Don't know if I fully understand 430-53 or not. I have several pieces of machinery to wire. One particular piece of machinery has 2 motors. One motor is 30 HP, and the other motor is just 3/4 HP. I was sizing the OCD by adding the FLA amperage of both motors, then multiplying that by the appropriate column of Table 430-152, which in this case is 250%. Article 430-53b says that the branch circuit short circuit device is not to exceed that allowed by Section 430-52 for the smallest rated motor etc.... Is this just saying that each motor has to have it's individual overload protection ( heaters ) or is it saying that the breaker supplying these cannot be figured as I stated above? sorta confused. Thanks again.... I gotta get this stuff right, pretty involved motor job I got going here.
430-53(b) says you can use 430-52 for determining the maximum branch circuit short-circuit protective device AS LONG AS the device selected (fuse, circuit breaker) is computed for the smallest rated motor....sounds pretty tough to do with a 30 HP and 3/4 HP motor on the same circuit.
Re: Several motors on one branch circuit#92210 03/08/0501:09 PM03/08/0501:09 PM
After I get through with this job, I should know motors wiring a whole lot better. I got at least 25 machines to wire, most of them have two motors on them. About 8 of them are 480 volt, ( I have to set a 400 amp. 480 volt service panel on an existing set of paralled conductors ) Most of the motors are "ancient", one with no name plate some that have been converted from 480 volts to 230 volts, at least one that has been converted from 575 volts to 230 volts and still has the original name plate that says 575 volts on it ( don't even give an option for another voltage, but someone done it anyway,) some with no motor overload protection, and all these are going into a building that already has it's share of violations Inspector is gonna love this one. Don't know exactly what to do now, just wire the circuits etc. and take it one day at a time. The owner is moving his business from one building to another one down the street. Safety inspectors, electrical engineers, architects, fire marshal and who knows who else has already been there. Inspectors told the owner they would work with him, but I don't think they know the extent yet, of what to expect from his machines. These are wood working machines, with sawdust covering them. He is working on getting the "dust collection systems" he needs that he don't already have. As far as my original post on the 30 hp. motor and 3/4 hp. motor, etc. , I don't know yet. I'm taking it one step at a time. Sure am studying the code book a lot. I appreciate all the input. I want to get it right the first time if possible. I don't like doing it the second or third time Steve...
Re: Several motors on one branch circuit#92212 03/08/0511:36 PM03/08/0511:36 PM
You could run a motor feeder to the machine and tap 2 separate branch circuits from there:
1. Get the motor FLC from Table 430.148.Assuming 480V 3 phase A 30-hp motor FLC is 40A. A 3/4-hp motor FLC is 1.6A.
2. Size the branch-circuit protection per the requirements of 430.52(C)(1), Table 430.52, and 240.6(A)] 30-hp: 40A x 2.5 = 100A. 3/4-hp: 1.6A x 2.5 = 4.0.
3. Size the feeder conductor per 430.24(A). The largest motor is 40A. (40A x 1.25) + 1.6A = 51.6A. Table 310.16 shows 6 AWG rated 55A at 60ºC as the smallest conductor with sufficient ampacity.
4. Size the feeder protection per 430.62. It must not be greater than the 100A protection of the branch circuit plus the 1.6A of the other motor (which is the total of all loads on that feeder). 100A + 1.6A = 101.6A. The next size down is 100A, so that is the breaker you choose.
Re: Several motors on one branch circuit#92213 03/09/0508:38 AM03/09/0508:38 AM
Thanks Ian, but would you have to label at the connections that there are two separate power sources? And also in figuring the amperage, what if the name plate is different from the table in code book? On some of these motors the amps is less and on others it is more. Do you take the table or the one that is the greatest? Thanks
Re: Several motors on one branch circuit#92214 03/09/0509:36 PM03/09/0509:36 PM
Since the small starters can't handle large let through currents of a short circuit on a 100A breaker (the contacts weld together), maybe you should put in a fused disconnect ahead of the manual starter that has the overloads in it for the smaller motor...assuming you're installing manual starters. I've seen a lot of size 0, or 00, starters running downstream from a 30 amp breaker, even though the manual starter labels clearly disallow anything more than 15 or 20 amp short circuit protective device. This is a common violation.
Re: Several motors on one branch circuit#92215 03/09/0510:29 PM03/09/0510:29 PM
I would use a disconnect for each motor with appropriately sized Short circuit and ground fault protection fuses for each motor. The 100Amp breaker is the short circuit and ground fault protection for the Feeders. In addition, each motor should have it's own short circuit and ground fault protection as well as overload protection. To answer your other question: nameplate rating is used to size overload protection. NEC FLA tables (430.147 thru 430.150) are used to size short circuit and ground fault protection.
Re: Several motors on one branch circuit#92216 03/09/0511:50 PM03/09/0511:50 PM
Thanks again for the input. Ian, I found the answer today on using the nameplate for the motor overload protection and tables for the branch circuit protection. Just didn't do enough reading. As far as the job goes, there may be some drastic changes now. The inspectors and the business owner must have had some misunderstandings of one another. I'm on temporary hold right now. The business owner was suppose to have a set of electrical archetectual drawings submitted to the inspection office, and he didn't understand that, and also since he is going to use more of the building for machinery than the inspectors realized, he may have to install a sprinkler system in it now. So before I get anymore involved, I'm going to wait and see what they come up with, when the dust settles. When it does, I may get one of the electrical inspectors to come out on the job, and see what is going on, before I pull to many wires and set disconnects etc. I figured I've got around 30 pieces of machinery to wire, hook up, and set the appropriate disconnects etc. on. Some already have them in the other plant, and some don't, and some have the wrong ones on them. Like I think I said before, most of these machines are probably 30 years old and probably still have some of the same saw dust still on and in them that they had 30 years ago. It's going to be a great challenge to say the least