Circuit Protection For Devices that Never Approach FLA

[Recondaddy]

I'm good with branch feeder and branch motor circuit calculations with respect to NEC. However, I've always had a nagging question about circuit protection methodology that I've seen engineers employ when it comes to control (non-motor) circuits. I don't get the opportunity to actually design and specify control panels, so I'm a bit puzzled by some of the things I've seen.

For example, when it comes to VFDs and servo drives, for example, I recognize that you forget about the motor, itself, and focus on the current that the drive is drawing.

As a simple example, imagine you have 5 servo drives that list an FLA of 9.4A, each. Now, in your application, none of your servos operate at full load, and your current draw at the input of the drive never even comes close to the listed FLA.

According to 210.20(A) and 215.3, the overcurrent device must be sized no less than 100% of the non-continuous load plus 125% of the continuous load. In this example, there are no continuous loads. The servos make infrequent, short duration moves, and the current draw of the drive never exceeds 0.5A. With such a paltry amount of current draw, can they even be considered a non-continuous load, by strict definition?

It seems to me that in order to be compliant with NEC, each drive circuit should be protected with a 10A fuse. I never see other engineers protecting circuits like these with such a "large" fuse. In fact, I'm looking at a set of prints, now, where seven servo drives are ALL being fed from a 15A fuse. What's the most correct approach? Protect each with a fuse that is based on their FLA?

One more question -- suppose I wish to protect the feeder circuit that supplies power to these five servo drives. Should I be looking at a 50A fuse?? While that seems to be what the code requires, I've never understood the way that other engineers size upstream circuit protection.

I mean, how is it reasonable to have 10 non-motor branch circuits, each protected with a 15A fuse, and have all those branch circuits fed from a 60A breaker?

Am I missing something?

Thanks for reading.

 

[templdl]

What is the intent of the OCPD, is it to protect the conductor? Isn',t the conductor sized to carry the load?
If so how is the rating of the OCPD which has been selected to protect the conductor related to the actual load?

 

[Recondaddy]

What is the intent of the OCPD, is it to protect the conductor? Isn',t the conductor sized to carry the load?
If so how is the rating of the OCPD which has been selected to protect the conductor related to the actual load?

That is a great way of putting it. I think that I'm just getting frustrated at seeing OCPD and conductor sizing in control panels that we are getting from our systems integrators that seem to be far removed from NEC rules. As I said, since I don't get the opportunity to practice applying these skills, very often, I tend to second-guess my understanding of the Code.

Let's take the example I presented:

The way that I would approach the design would be this:

1. For the feeder circuit that is supplying the five servo drives, I would calculate the continuous and non-continuous loads. In this case, I have no continuous loads. The non-continuous loads, though they never come close to the rated FLA of 9.4A, would still be summed. So, the feeder OCPD, in this case would have to be capable of handling no less than 47A of current (9.4A x 5 drives x 100%). According to 240.6, the next standard size up would be 50A.
2. The conductor for the feeder circuit would also need to be capable of supplying no less than 47A of current. According to Table 310.16, the next highest rated copper conductor that can handle the job is #6 (55A).
3. I would then ensure that the OCPD I have selected is sufficient to protect the conductor I specified. In this case, I have a 50A OCPD protecting a conductor rated for 55A. Protection is assured.

As for the branch circuits for the individual servo drives, I would follow the same steps.

1. In each case, there is a maximum non-continuous load of 9.4A. According to 240.6, the next standard size up would be 15A.
2. The conductor for the branch circuit would need to be capable of supplying no less than 9.4A. According to Table 310.16, the next highest rated copper conductor would be #14 (20A).
3. Finally, I would ensure that the OCPD I have selected is sufficient to protect the conductor I have specified. A 15A OCPD is sufficient to protect conductor rated at 20A.

Does my thinking seem to be correct?

Thanks for your help.

 

[iwire]

It seems to me that in order to be compliant with NEC, each drive circuit should be protected with a 10A fuse.

What section of the NEC do you feel restricts your example to a 10 amp fuse? :?

I mean, how is it reasonable to have 10 non-motor branch circuits, each protected with a 15A fuse, and have all those branch circuits fed from a 60A breaker?

What do you find unreasonable about it?