Cable Ampacities vs. OCPD rating

[mr_rhino]

Apologies in advance for the lengthy post.

I am looking for recommendations from my fellow consulting colleagues regarding a situation that recently came to my attention while reviewing a client?s wiring documentation.

My client is in the elevator industry, and like virtually all companies in this trade, uses flexible multi-conductor ?travel cables? to provide communications, control, signaling and power between the machine (control) room and the moving elevator car. NEC Article 620 governs details for elevators; however, because nothing is specifically identified in 620, I?m left to rely on Article 725 (Class 1 circuitry) and Articles 310 & 400 for guidance regarding proper ampacity adjustment of the individual conductors in the travel cable.

A typical elevator travel cable has three (or possibly more) different types of circuits:

1. ?Lighting circuit(s)? ? Provides power for in-car (passengers) and car-top (service) lighting, in-car ventilator, and in-car and car-top outlets. The in-car lighting and ventilator are considered continuous use, while the others are typically intermittent/non-continuous loads. There are typically <1 dozen conductors of this type in the travel cable.

2. ?Safety and control circuits? (Class 1): typically ~3 dozen conductors of this type in the travel cable.

3. ?Signaling and communications circuits? (Class 1): typically ~2 dozen conductors of this type in the travel cable.

I am particularly concerned about the typical 20A circuit breaker rating vs. the derated conductor ampacities for the typical ?lighting circuit? conductors in the travel cable.

Several of my client?s electrical engineers (employees) believe that the 12AWG lighting circuit conductors can be protected by a 20A breaker regardless of the number of conductors in the travel cable (or any additional considerations). I disagree, pointing to the following adjustment contributors:
1. Number of current-carrying conductors in the cable,
2. Temperature rating of the conductor insulation (60C),
3. Minimum temperature rating of the terminals that the conductors are wired to (60C), and
4. Maximum ambient temperature (40C).

There is some debate between one of the engineers and myself as to which conductors must be considered ?current carrying?. My understanding, per NEC Article 725.28 (B), is that any of the Class 1 conductors carrying more than 10% of their temperature-adjusted ampacities are considered to be current carrying, and must therefore be included when calculating the adjustment factor in Table 310.15 (B)(2)(a).

Let?s determine the maximum OCPD ratings for each of the circuit conductors. As a hypothetical exercise, let?s assume the following to make sure that everything is being calculated correctly:
1. 6 lighting circuit conductors, 12AWG each; all can be considered current carrying.
2. 3 equipment grounding conductors, 12AWG; none are considered current carrying.
3. 32 safety and control conductors, 16AWG each; Based upon NEC 725.28(B), 75% can be considered current carrying.
4. 22 signaling and communications conductors, 20AWG each; none will be considered current carrying. (typically <10mA each).

In this example, we have (6 x 100%) + (32 * 75%) = 30 current-carrying conductors in the travel cable. Based upon my calculations, the maximum allowable continuous load on each 12AWG lighting circuit conductor is ~9.2 Amperes, with a maximum end-to-end length of ~100 feet for less than 3% voltage drop. It seems clear to me that you wouldn?t protect these lighting circuit conductors with 20A circuit breakers.

I am recommending that the client install inline 12A fuse between the 20A circuit breaker(s) and the travel cable 12AWG lighting circuit conductors. Do you agree? If not, why? What am I missing?

Thanks,
Jeff

 

[steve66]

I would think 310.15(A)(2), Exception #1 would make the control and signal cables non-current carrying.

I'm also not sure the NEC even really applies. Aren't elevators listed as an entire unit (much like an appliance)? So aren't the traveling cables and associated OCP covered by that listing?

Steve

 

[mr_rhino]

Please keep the input coming!

Please keep the input coming!

Steve66,

1. I missed how you arrived at the conclusion that control and signal wires are not current-carrying based upon 310.15(A)(2) Exception 1. The example following the exception in the 2005 Handbook only talks about multiple ampacities of the same conductor depending upon ambient temperature variations along a given section of wire.

2. Elevators are listed like appliances in Europe, but not here in North America. NEC 620.11 and 620.12 specifically discusses Travel Cables. In addition, Table 400.4 Note 5 and Table 400.5(A) also discuss Elevator Travel cables. Any wiring outside of a UL/CSA-listed piece of equipment must comply with the NEC (or CEC in Canada).

