110.14(c)(1) Equipment Provisions.

[msu-ee]

Quick question.

Wire ampacity, when running in conduit, must be sized per Table 310.15(B)(16) (2017 NEC)
When is it appropriate to apply ampacities listed in Table 310.15(B)(17)?

If I'm running power to a Duct Heater, my conductors must be 1.25 x the ampacity of Table 310.15(B)(16).
Now once inside the control panel are the factory wires considered run in open air and fall under Table 310.15(B)(17)?

I have a client that has serveral burnt up factory conductors because the factory seems to have used Table 310.15(B)(17) when sizing the wire from the main disconnect to each of the heating elements.

I can't find where the code handbook actually says what is considered open air.
Maybe I'm just missing it.

thanks for the help.

 

[jumper]

Inside an enclosure is not free (open) air. Free air is outside, for the most cases, overhead and swinging in the wind.

I hope no one is also trying to use the 90C column for final ampacity either.

 

[msu-ee]

Free air, Single conductors

Free air, Single conductors

Overhead and swinging in the wind is typically utility lines which are sized per National Electric Safety Code, not the NEC.
These conductors are also typically triplex wires where we are talking about single conductors.
Where would you apply Table 310.15(B)(17)?

 

[jumper]

Overhead and swinging in the wind is typically utility lines which are sized per National Electric Safety Code, not the NEC.
These conductors are also typically triplex wires where we are talking about single conductors.
Where would you apply Table 310.15(B)(17)?

To small buildings from main structure, overhead festival lights, whatever. Trenching and conduit is more reliable, not as subject to damage, but overhead is way cheaper. Set some poles and roll.

 

[Carultch]

Inside an enclosure is not free (open) air. Free air is outside, for the most cases, overhead and swinging in the wind.

I hope no one is also trying to use the 90C column for final ampacity either.

When you do have free air conductors, that eventually terminate inside of standard 75C rated equipment, do you still ultimately have to comply with the 75C column of 310.15(B)(16) for the terminations? Part of the circuit will eventually be in equipment, or in a riser conduit, where 310.15(B)(16) would apply instead of 310.15(B)(17).

In such a situation, what would be the value of taking credit for 310.15(B)(17)? All I can think of, are if that part of it terminates at open air splices. Such that you can rise with large wire, and splice to smaller wire for the open air portion.

 

[jumper]

When you do have free air conductors, that eventually terminate inside of standard 75C rated equipment, do you still ultimately have to comply with the 75C column of 310.15(B)(16) for the terminations? Part of the circuit will eventually be in equipment, or in a riser conduit, where 310.15(B)(16) would apply instead of 310.15(B)(17).

In such a situation, what would be the value of taking credit for 310.15(B)(17)? All I can think of, are if that part of it terminates at open air splices. Such that you can rise with large wire, and splice to smaller wire for the open air portion.

10 percent of conductor length ampacity rule. I can look it up if you do not know what I am referring to.

 

[Jraef]

If that duct heater is a listed assembly, the internal wire sizing and selection is part of the listing process, along with any OCPD requirements. There are different rules in that process compared to what we must do in field installations. If his wires burned up it was either defective, not a listed assembly, or something about the way it was used or installed was incorrect.

 

[jumper]

If that duct heater is a listed assembly, the internal wire sizing and selection is part of the listing process, along with any OCPD requirements. There are different rules in that process compared to what we must do in field installations. If his wires burned up it was either defective, not a listed assembly, or something about the way it was used or installed was incorrect.

I totally forgot about the duct heater when we sidetracked to free air ampacity.:slaphead:

 

[Carultch]

10 percent of conductor length ampacity rule. I can look it up if you do not know what I am referring to.

I'm aware of the 10ft/10% rule, and that takes care of the portion that is inside the riser conduits. But that still does not address the termination rating rule. If it did, the termination rule would be a moot point in all other cases, since terminations are less than both 10 ft long and 10% of the circuit length.

 

[msu-ee]

Open Air

Open Air

To small buildings from main structure, overhead festival lights, whatever. Trenching and conduit is more reliable, not as subject to damage, but overhead is way cheaper. Set some poles and roll.

I don't see where any of these scenarios are Single wires. Something run overhead on a pole is typically multiple wires spun together, see table discription.

