Feeder sizing in UL508 panel

[Kiljoy]

Help me out. I’m a bit confused. We build industrial control panels for industrial machinery per UL508a. We use an instantaneous circuit breaker as the machine/panel disconnect. The current panel I am designing has two 30HP motors, one 0.5HP motor, and a 500VA control transformer. All of the motor branches use Type E self protected starters.
Here is where I am confused. The code states that the disconnect can not be sized for more than 80% of it’s rating. I’ve had to bump up to a 150A disconnect. The FLA on the panel is 94A. I’ve multiplied the two 30HP motors by 125% due to our machines being designed to use service factor during part of the process. Do I size the wire from the bottom of the breaker to the distribution block based on the FLA (94A) or do I size it for the size of the breaker (150A)? Could this be considered feeder protection as well as disconnecting or will the customer need to supply feeder protection in the field?

 

[waross]

The sizing of field wiring is based on:
100% of the full load rating of all components including service factors. (There may be an exception if not all components will operate simultaneously, such as two pumps that alternate but will never operate together.)
Plus 25% of the largest motor,
OR
Plus 25% of the largest group of motors that will start simultaneously.
Whichever is larger.
Determine the overload protection your conductors require. If the breaker is suitable it may be the overload protection in some cases. This scheme is most often used for transformer installations. Not recommended for panel feeds.
BUT
Over-current protection must be installed at the supply end. You are better to use over-current protection that also provides overload protection at the supply end of the feeder.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[REDDOG]

I thought that only listed thermal magnetic circuit breakers, could be used as a panel disconnect and over current protective devise. Magnetic only (MCP) are UL recognized devise and can used only with a listed starter combination that has been evaluated by UL.
The 150 amp thermal magnetic circuit breaker will have to have 150 ampere at 75 degree C rated Ampacity wire from its load terminals to your power distribution block.

 

[Kiljoy]

The code states that an instantaneous trip breaker can be used as a panel disconnect in section 30.1.

Let me make sure I have this right. Since my FLA is 94A, I’d like to use a 100A breaker. But because of the 80% rule, I have to bump up to the next size, which is 150A. In that case I need wire rated for at least 150A between the load side of the breaker and my panel distribution block. Correct? BTW, is this wire considered a feeder?

This also leads me to one more question. Let’s say I do not intend our machine disconnect breaker to protect the feeder supplying the machine. Do I have to list, on my electrical print, the size and rating of the feeder protection? I do not know if an end user will have a breaker or fuses. My thought is that it’s up to the installer of the equipment to properly size the service and feeder protection based on his local codes.

 

[DRWeig]

Kiljoy,

Our UL inspector would require a label in the panel -- all you have to do is specify feeder size (conductor), voltage, and protection (amps), the customer can choose breaker or fuse.

Good on ya,

Goober Dave

 

[jraef]

The code states that an instantaneous trip breaker can be used as a panel disconnect in section 30.1.

What code are you reading? 30.1 is not an NEC reference as far as I know.

You cannot use instantaneous only circuit breakers for anything other than listed factory assembled combination motor starters in a very narrowly specific tested combination with specific other devices (did I mention specific and narrow?). Using them as a main disconnect is not one of those applications, unless you have submitted and tested them to UL test standards, something that would likely cost around $20,000 per line item combination, which effectively limits it to the major motor control manufacturers. I think maybe you are misinterpreting something.

Unless I am...

In the NEC, this is covered under Article 430.52.3, and says IT breakers are allowed "...if part of a listed combination motor controller having coordinated motor overload and short-circuit and ground fault protection...". The "listed" part of that then falls back to some NRTL, all of which will use UL508c as the test standard. Note that is UL508c, not 508a. UL508a has no provisions that I know of for allowing the use of UR short circuit devices such as this without submitting to UL for testing and approval, and I guarantee you, they will want their $20K... If you have found a UL inspector who is letting you get away with that, you have found a gold mine! But be forewarned, if anyone else in the industry finds your panels and asks UL about it, you are toast.


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[Kiljoy]

Jref,

30.1 is in the UL508a standard for industrial control panels. Our company is certified to produce panels for our machines that comply with the 508a code. As part of our program, we pay an annual fee, pay for the UL cUL stickers, and pay for 4 random quarterly inspections. As long as we build the panels using UL approved components and follow the 508a code, I can assure you we are in full compliance and there is no roasting of bread involved. Believe me, UL gets their money.

 

[jraef]

Sorry, I no longer have my UL508 documents.