Thanks for the inputs!
Jeff

 

[don_resqcapt19]

Jeff,
The control and signal wires are not "power and lighting conductors". For the purposes of derating you are only required to count the power and lighting conductors as current carrying conductors.
Don

 

[Smart $]

Jeff,

The area of your analysis I most question is the 75% determination of the safety and control conductors (Class 1, non-power, non-lighting) as powering continuous loads over 10% of their temperature-adjusted ampacity? Its impossible for anyone on this end of your thread to either confirm or contradict your conclusion without knowing the circuitry... and if the design is as described, I'd say the engineers need to "put it back on the drawing table"

What ampacity are you assigning to the 16 AWG conductors? TTBOMK, the NEC doesn't list an ampacity for 16AWG@60?C anywhere.

Additionally, I have to question your use of 40?C maximum ambient temperature as the ambient adjustment temperature. Under what conditions will the temperature in the elevator shaft reach and sustain a temperature of 104?F? Are you saying the elevator car has its own AC unit or the passengers just have to suffer while they ride? ...or did the designers plan for the doors to open regularly for an air exchange? :grin:

 

[haskindm]

Look at 240.4(B). This allows you to use the next standard size circuit breaker when your calculated ampacity does not match a standard size. The smallest standard size circuit breaker is 15-amps, so that would be usable on the 12-gauge wire, even if we accept your derating to 9.2 amps. There is nothing in the NEC that would require in-line fuses of a smaller ampacity. Also keep in mind that the purpose of circuit breakers is to protect conductors, they should not be used to limit loads. This load should be stable and predictable, it is not as if you are supplying a circuit with multiple receptacles with no control over what will be connected to the circuit. In this case the designer can determine exactly what the load will be and design the circuits accordingly. Smaller overcurrent devices should not be used as a substitute for good design.

 

[mr_rhino]

Response to don_resqcapt19

Response to don_resqcapt19

Don-
You stated:
?The control and signal wires are not ?power and lighting conductors?. For the purposes of derating you are only required to count the power and lighting conductors as current carrying conductors.?

Can you point to somewhere in the NEC that confirms your statement? Unless I missed something here, it appears that NEC 725.28 contradicts your statement.

Thanks,
Jeff

 

[mr_rhino]

Response to Smart $

Response to Smart $

Smart $ -
You stated:
1. ?What ampacity are you assigning to the 16 AWG conductors? TTBOMK, the NEC doesn?t list an ampacity for 16AWG@60?C anywhere.?

- NEC Table 400.5(A) lists 16AWG as 10A (if only 3 current-carrying conductors in ETT cable) or 13A (if only 2 current-carrying conductors in ETT cable).

- Manufacturer (who shall remain nameless) rates all conductors (including 16AWG) in the ETT cable for 60C. (You?ll have to assume that this is correct)

- Manufacturer specified baseline ampacity of 16AWG in ETT cable as 10A per Table 400.5(A). (You?ll have to assume that this is also correct)

2. ?Under what conditions will the temperature in the elevator shaft reach and sustain a temperature of 104?F? Are you saying the elevator car has its own AC unit???

- The client?s product marketing spec identifies a maximum operating temperature of 40degC, although the recommended maximum operating temperature is 30degC (86degF). The system is listed for operation up to 40degC (hoistway and machine room), and therefore all wiring must be adjusted accordingly.

- Is there somewhere in the NEC that differentiates between reaching and sustained max temp adjustment factors? From what I understand, the thermal time constant of elevator travel cables and most common building wire is somewhere in the order of seconds or minutes, not hours or days. If the temp can reach 104degF (but not go higher), from my perspective it really doesn?t matter how long it stays there.

3. ?Its impossible for anyone on this end of your thread to either confirm or contradict your conclusion without knowing the circuitry??

- Unfortunately a Non Disclosure Agreement (NDA) regarding internal circuit details binds me from providing more information. You?ll have to take my word that 24 of the 32 16AWG Control conductors have continuous currents that exceed 10A * 0.82 * 10% = 0.82A. (where 10A comes from cable manufacturer, 0.82 comes from 60C correction factor for 60C insulation in 40C ambient per NEC Table 310.16, and 10% comes from NEC 725.28).


