NEC 430.53 (D) and NFPA 79 7.2.8 (1)

[paulbr]

We are machine builder. Our machines are 460VAC machines. Our electrical enclosures contain multiple motor controllers (35-45). Several motors (3-5) are 30 -50hp motors, and rest of it is from 10hp to 0.5hp. Our enclosure is having main molded case circuit breaker, usially 500 - 700amp. Each power motor circuit is having its own branch circuit protective device sized per motor size.
Question we have is about size of conductors from main circuit breaker to the line side of each branch circuit protective device. According to NEC 430.53(D) these conductors have to be sized not less than 10% of main circuit breaker. If main circuit breaker is 600amp, then conductor to the branch circuit device of 1Hp (4.2A) motor has to be at least 6AWG.
However NFPA 79 2007 paragraph 7.2.8(1) is "reading" that overcurrent protection of conductor at the supply is not required under sertain conditions, and conductors from main circuit breaker to line side of branch circuit protective device of for example 1hp motor (4.2A) can be 14AWG.
We would like to hear opinions of what is right and what other people / companies doing in similar situation.

Thank you

PaulBr.

 

[GoldDigger]

We are machine builder. Our machines are 460VAC machines. Our electrical enclosures contain multiple motor controllers (35-45). Several motors (3-5) are 30 -50hp motors, and rest of it is from 10hp to 0.5hp. Our enclosure is having main molded case circuit breaker, usially 500 - 700amp. Each power motor circuit is having its own branch circuit protective device sized per motor size.
Question we have is about size of conductors from main circuit breaker to the line side of each branch circuit protective device. According to NEC 430.53(D) these conductors have to be sized not less than 10% of main circuit breaker. If main circuit breaker is 600amp, then conductor to the branch circuit device of 1Hp (4.2A) motor has to be at least 6AWG.
However NFPA 79 2007 paragraph 7.2.8(1) is "reading" that overcurrent protection of conductor at the supply is not required under sertain conditions, and conductors from main circuit breaker to line side of branch circuit protective device of for example 1hp motor (4.2A) can be 14AWG.
We would like to hear opinions of what is right and what other people / companies doing in similar situation.

Thank you

PaulBr.

I look forward to reading the response on this, because of some problems understanding the wording of 430.53.

The Article 100 definition of branch circuit is short and clear:

Branch Circuit. The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

The additional definitions of specific types of branch circuits qualify that but do not change it.

However in 430.52, we suddenly have a new concept of branch circuit wiring that starts at a "branch-circuit short-circuit and ground-
fault protective device" and continues, still as a branch circuit, through one or more "branch protective devices" or motor overcurrent protection devices.


My biggest problem lies in the wording in (D)(3) [2011]:

Conductors from the branch-circuit short-circuit and ground-fault protective device to a listed manual motor controller additionally marked ?Suitable for Tap Conductor Protection in Group Installations,? or to a branch-circuit protective device,....

Is "branch-circuit protective device" being used here as an abbreviation for the full wording, "branch-circuit short-circuit and ground-
fault protective device", even though the full wording is used everywhere else but here? Or is it introducing an additional device type. Maybe short-circuit protection only or ground-fault protection only or overcurrent protection only?

The reason that is an issue is whether the exception in (3) is intended to apply only downstream of a "branch-circuit short-circuit and ground-
fault protective device" (BSGP for short) or is specifically worded to apply to conductors tapping directly off feeder or main service conductors and leading to a BSGP?
The reason this is important is in what the reference point is for "an ampacity not less than one-tenth the rating or setting of the branch-circuit short-circuit and ground-fault protective device. "
If this specifically refers to a BSGP located ahead of the tap wires in question, it makes some sense, and it does not allow the tap to be made to either a feeder or a main conductor.
If this refers to a BSGP located downstream of the tap conductors, it makes absolutely no sense to allow that protection to be 10 times the conductor ampacity.

I think that you are not being given good advice about what is and is not allowed.

The condition which allows motor circuit conductors to have an ampacity lower than that of the branch circuit BSGP, as long as it is higher than the level of the motor overload protector, is conditioned on the fact that the overload protection will take care of a situation where the current is only slightly above the ampacity of the wires, and that such a condition involving the motor could not be sustained indefinitely. While the short circuit protection is still effectively provided by the higher rated BGSP, as long as the conductors are heavy enough to carry the fault current until the fuse or inverse time device trips.

Putting smaller wires directly on a feeder or main conductor would be limited by the tap rules contained elsewhere in the code (primarily 240.21 and later) and not specific to motor circuits. And in that case, I do not think that you are allowed to have just a 10% ampacity rating for taps longer than 10 feet. Also that section specifically refers to the ampacity of feeder breakers or feeder wires, not branch circuit wires.

My interpretation of (D) is that it applies to tap connections made on the load side of a BSGP, with each tap going to individual motors or groups of motors which are all overcurrent protected at the far end of the tap at a level much lower than the BSGP provides for. It does not allow you to avoid having a BGSP above the tap wires.

