Conductor ampacity

[david luchini]

I don't see why not. 110.14(C) is just telling us Table 310.16 is the default. Equipment with terminations outside of enclosures for which it makes sense to allow termination ampacities from other Tables simply needs to so "listed and marked" to allow that to happen.

There is no requirement in 110.14(C) for terminations outside of enclosure to be "listed and marked" at all. They can be the same terminations that are used within enclosures.

 

[wwhitney]

There is no requirement in 110.14(C) for terminations outside of enclosure to be "listed and marked" at all.

Agreed, and what you quoted from me does not say otherwise. What it says is that if such equipment wants to use termination ampacities other than from Table 310.16, then it would need to be so listed and marked, per 110.14(C).

Cheers, Wane

 

[solarken]

Terminations means at equipment, like a PV inverter or a breaker or utilization equipment, yes? Then typically those terminations will be in an enclosure, so referencing Table 310.16 as the default seems perfectly reasonable to me. If you have some atypical equipment where the terminations are actually in free air, the language allows the equipment to listed for other termination ampacities.

So I'm not seeing the problem. For your outdoor free air PV application, you can use say an inverter with a conduit riser and weatherhead, and splice at the weatherhead. The conductors between the splice and the inverter would likely be governed by the termination ampacity within the inverter. While the other conductors may be in free air and governed by Table 310.17.

Cheers, Wayne

You can have an enclosure with free air running through it. Free air in the definitions is "Open or ventilated environment that allows for heat dissipation and air flow around a conductor. It is not the same as 3 current carrying conductors in a conduit. It sure seems like there should be some room for reason here. Single conductors inside a large container (relative to the cross section of the conductor) can certainly have a lot of air flow around them, and heat could dissipate into that air.

Many inverters or other equipment have active cooling, with fans and air circulation. It seems odd that 110.14(c) does not just keep to specifying the temperature limits, since that is the subject of that section, and leave the ampacity up to where it belongs, namely 310.

 

[solarken]

What it says is that if such equipment wants to use termination ampacities other than from Table 310.16, then it would need to be so listed and marked, per 110.14(C).

Cheers, Wane

What would the listing and marking look like? "These terminals can use either Table 310.16 or Table 310.17 to calculate ampacities"?

 

[wwhitney]

You can have an enclosure with free air running through it. Free air in the definitions is "Open or ventilated environment that allows for heat dissipation and air flow around a conductor

Thanks for pointing out that definition, it's new in the 2020 NEC and I never noticed it. I wonder what ventilation rate would be required through a 4" conduit to allow the use of Table 310.17. : - )

Anyway, equipment using a ventilated enclosure can choose to pursue a listing allowing a marking for termination ampacity based on something other than Table 310.16.

Many inverters or other equipment have active cooling, with fans and air circulation. It seems odd that 110.14(c) does not just keep to specifying the temperature limits, since that is the subject of that section, and leave the ampacity up to where it belongs, namely 310.

It's only specifying Table 310.16 as a default with respect to the termination ampacity, seems reasonable to me. It doesn't preclude the use of other tables for other portions of the circuit.

Cheers, Wayne

 

[Jpflex]

I agree the code as written is confusing. 110.14(C)(1) states "unless the equipment is listed and marked ... conductor ampacities shall be based on Table 310.16 as ... modified by 310.12." 110.14(C) is intended to specify temperature limitations when selecting conductor ampacities, as I read it in order to protect against exceeding rated terminal limits. It really should have nothing to do with the sizing of the conductor, which is addressed in 310. Yet it directly references Table 310.16 which specifies ampacities for a specific situation, which is conductors in conduit or in a cable. I don't think it is about Electricians trying to squeeze more ampacity out, but there are applications where we have single conductors in free air, and that should use Table 310.17, not 310.16.

It is common in Solar PV and battery storage systems to use single conductors in free air, and maybe also other applications. The 75C ampacity of 1AWG copper with 3 CC in conduit from Table 310.16 is 130A, while in 310.17 it is 195A. We should be able to use 195A when using single conductors in free air, don't you agree?