With the above information, do you have enough to disagree or agree with my conclusions?

Thanks,
Jeff

 

[mr_rhino]

Response to haskindm

Response to haskindm

Haskindm -

You stated:

?Look at 240.4(B). This allows you to use the next standard size circuit breaker when your calculated ampacity does not match a standard size. The smallest standard size circuit breaker is 15-amps, so that would be usable on the 12-gauge wire, even if we accept your derating to 9.2 amps. There is nothing in the NEC that would require in-line fuses of a smaller ampacity. Also keep in mind that the purpose of circuit breakers is to protect conductors, they should not be used to limit loads. This load should be stable and predictable, it is not as if you are supplying a circuit with multiple receptacles with no control over what will be connected to the circuit.?

- I apologize for not making the situation clearer. NEC 240.4(B) is only valid when you do not have a multi-outlet branch circuit. Unfortunately one pair of 12AWG conductors within the travel cable provide power to outlets within the elevator car (typically for building cleaning personnel) and on top of the elevator car (for elevator service mechanics).

- I should make it clear that the adjusted ampacity of the single 12AWG conductor is 9.2A continuous, or 11.25A non-continuous. If 240.4(B) was applicable, I could use a 15A breaker or fuse. However, because it is not applicable, I must use a 10A fuse.

- Like all of us, I am fully aware that OCPDs are used to protect conductors, not loads. My intention of installing a 10A fuse [a standard rating ? see 240.6(A)] is to protect the 12AWG conductors? temperature-adjusted and bundling-adjusted ampacity with a suitably-rated OCPD.

- If my client chooses not to replace travel cables already deployed (a very-costly option), my proposal is to add a 10A in-line fuse is for all systems already deployed in the field. Probably like most of you, I too am concerned that the fuse may blow when the cleaning or service personnel plug in a vacuum or a tool ? since the circuit should have a capacity of providing 16A continuous or 20A max. (with 20A-rated outlets)

- For new systems I am recommending to my client that 2 pairs of 12AWG conductors be paralleled (as allowed per 620.12(A)(1) to provide an adjusted combined ampacity of 23A, which exceeds the 20A rating of the upstream breaker.

With the above information, do you have enough to agree or disagree with my conclusions?

Thanks,
Jeff

 

[steve66]

I would think 310.15(A)(2), Exception #1 would make the control and signal cables non-current carrying.

I'm also not sure the NEC even really applies. Aren't elevators listed as an entire unit (much like an appliance)? So aren't the traveling cables and associated OCP covered by that listing?

Steve

My mistake, it is 310.15(B)(2) Exception #1: Where conductors of different systems.. are installed in a common raceway or cable, the derating factors shown...shall apply only to the number of power and lighting conductors."

You can check the references I omitted from the quote. See if that doesn't make the installation look any better.

I don't see how wiring for a receptacle would need to be derated when it is only used for cleaning and service. I would assume this would only occur when the elevator isn't running.

And I don't think the NEC will permit a 10A fuse to be installed in a receptacle circuit.

Steve

 

[mr_rhino]

Derating factors for Power/Lighting only

Derating factors for Power/Lighting only

Steve,

Thanks for the clarification. After reviewing exception 1 to 310.15(B)(2) and the discussion in the HDBK, you are correct that the situation appears much better.

"I don't see how wiring for a receptacle would need to be derated when it is only used for cleaning and service. I would assume this would only occur when the elevator isn't running." -

As it turns out, the total number of continuous "current-carrying" conductors is reduced to 6 when the car is being serviced or being cleaned. Four of those conductors are the 16AWG control & signaling conductors; the other two are for the outlets/lighting. The four current-carrying 16AWG conductors should not significantly increase the temperature of the power and lighting conductors, and therefore per exception 1 to 310.15(B)(2), can probably be ignored for derating purposes of the lighting/power conductors.

I think we're all in agreement that the NEC wouldn't permit a 10A fuse on a receptacle circuit.

Thanks once again,
Jeff

 

[LarryFine]

I just got off the phone with a good friend who used to build elevators and now designs and builds processor-controlled selectors. Here's the gist of our conversation:

1. He's never seen a travel cable with more than a single power circuit, and almost never a car with an internal receptacle, except for "weird government jobs."