But... I don't think you can do what you are claiming without SPECIFIC testing of the combination of devices you are using. As I understood it when I owned a 508a shop, 508a is for control panels using LISTED components. So for instance, you can use Thermal-Mag circuit breakers, because those ARE listed by UL. IT breakers are not, they are only RECOGNIZED components. You can use recognized components (UR) by adding them to your procedure and submitting to UL, but my experience is that they do NOT allow IT breakers to be used without testing, which is above and beyond what 508a covers; the testing procedures are spelled out in 508c, which is what the manufacturers build to.

I will be at another 508a shop tomorrow, I'll ask if there has been a recent change I wasn't aware of in that regard.




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[Kiljoy]

Maybe I misspoke when I said instantaneous trip. The breakers we use include a fixed thermal and fixed magnetic trip. I think the larger breakers have an adjustable trip. We were using the Siemens ED series and now are using the AB 140U series. Since they are not inverse trip, I assumed they were instantaneous. Regardless, the section 30.1 says you may use an IT breaker as a main disconnect if it complies with some other UL standard. I also don’t have my docs in front of me. I’ll look it up in the morning.

 

[Kiljoy]

Adjustable magnetic trip that is...

 

[jraef]

If it has a thermal trip then it is not an IT only breaker, you can use that just as you are.


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[Kiljoy]

30 Disconnect Switches
30.1 Component requirements
30.1.1 An inverse-time or instantaneous-trip circuit breaker shall comply with the requirements in the
Standard for Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures, UL
489.

In the mean time, I went and created part numbers for in between size breakers, so I don't have to jump so much.

I'm still a bit confused as to the sizing of the wire between the breaker and distribution block. Is that considered a feeder or branch circuit? Does the wire need to be sized equal or greater than the breaker or is a little below OK?

Jref, If my thermal magnetic breaker is not in inverse time and it’s not an instantaneous trip, what is it? The UL code only mentions the two types. Am I missing something?

 

[jraef]

A thermal-mag breaker has both. The thermal trip is the inverse time (I²t) trip, the magnetic trip is the instantaneous trip. The I²t trip is intended to protect the conductors from long-time thermal overloads. The instantaneous trip is intended to protect against short circuits and grounded faults (although not in the sense of an Ground Fault Sensor trip). The reason why IT only breakers are not suitable for main circuit breakers is that they provide NO long term I²t protection. Their intended use is only for where you have ANOTHER thermal (I²t) device somewhere else in the circuit, such as an overload relay in a motor starter. But UL requires that the two devices be tested together.

I don't remember seeing that reference before, it's either something new or it is out of context, I have a hard time with UL stating that it is OK to use an IT only breaker as a main as long as it meets only the requirements of an MCCB, in fact I would take that as a given. So this may be a semantics issue.

But to your specific issue however, the wire from the breaker to the line side of the distribution block would need to be sized to be protected by that breaker. But the way to do it is to look at your total load, size the conductor to 125% then size the breaker to protect the conductors. You should end up with a 125A breaker IMHO. The wires from the distribution block to the Type E starters are considered to be protected by the Type E starter trip devices.


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[Kiljoy]

Great thanks!

I think it's OK, because other sections state that if over current or branch circuit protection is not included, the manufacturer must list that it is to be supplied by the end user and to specify the proper size. You could also use a simple disconnect switch if you’ve included the proper fusing down stream.

 

[LionelHutz]

Why don't you quote the whole paragraph and not put things out of context...

31.1.1 An inverse-time or instantaneous-trip circuit breaker shall comply with the requirements in the
Standard for Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures, UL
489. An instantaneous-trip circuit breaker, in combination with the motor controller and motor overload
device, shall additionally comply with the requirements for combination motor controllers in the Standard
for Industrial Control Equipment, UL 508.

And also in SA1.1 - Instantaneous trip breaker = Procedure described only.

So, you CAN NOT just use an Instantaneous trip breaker however you please.


I would use a 125A thermal magnetic breaker, put #2 wire to the distribution block and appropriately sized wire from there to each motor starter.

 

[Kiljoy]

Lionel,
I was not attempting to show anything out of context. If you read my previous posts more closely, you will see that Jref and I were trying to establish that you can use an IT breaker as a panel/machine disconnect, which you can. Further down, I post that I AM using a thermal magnetic trip breaker and not an instantaneous breaker. I was mistaken as to the definition of an IT breaker and thought I was using one. I am not.
I do not have the code here, but I think the section you are listing is for a branch circuit, not a main disconnect. I listed 30.1, not 31.1.