It seems that 110.14(C)(1) could instead just reference conductor ampacities per 310, and not specifically reference Table 310.16, and that would allow the use of Table 310.17 for appropriate instances.

I used to thinl the ampacoties in other tables could be used based on the type of wiring method. This wpuld have included higher ampacity for coductors in free air


However based on the nec explanation, components to be terminated are ul tested with the 60 degree column in table 310.16. Therefore the other tables can only be used to start with a higher ampacity but must be reduced to table 310.16 table after temperature cotrection and adjustments. That is how its worded in code

 

[Jpflex]

Thanks for pointing out that definition, it's new in the 2020 NEC and I never noticed it. I wonder what ventilation rate would be required through a 4" conduit to allow the use of Table 310.17. : - )

Anyway, equipment using a ventilated enclosure can choose to pursue a listing allowing a marking for termination ampacity based on something other than Table 310.16.


It's only specifying Table 310.16 as a default with respect to the termination ampacity, seems reasonable to me. It doesn't preclude the use of other tables for other portions of the circuit.

Cheers, Wayne

Not according to nec

 

[wwhitney]

Not according to nec

I think you may be mistaken, although I'm not clear on what you mean. If you have a termination that isn't listed and marked otherwise, you need to Table 310.16 to determine the termination ampacity. The circuit ampacity will be limited to that value, you can't exceed it. But elsewhere in the circuit, if the conductors meet the requirements of one of the other ampacity tables, you can use that table to determine the conductor ampacity.

So for example, say you want to run a 150A overhead feeder from one building to another, using a messenger wire. And you have risers and weatherheads on each building, and use a 90C rated splice at each weatherhead to transition from the wire in the conduit to the wire on the messenger. And say the outdoor high temperature is 40C.

If we use 90C conductors throughout, then portions of the circuit up to each weatherhead will likely be governed by equipment terminations and Table 310.16. As the equipment will have 75C terminations, we will need 1/0 Cu or 3/0 Al to have a 75C Table 310.16 ampacity of at least 150A. And both of those are fine in the outdoor risers exposed to 40C ambient, as with 90C conductors the temperature correction factor is 0.91, and those sizes each have a 90C ampacity in excess of 150/0.91 = 165.

But for the portion of the circuit on the messenger wire, we can use Table 310.20. As we have 90C rated splices at each end of the conductors on the messenger wires, we can use the full 90C ampacity of those conductors; Table 310.20 is based on 40C ambient, so no temperature correction is required So we only need to use #2 Cu or 1/0 Al on the messenger wire.

Cheers, Wayne

 

[david luchini]

However based on the nec explanation, components to be terminated are ul tested with the 60 degree column in table 310.16. Therefore the other tables can only be used to start with a higher ampacity but must be reduced to table 310.16 table after temperature cotrection and adjustments. That is how its worded in code

That's not how it is worded in the Code at all.

The Code says... Ampacities for conductors rated 0 volts to 2000 volts shall be as specified in the Ampacity Table 310.16 through Table 310.21, as modified by 310.15(A) through (F) and 310.12.

There's no wording in the Code about the other ampacity tables being used to start with a higher ampacity but reduced to table 310.16.

 

[Jaybone812]

I used to thinl the ampacoties in other tables could be used based on the type of wiring method. This wpuld have included higher ampacity for coductors in free air


However based on the nec explanation, components to be terminated are ul tested with the 60 degree column in table 310.16. Therefore the other tables can only be used to start with a higher ampacity but must be reduced to table 310.16 table after temperature cotrection and adjustments. That is how its worded in code

Thatd not close to how he code reads at all

 

[don_resqcapt19]

Thanks for pointing out that definition, it's new in the 2020 NEC and I never noticed it. I wonder what ventilation rate would be required through a 4" conduit to allow the use of Table 310.17. : - )

Anyway, equipment using a ventilated enclosure can choose to pursue a listing allowing a marking for termination ampacity based on something other than Table 310.16.