Table*310.15(B)(17) (formerly Table 310.17) Allowable Ampacities of Single-Insulated Conductors
Rated Up to and Including 2000 Volts in Free Air, Based on Ambient Temperature of 30°C (86°F)*

 

[jumper]

I'm aware of the 10ft/10% rule, and that takes care of the portion that is inside the riser conduits. But that still does not address the termination rating rule. If it did, the termination rule would be a moot point in all other cases, since terminations are less than both 10 ft long and 10% of the circuit length.

I never said ignore the termination rule,,,,sheesh.

I addressed one aspect and now I will address the other.

One would still be limited to the 75C column at best for final ampacity.

 

[jumper]

I don't see where any of these scenarios are Single wires. Something run overhead on a pole is typically multiple wires spun together, see table discription.

Table*310.15(B)(17) (formerly Table 310.17) Allowable Ampacities of Single-Insulated Conductors
Rated Up to and Including 2000 Volts in Free Air, Based on Ambient Temperature of 30°C (86°F)*

Um, I was just tossing out random scenarios uI never said this Applies to you, go back and look-I said you conductors are not in free air.

 

[Carultch]

I never said ignore the termination rule,,,,sheesh.

I addressed one aspect and now I will address the other.

One would still be limited to the 75C column at best for final ampacity.

75C column of 310.15(B)(16)?

 

[jumper]

75C column of 310.15(B)(16)?

Yes. Inside an enclosure that would be correct, could be 60C depending on lug ratings and such.

 

[kwired]

If that duct heater is a listed assembly, the internal wire sizing and selection is part of the listing process, along with any OCPD requirements. There are different rules in that process compared to what we must do in field installations. If his wires burned up it was either defective, not a listed assembly, or something about the way it was used or installed was incorrect.

I agree, and will say that my experience with similar items is the overheating is only at terminations and 99% of the time is cheap "quick connect" style connectors - I blame the method/product used more then the conductor not being suitable. If the entire length of conductor has melted insulation then the insulation wasn't a type that could handle the heat it encountered, if only a couple inches at terminals is melted - the terminal was operating too hot.

 

[msu-ee]

I agree, and will say that my experience with similar items is the overheating is only at terminations and 99% of the time is cheap "quick connect" style connectors - I blame the method/product used more then the conductor not being suitable. If the entire length of conductor has melted insulation then the insulation wasn't a type that could handle the heat it encountered, if only a couple inches at terminals is melted - the terminal was operating too hot.

You are exactly correct.
It was the spade crimp on connectors that failed, not the entire length of wire.
After reviewing my pictures closer, they did sticker the front panel with the following (suitable for at least 75 degrees cent.)
More about the heater.
It is a 16000-watt duct heater fed at 208/3p.
The heater is three units connected in a delta pattern with three contactors controlling the stages.
Each main conductor from the disconnect to the contactor (delta corner) can see up to 44.4 amps and the conductors from the contactor to the heating elements see 25.6 amps.
The branch conductors to the unit are sized correctly with no problems.
In the control panel after the main disconnect, the conductors are #10 & #12 cu.
The duct heater is, from what I would consider, a reputable manufacture.
I just don't get it.

 

[Jraef]

You are exactly correct.
It was the spade crimp on connectors that failed, not the entire length of wire.
After reviewing my pictures closer, they did sticker the front panel with the following (suitable for at least 75 degrees cent.)
More about the heater.
It is a 16000-watt duct heater fed at 208/3p.
The heater is three units connected in a delta pattern with three contactors controlling the stages.
Each main conductor from the disconnect to the contactor (delta corner) can see up to 44.4 amps and the conductors from the contactor to the heating elements see 25.6 amps.
The branch conductors to the unit are sized correctly with no problems.
In the control panel after the main disconnect, the conductors are #10 & #12 cu.
The duct heater is, from what I would consider, a reputable manufacture.
I just don't get it.

As I said, different rules. The NEC rules are all about safe INSTALLATIONS and tend to run conservative, because the real world is full of what-ifs and inconsistancies. When manufacturing a product, the design and performance expectations are finite, the manufacturer KNOWS what will transpire inside of the box and what everything must be capable of. So if they want to use wire of a smaller gauge, and UL tests and accepts it under their rules and the expected performance, it's done. WE don't have to concern ourselves with the internal wiring, just what we run to it and how we terminate it.

Classic case in point; open up a large motor or transformer and look at the size of the conductors used in the windings. ALL of the power going to those windings comes from our conductors, sized per the NEC. But inside of the motor or transformer, the magnet wire, which is still carrying ALL of the same current, is 2 or 3 sizes smaller than what we must run. That's because that motor or transformer was built, tested and listed for what it is expected to do. Our wiring in the field is not.