 

[petersonra]

IIRC, there is no length limit on taps that don't leave the enclosure.

I generally add a feeder breaker for the smaller motors so they basically become their own motor group. Easier to wire and most times the feeder breaker with distribution lugs is the simplest way to get the SCCR and often the least expensive.

 

[GoldDigger]

IIRC, there is no length limit on taps that don't leave the enclosure.

And the word "main" circuit breaker and "branch-circuit" now take on totally different meanings from those of the NEC.

 

[petersonra]

And the word "main" circuit breaker and "branch-circuit" now take on totally different meanings from those of the NEC.

does the NEC use the phrase "main circuit breaker"?

 

[mgookin]

We are a manufacturer and we put heavy loads (usually 10x anticipated) on our designs, sustain it, cycle it, etc. in an environmental chamber and record the temps, lifecycle, etc. We set our in-house tolerance at the rating of the component with the lowest value and keep a cushion below that.

NEC does not apply but it's a valid starting point to size a conductor.

 

[milmat1]

I have had questions related to this as well. Mainly because we also are machine builders and the code is very much geared for commercial / residential wiring and not for an industrial control situation. We try to use the code as a guideline however we are not governed by either code, and it comes down to me deciding what is safe and what presents a danger. I have questioned the practice you described before. If you have a main MCCB of say 100 amps then the conductors following that breaker could see 100 amps. And in cases where you are feeding a 20A breaker for a motor circuit. What size conductors need to go from the main CB to the branch circuit CB..In a short circuit condition these feeder wires could likely melt before the main breaker trips..

 

[kingpb]

I would think that the equipment would be governed more by ANSI/IEEE standards since those are related to design, and the NEC is simply for safety related to the general public. Similar to manufacturing of switchgear, transformers or anything else electrical.

 

[mgookin]

I have had questions related to this as well. Mainly because we also are machine builders and the code is very much geared for commercial / residential wiring and not for an industrial control situation. We try to use the code as a guideline however we are not governed by either code, and it comes down to me deciding what is safe and what presents a danger. I have questioned the practice you described before. If you have a main MCCB of say 100 amps then the conductors following that breaker could see 100 amps. And in cases where you are feeding a 20A breaker for a motor circuit. What size conductors need to go from the main CB to the branch circuit CB..In a short circuit condition these feeder wires could likely melt before the main breaker trips..

Look at the wiring in a ceiling fan. What is that something like 18 or 22 awg? It's on a 20A ocpd. And that fan is UL Listed.

 

[GoldDigger]

Look at the wiring in a ceiling fan. What is that something like 18 or 22 awg? It's on a 20A ocpd. And that fan is UL Listed.

And under some circumstances it emits bad smelling smoke without tripping a breaker. But at least it is not hidden inside a wall and more likely to start a fire unnoticed.

For general wiring, the NEC cannot be detailed enough to cover all possible installation and use conditions, and so the exceptions are very limited. The wiring inside a UL listed device can be specifically tested for its behavior under various fault conditions.

 

[weressl]

We are machine builder. Our machines are 460VAC machines. Our electrical enclosures contain multiple motor controllers (35-45). Several motors (3-5) are 30 -50hp motors, and rest of it is from 10hp to 0.5hp. Our enclosure is having main molded case circuit breaker, usially 500 - 700amp. Each power motor circuit is having its own branch circuit protective device sized per motor size.
Question we have is about size of conductors from main circuit breaker to the line side of each branch circuit protective device. According to NEC 430.53(D) these conductors have to be sized not less than 10% of main circuit breaker. If main circuit breaker is 600amp, then conductor to the branch circuit device of 1Hp (4.2A) motor has to be at least 6AWG.
However NFPA 79 2007 paragraph 7.2.8(1) is "reading" that overcurrent protection of conductor at the supply is not required under sertain conditions, and conductors from main circuit breaker to line side of branch circuit protective device of for example 1hp motor (4.2A) can be 14AWG.
We would like to hear opinions of what is right and what other people / companies doing in similar situation.

Thank you

PaulBr.

NEC is not applicable to assembled units. You may want to build it under UL 508.

 

[wallypiper]

I have had questions related to this as well. Mainly because we also are machine builders and the code is very much geared for commercial / residential wiring and not for an industrial control situation. We try to use the code as a guideline however we are not governed by either code, and it comes down to me deciding what is safe and what presents a danger. I have questioned the practice you described before. If you have a main MCCB of say 100 amps then the conductors following that breaker could see 100 amps. And in cases where you are feeding a 20A breaker for a motor circuit. What size conductors need to go from the main CB to the branch circuit CB..In a short circuit condition these feeder wires could likely melt before the main breaker trips..