2. The greatest continuous current in any conductors is that of the safety circuit (basically a large loop similar to a normally-closed alarm circuit), and that's usually less than 0.1a

3. The lighting and fan load rarely exceeds 3a. and he's never, ever felt a warm travel cable. (Yes, I explained that we're discussing OCP and not conductor loading.)

4. The greatest "continuous" car loads, other than the lights and fan, are the panel indicator bulbs, which at 120v and 3w, have a current of (3/120) 0.025a.

5. The current in the "safety and control circuits" was less than 0.1a in relay-based controllers, and more like 0.01a with modern processor-controlled systems.

6. The building electricians always supplied whatever they supplied, typcially a standard 20a circuit, with no regard for the size of the cable conductor size (12 or 14ga.)

Bottom line: he considers this a non-issue, and believes that derating does not apply.

 

[Smart $]

Smart $ -
...
With the above information, do you have enough to disagree or agree with my conclusions?

I have to agree... but with reservations due to undisclosed information.

Is there no other measures that can be taken than reduction of OCPD ratings? Perhaps increasing conductor sizes. Would increasing some or all of the 16AWG safety and control conductors to 14AWG (15A * 0.82 * 10% = 1.23A) decrease the number of ccc's significantly? Why are these circuits conducting at such high (comparatively speaking) current levels? Does the control circuitry use PLC technology?

 

[haskindm]

Haskindm -


- I apologize for not making the situation clearer. NEC 240.4(B) is only valid when you do not have a multi-outlet branch circuit. Unfortunately one pair of 12AWG conductors within the travel cable provide power to outlets within the elevator car (typically for building cleaning personnel) and on top of the elevator car (for elevator service mechanics).

- For new systems I am recommending to my client that 2 pairs of 12AWG conductors be paralleled (as allowed per 620.12(A)(1) to provide an adjusted combined ampacity of 23A, which exceeds the 20A rating of the upstream breaker.

With the above information, do you have enough to agree or disagree with my conclusions?

QUOTE]

Jeff,
I believe you are grossly over complicating the situation and seeing problems where none exist. If 240.4(B) does not apply because it is a multioutlet circuit, then you will need to go to 210.24 and table 210.24 which does cover multi-outlet branch circuits. In this case you would install a 20-amp overcurrent protection on a multioutlet branch circuit using #12 AWG wire. Again, I don't recall anything in the code that would require the use of a circuit breaker smaller than the standard size of 15-amps.
I believe you are really stretching the permission granted in 620.12(A)(1) that allows you to parallel small conductors. This permission is to allow the use of paralleled conductors of 20-gauge and larger to equal at least a number 14-AWG wire. Since your basic wire already exceeds 14-AWG I don't think this section applies. 310.4 prohibits the paralleling of conductors smaller than 1/0 AWG and it is my belief that this would take precedence.
It appears that you have decided that there is a violation here, where there really is none, and are trying to creatively interpret the code back you up.

 

[mr_rhino]

Haskindm,

Much thanks for responding back.

QUOTE: "I believe you are really stretching the permission granted in 620.12(A)(1) that allows you to parallel small conductors. This permission is to allow the use of paralleled conductors of 20-gauge and larger to equal at least a number 14-AWG wire. Since your basic wire already exceeds 14-AWG I don't think this section applies. 310.4 prohibits the paralleling of conductors smaller than 1/0 AWG and it is my belief that this would take precedence."

NEC 90.3: "Chapters 1,2,3, & 4 apply generally; Chapters 5, 6, & 7 apply to special occupancies, special equipment, or other special conditions. These latter chapters supplement or MODIFY the general rules." This allows 620.12(A)(1) to supercede 310.4.


QUOTE: "Jeff,
I believe you are grossly over complicating the situation and seeing problems where none exist....It appears that you have decided that there is a violation here, where there really is none, and are trying to creatively interpret the code back you up."

No, I'm not looking for a violation, and I'm truly not looking to overcomplicate the situation. I'm attempting to be as thorough as possible so that my client is not liable for any repercussions. I find that far too often people oversimplify things because they are either lazy, ignorant, or just don't care. I just want to do my "homework".

I really appreciate all the valuable input that you and others have provided so far. At this point I'm inclined to agree with Steve's inputs in a prior message, where 310.15(B)(2) exception 1 helps the situation, though I see a possibility that 725.28(B) could take precedence over 310.15(B)(2).

