from tripping before the branch circuit?

The point of my question is when using NEC 430 to get the minimum feeder OCPD, how do should I approach selecting the minimum feeder service and moving up to the next size service? Is there a general rule of thumb to not have the FLA of a machine and the main service less than 5 amps if the machine FLA is over 35A? I hope I am clear enough in my question. I want to use NEC 430 without selecting too small of a mains service. Sometimes the calculations come out nice and I have a comfortable margine, but sometimes they do not and the minimum mains is close to the FLA of the machine.

Thanks in advance.

 

[Dennis Alwon]

Why are you installing overcurrent protective device on the machine? Can you just allow that to be done in the field?

 

[fifty60]

I have to state the FLA and Mains service for the machine. The FLA is straight forward enough, but the Mains is what I am having concerns about. I am using the largest motor OCPD plus the FLA's of everything else. Sometimes this gives me a nice "comfortable" number but sometimes it is a close, like 40A FLA and 45A main service.

 

[augie47]

Unfortunately the solution is not as simple as "sizing the main". To have some assurance the 'main' won't trip first you are going to need a coordination study and compare trip curves, etc.
Simply selecting amperes is not going to assure sequence. I think most of us have seen a 100 or 200 amp feeder breaker open before or along with a 15 or 20 amp branch breaker.

 

[fifty60]

What is the best type of mains protection to use for an Industrial machine that has motors? If i want to size it close to the FLA of the machine, using the minimum amps allowed by NEC 430, then it most certainly would have to be some kind of time delay dual element fuse or a thermal magnetic breaker. This machine has a 5KA SCCR, so that will limit the type of overcurrent protection to one that is very current limiting right?

 

[bob]

This machine has a 5KA SCCR, so that will limit the type of overcurrent protection to one that is very current limiting right?

No. This means the machine is rated for a fault of 5000 amps. Have you done a fault current calculation? If the fault is greater that 5 ka, you will have to do something to limit the fault to 5 ka.

 

[fifty60]

I am not installing the machine. The customer will have to adhere to our 5KA SCCR. I am trying to not create a situation where the main breaker trips before a branch OCPD. Also, I am trying to forsee what problems may arise for declaring the minimum SCCR.

I am using UL 489 with type D tripping characteristics. So if my machine has a 35A FLA and a recommended main service of 40A, if the customer uses a dual element time delay fuse that has a greater tripping time than my type D circuit breakers then there will not be a problem.

I am trying to succesfully use article 430 to size the main service for my machine. If the numbers come out like 32A with a 40A minimum service, I am comfortable with that. But if the calculations come out to 35A with a 40A main, I wonder if I should recommend a 45A service. After looking into it further (thanks in large part to advice from the forum) It seems that success here is not raising the size of the mains but actually selecting a fuse with a slower tripping characteristic than the breakers I am using on my machine.

So I should not be hesitant to use the minimum mains services as calculate from NEC 430. The customer will need to selectively coordinate his mains OCP.

 

[petersonra]

I would not worry about it much. If you have anything trip in your panel in will shut the whole machine down anyway. It is very difficult to coordinate between a machine main and the feeder circuit.

It just is not worth the effort in most cases for such a small load.

 

[fifty60]

Thanks for the answers. For an industrial machine it is ok to use NEC 430 to find the minimum mains service by using the OCPD size of the largest motor plus the FLA's of all simultaneous loads. I should be comfortable using this minimum service, or are there instances where I should go to the next larger size?

On a straight motor load, the NEC gives me a maximum feeder OCPD, on a straight resistive load the NEC gives me a maximum feeder OCPD, but on a mixed motor/resistive load I am given a minimum. This is what concerns me. It would not be good if the mains have to be upsized in the field, but at the same time lower amps machines sell better than the same machine with higher a higher amp mains service requirement.

 

[jumper]

This might be a dumb question, but I do not understand your use of "mains".

Is this the feeder to a panel in which the motor is fed from?

The branch circuit to the controller to the motor?

:?

 

[petersonra]

There is no real benefit from telling the end user what size feeder (presumably that is what you mean by service and mains) to use.

There is no real benefit to making the CB on the feeder 5 or 10 A more. It just won't change anything.

Just tell them what the FLA is and the largest motor and they can figure out what size to make the feeder.

If they want to make the feeder breaker larger for whatever reason, that is up to them.

 

[fifty60]

What I mean by "Mains" is the service that is feeding the main sub panel of the machine. It is the minimum amps service that will supply the machines control circuit, motor branch circuits, and heater branch circuits. We supply the customer with the FLA of the machine, and then the mains service that they will need to install prior to the machine's arrival.

For example, the machine has an FLA of 42A's. I then calculate the minimum service to the machine by adding the largest motor overcurrent device to all the concurrent FLA's of everything else (other motors, heaters). If I do this, for example, I get a main service of 50A.

Here is one thing (of many) that I am not sure about. If I have a mains service of 50A, can I then use a 60A breaker to protect this main service if the conductors are sized accordingly? Or is a 50A service protected by a 50A OCPD that would have to be selectively coordinated with everything downstream so that it does not trip before the branch OCPD.

I am getting some advice here (which is great advice) to let the customer worry about the main service. The problem with that is that we have to recommend a mains service for them.

 

[petersonra]

What I mean by "Mains" is the service that is feeding the main sub panel of the machine. It is the minimum amps service that will supply the machines control circuit, motor branch circuits, and heater branch circuits. We supply the customer with the FLA of the machine, and then the mains service that they will need to install prior to the machine's arrival.

For example, the machine has an FLA of 42A's. I then calculate the minimum service to the machine by adding the largest motor overcurrent device to all the concurrent FLA's of everything else (other motors, heaters). If I do this, for example, I get a main service of 50A.

Here is one thing (of many) that I am not sure about. If I have a mains service of 50A, can I then use a 60A breaker to protect this main service if the conductors are sized accordingly? Or is a 50A service protected by a 50A OCPD that would have to be selectively coordinated with everything downstream so that it does not trip before the branch OCPD.

I am getting some advice here (which is great advice) to let the customer worry about the main service. The problem with that is that we have to recommend a mains service for them.

I think you need to stop using the words mains and service. You have a feeder. It is not nor can it be a main anything or a service.

The feeder conductors have to be sized for the FLA of the control panel nameplate plus 25% of the FLA of the largest motor. The feeder conductor size sets the maximum size of the feeder CB. If they make the feeder conductors bigger they can make the feeder bigger. If FLA + 25% of the largest motor FLA of your machine is <50A, tell them minimum circuit ampacity of 50A. That has a specific meaning to an electrician and he will know what to do with it.

You also cannot size the feeder conductors for him. Normally a 50A circuit would be 75C conductors at #8. But depending on what wire was run and how it was run, he might have to derate the conductors and make them larger.

You are overthinking this.

You cannot possibly recommend the size of either the main or the service since it would include other things you have no way to know.

You cannot do any selective coordination either. It is not something that can be done from the machine end. There is no difference in the coordination capability between a typical 50A or a 60A CB. The trip curves overlap so it just does not matter which one was picked for the feeder CB. In any case, it will vary somewhat from CB model to model.