It's only specifying Table 310.16 as a default with respect to the termination ampacity, seems reasonable to me. It doesn't preclude the use of other tables for other portions of the circuit.

Cheers, Wayne

Of course not, but all conductors terminate as some point, so you either violate the code or slice in a different conductor for the termination.
With the cablebus installations I have seen, the code on this issue is just ignored.

 

[wwhitney]

Of course not, but all conductors terminate as some point, so you either violate the code or slice in a different conductor for the termination

Sure, so it's easiest to take advantage of when you already have splices in the planned circuit. Occasionally I expect it could be worth adding splices to take advantage of this.

Cheers, Wayne

 

[solarken]

However based on the nec explanation, components to be terminated are ul tested with the 60 degree column in table 310.16. Therefore the other tables can only be used to start with a higher ampacity but must be reduced to table 310.16 table after temperature cotrection and adjustments. That is how its worded in code

110.14(C)(1) states determination of termination (wow-tongue twister) provisions of equipment shall be based on (a) or (b) in that same section. (a) addresses 14AWG thru 1AWG and (b) addresses > 1AWG. (a)(1) says to use 60C basis for ampacity, but (a)(3) allows higher temp ratings "if the equipment is listed and identified for use with such conductors". I assume that means if the terminals are listed/identified as 75C (or 90C), then you can use the 75C (or 90C) ampacities. (b) defaults to 75C basis but has similar provision for higher.

Still, there are 60C, 75C, and 90C columns in both Table 310.16 and 310.17. So in my opinion 110.14(c) is not clear in direction when it says to base ampacities on Table 310.16 which is specifically for up to 3 CC's in conduit for ampacity, "unless the equipment is listed and marked otherwise". If a terminated connection and the conductor that is terminated has free air flowing around it, then it sure seems like we should be able to use Table 310.17 and not 310.16.

 

[wwhitney]

If a terminated connection and the conductor that is terminated has free air flowing around it, then it sure seems like we should be able to use Table 310.17 and not 310.16.

Apparently the CMP in charge of 110.14 feels that decision should be made by the listing process, not by the electrician in the field.

Cheers, Wayne

 

[Therealcrt]

Alrighty.. so is #12 THHN rated for 20amps only, in conduit if there are more than 3 CCC?

When would #12 THHN be rated for 25a or 30a?

 

[LarryFine]

Alrighty.. so is #12 THHN rated for 20amps only, in conduit if there are more than 3 CCC?

When would #12 THHN be rated for 25a or 30a?

See NEC 240.4(D) and what follows. There is a difference between a wire's nominal ampacity and what OCP may be used to protect it under various circumstances.

 

[Therealcrt]

See NEC 240.4(D) and what follows. There is a difference between a wire's nominal ampacity and what OCP may be used to protect it under various circumstances.

Ok but this contradicts EVERYTHING for the 310 tables for ampacity.. if 12 gauge is only to be protected by 20 amps then why even have the tables? Why even show the option for it to be rated for 25 or 30 amps if another code reference prohibits it ever being protected by larger than 20 amps

 

[Jpflex]

See NEC 240.4(D) and what follows. There is a difference between a wire's nominal ampacity and what OCP may be used to protect it under various circumstances.

Wires for motors as well as some other equipment have their own section in nec and may not be subject to the small conductor ampacity rule.

However as its curremtly worded for everything else wiring ampacity to termination must only be based on table 310.16

You may start off with higher ampacity from using other tables but after temp correction and derating you must end with the ampacity on table 310.16

If this is not what nec intended, then they need to change their wording

 

[Jpflex]

Ok but this contradicts EVERYTHING for the 310 tables for ampacity.. if 12 gauge is only to be protected by 20 amps then why even have the tables? Why even show the option for it to be rated for 25 or 30 amps if another code reference prohibits it ever being protected by larger than 20 amps

Motors dont follow small conductor ampacity rule and may use hihher ampacity. They have their own section in nec

Also as explained other tables can be used for ampacity correction etc

 

[Jpflex]

It

Thatd not close to how he code reads at all

It is and i even submited the necs explanation as to why. See pictute of nec text