Regards,
Jeff

 

[kingpb]

The NEC does not apply to elevator designs or the internal wiring. It only applies to the wiring up to the elevator. No different then say an HVAC unit.

I have to question the basis for the OP, as it seems the intent was to merely post a question and argue a trivial point that has no basis in the NEC anyway.

Just my HO

 

[haskindm]

Jeff,
Look again at 620.12(A)(1). I know about the later chapters modifying chapters 1-4. The first sentence states that wiring for lighting circuits must be #14 AWG copper, period. The next sentence allows you to parallel wires SMALLER than #14 in the traveling cables so that they equal at least a #14 AWG. Since your wires are #12, they already equal (indeed exceed) the required #14. Also this section ONLY applies to lighting circuits, so it does NOT apply to the circuit for receptacles. There is NOTHING in this section that overrides the basic restriction against paralleling wires as small as #12.

 

[Smart $]

I really appreciate all the valuable input that you and others have provided so far. At this point I'm inclined to agree with Steve's inputs in a prior message, where 310.15(B)(2) exception 1 helps the situation, though I see a possibility that 725.28(B) could take precedence over 310.15(B)(2).

Now I remember what my short-lived train of thought was... but let's recap first.

Jeff,
The control and signal wires are not "power and lighting conductors". For the purposes of derating you are only required to count the power and lighting conductors as current carrying conductors.
Don

Jeff,

The area of your analysis I most question is the 75% determination of the safety and control conductors... Its impossible for anyone on this end of your thread to either confirm or contradict your conclusion without knowing the circuitry...

Smart $ -
You stated:
...
3. ?Its impossible for anyone on this end of your thread to either confirm or contradict your conclusion without knowing the circuitry??

- Unfortunately a Non Disclosure Agreement (NDA) regarding internal circuit details binds me from providing more information. You?ll have to take my word that 24 of the 32 16AWG Control conductors have continuous currents that exceed 10A * 0.82 * 10% = 0.82A. (where 10A comes from cable manufacturer, 0.82 comes from 60C correction factor for 60C insulation in 40C ambient per NEC Table 310.16, and 10% comes from NEC 725.28).

My train of thought prior to hitting the NDA snag was to distinguish whether or not the safety and control conductors which you are counting as ccc's are in fact of a different system or systems:

Is the current passing through these conductors a result of a direct electrical connection to the same source as power and lighting?

...or...

Do the currents pass through closed-loop circuits which are only inductively coupled to the source of power and lighting (read: powered through a control transformer, for example)?​

Conductors of the latter circuits are of a different system.

 

[mr_rhino]

Is the current passing through these conductors a result of a direct electrical connection to the same source as power and lighting?

...or...

Do the currents pass through closed-loop circuits which are only inductively coupled to the source of power and lighting (read: powered through a control transformer, for example)?​

Conductors of the latter circuits are of a different system.

A17.1 elevator code requires that the lighting/receptacle circuit be sourced directly by the building, separate from any elevator control circuits. So to answer your question, yes they are of different systems. Therefore I'm inclined to follow 310.15(B)(2) exception 1 and ignore the impact of the 16AWG control conductors in terms of bundling ampacity adjustment factor.

Thanks to Smart $, Haskindm, Larry Fine and Steve66 for taking time to walk thru this with me, and help interpret the code. Folks like you make these forums truly useful.

It's unfortunate that some folks may have felt that my "intent was to merely post a question and argue a trivial point that has no basis in the NEC." Also, while many of you may never have to deal with providing more than "the wiring up to the elevator", NEC Article 620 makes it clear that the elevator designer/installer still needs to follow the NEC for interconnections outside of UL/CSA-listed equipment (such as between the controller and the hoist machine, or between the controller and elevator car).

Regards,
Jeff

 

[kingpb]

Golly, jeepers! I guess because I didn't role over and agree, I better pick up my marbles and scurry on home.

So, I went back and re-read the posts, and came to the same conclusion....

The original post, and arguments/rebuttals to each post thereafter, appears that the intent is to merely try and justify a decision or recommendation made to a client, and consequently looking for back-up to support the original opinion, since the recommendation was possibly met with disagreement.

On that basis, I will not go home