Nevertheless, the tap rules allow you to size the tap conductors based on the load on that tap as long as they meet the tap rule limits. I do not believe that the 10' limit is waived inside an enclosure. At least, more than one UL panel shop I deal with has installed a breaker to protect wires which will exceed the 10' limit, even inside the enclosure. I deal with a lot of industrial control panels and the practice of running many small taps off of a large main switch (does not have to be a breaker, that protection can be further upstream but the panel label should specify the maximum rating for that breaker) is nearly universal. Typically, I see it done in one of two ways. First is the use of a power distribution block. Wire from the main switch to the PDB is sized according the MCA of the entire panel. Wire from the PDB to individual loads is sized based on those loads. Second is the use of multi tap lugs on the load side of the main switch which just eliminates the short run of large wire from the switch to a PDB.

The questionable practice I see all the time is the use of a non-fused disconnect as the terminating point of the tap conductors. A fuse block is then connected to the load side of that disconnect using the same size conductors. That doesn't really meet the requirement that the conductors terminate in an OCPD, although I suppose if the switch is rated for at least as much current as the conductors, you can argue that it's just part of the tap. I've never seen a panel flagged for it though few ever get any inspection attention, especially if they have a UL label.

Nearly every panel I see is UL508, sometimes ETL, almost never UL listed (not a practical label for custom panels). ETL is common on manufactured equipment such as chillers and compressors. ETL allows minor modifications to a design without voiding the listing, depending on the modification. UL does not. Once a device is UL listed, virtually any change to the design voids the listing requiring expensive and time consuming retesting and re-listing. That is a serious limitation to industrial equipment manufacturers who want to improve the design of their product continuously. Most of the time, the changes are not safety related at all, different connectors or something, but would void a UL listing.

 

[petersonra]

Nevertheless, the tap rules allow you to size the tap conductors based on the load on that tap as long as they meet the tap rule limits. I do not believe that the 10' limit is waived inside an enclosure. At least, more than one UL panel shop I deal with has installed a breaker to protect wires which will exceed the 10' limit, even inside the enclosure. I deal with a lot of industrial control panels and the practice of running many small taps off of a large main switch (does not have to be a breaker, that protection can be further upstream but the panel label should specify the maximum rating for that breaker) is nearly universal. Typically, I see it done in one of two ways. First is the use of a power distribution block. Wire from the main switch to the PDB is sized according the MCA of the entire panel. Wire from the PDB to individual loads is sized based on those loads. Second is the use of multi tap lugs on the load side of the main switch which just eliminates the short run of large wire from the switch to a PDB.

The problem with a PDB is they normally have relatively low SCCR. Some are tested with specific fuses or CBs and can be higher, but most times you end up using the default rating with i think is 10kA if you use CBs.

The questionable practice I see all the time is the use of a non-fused disconnect as the terminating point of the tap conductors. A fuse block is then connected to the load side of that disconnect using the same size conductors. That doesn't really meet the requirement that the conductors terminate in an OCPD, although I suppose if the switch is rated for at least as much current as the conductors, you can argue that it's just part of the tap. I've never seen a panel flagged for it though few ever get any inspection attention, especially if they have a UL label.

IMO, a tap has to terminate at an OCPD except for motor groups where there is a special rule.

Nearly every panel I see is UL508, sometimes ETL, almost never UL listed (not a practical label for custom panels). ETL is common on manufactured equipment such as chillers and compressors. ETL allows minor modifications to a design without voiding the listing, depending on the modification. UL does not. Once a device is UL listed, virtually any change to the design voids the listing requiring expensive and time consuming retesting and re-listing. That is a serious limitation to industrial equipment manufacturers who want to improve the design of their product continuously. Most of the time, the changes are not safety related at all, different connectors or something, but would void a UL listing.

I never heard of an industrial control panel listed by ETL. I suppose it could be, but never seen one. UL508 is the UL standard for industrial control components. There are no control panels listed to UL508.

UL508a is the UL standard for industrial control panels. It is very flexible and a listed shop just has to follow the rules. Mostly this involves using parts approved by UL for use in UL508a listed control panels and following the rules of the NEC. There are a few minor differences, along with some special rules such as wire color coding. We have built many UL listed control panels and while sometimes we have to get creative, we have never had much trouble incorporating design changes within the standard. UL508a is designed around being able to build custom control panels, just like the UL switchboard and MCC standards allow a great deal of flexibility for those manufacturers.

I have trouble with the group motor rules as well. They are very convoluted. One of the things on my list is to come up with some standard designs that use group motor rules to our advantage.

IMO, the tap on a standard group motor installation extends from the Cb it is fed from all the way to the motor. I have been dealing with part of the tap issue by using relatively small breakers with distribution lugs to feed a full size conductor to the motor starter. Therefore there is no tap to the MS. The starters we are using are listed for tap protection so I think, but am not sure that eliminates the conductor downstream of the motor as being a tap. However, there is an odd marking on the side of the starters we are using that says something like "max SCPD acc. NEC". I have talked to both UL and Siemens about what this actually means. I have yet to get an answer that makes all that much sense to me.

I think it means you have to size the feeder breaker according to the max size in the NEC for motor groups. However, in some cases that comes out to less than 15A, and there are no 3 phase breakers we can use that are smaller than that. It is a confusing situation. I am not even really convinced I am doing the calculation correctly.

430.53 is a tough read.