HVAC wire size?

[don_resqcapt19]

Seems to me that when the ampacity is required to be 16.6A, you need an installation that could carry 16.6A without violating any temperature limits. An LFNC installation in a wet location with #14 conductors would not meet that standard.

Cheers, Wayne

The rule says where the conductor operates above the 60°C ampacity....it does not require me to include the fictitious current as it does not operate at that current.

 

[wwhitney]

The rule says where the conductor operates above the 60°C ampacity....it does not require me to include the fictitious current as it does not operate at that current.

So we have a conductor whose ampacity is 25A, but we aren't allowed to use it for more than 15A? That restriction to 15A is not a restriction of ampacity?

I guess that's logically consistent, just seems unnecessarily complicated to me, vs seeing as 356.10(9) as limiting the ampacity.

I put in a 2026 PI to adjust the definition of ampacity to read ". . .without exceeding the temperature rating of it [the conductor] or its wiring method." That would make it clear that 356.10(9) has the effect of limiting the ampacity.

The PI was rejected with the statement "Additional restrictions to ampacity are limited by the applications that are being used." Which statement frustratingly doesn't make it clear if the 356.10(9) is meant as a limit on ampacity or not. You have to decide whether the "application being used" includes the choice of wiring method.

Cheers, Wayne

 

[tortuga]

Seems to me that when the ampacity is required to be 16.6A, you need an installation that could carry 16.6A without violating any temperature limits.

Inspectors here interpret it that way, MCA = Minimum circuit ampacity after any adjusting derating etc.

 

[Ken_S]

I don't agree. The operational load is 13.4 amps, not 16.6 amps, and that does not exceed the 60°C ampacity of 14 AWG. We do not need to use the MCA for this as that number includes a fictitious 25% increase to the compressor current.

Could you provide the code citation for this? Part 4 of 440 seems to indicate that the 25% would be included in the conductor size. Unless I'm missing something.

I agree with the use of 14, I'm just not understanding ignoring of the MCA.

 

[Greg1707]

You can't use #14NM but you could use #14 MC, etc unless the terminals are marked for 60 deg termination

I thought the small conductor rule 240.4.D.3 limited #14 to 15 amps?

 

[infinity]

I thought the small conductor rule 240.4.D.3 limited #14 to 15 amps?

240.4(G) would override the 15 amp limit in 240.4(D)(3).

 

[LarryFine]

I believe the low rating for NM and UF is largely based on how much they get bundled and buried in insulation in addition to their construction as cables.

 

[Eddie702]

This post is an example of where the CODE fails. The code is a failure in many areas.

We have had over 20 posts on what should be a simple wire sizing example with one of the most common pieces of equipment we all install and the answer should be easy. Why do we have to struggle with something so simple because they don't know how to write the code?

 

[jim dungar]

This post is an example of where the CODE fails. The code is a failure in many areas.

We have had over 20 posts on what should be a simple wire sizing example with one of the most common pieces of equipment we all install and the answer should be easy. Why do we have to struggle with something so simple because they don't know how to write the code?

Yes the code can be exasperating, but I see this as more as we don't know how to provide the pertinent data when we ask questions. The

What size wire do I need for my 15A HVAC will get a whole lot of different answers than; What size wire do I need for my HVAC that has an MCA of 15A, with indoor NM cable transitioning to NM flex at the outdoor disconnect?

 

[don_resqcapt19]

Could you provide the code citation for this? Part 4 of 440 seems to indicate that the 25% would be included in the conductor size. Unless I'm missing something.

I agree with the use of 14, I'm just not understanding ignoring of the MCA.

I am not ignoring it...I am simply saying for the operating current, we do not use the MCA. We use the full load curent.

 

[don_resqcapt19]

So we have a conductor whose ampacity is 25A, but we aren't allowed to use it for more than 15A? That restriction to 15A is not a restriction of ampacity?

I guess that's logically consistent, just seems unnecessarily complicated to me, vs seeing as 356.10(9) as limiting the ampacity.

I put in a 2026 PI to adjust the definition of ampacity to read ". . .without exceeding the temperature rating of it [the conductor] or its wiring method." That would make it clear that 356.10(9) has the effect of limiting the ampacity.

The PI was rejected with the statement "Additional restrictions to ampacity are limited by the applications that are being used." Which statement frustratingly doesn't make it clear if the 356.10(9) is meant as a limit on ampacity or not. You have to decide whether the "application being used" includes the choice of wiring method.

Cheers, Wayne

I have zero idea of how this relates to the operating current of the equipment.

I am simply saying for the purpose of 350.10(4) or 356.10(9) we use the full load currents and not the MCA. The conductor temperature rise is based on the actual current and does not include the fictitious additional current that the code requires us to use for the selection of the conductor size.

Once again I think we have reached an impasse.

 

[tortuga]

I believe the low rating for NM and UF is largely based on how much they get bundled and buried in insulation in addition to their construction as cables.

The issue at hand is Liquidtight Flexible Nonmetallic Conduit (LFNC) appears to impose a 60C ampacity limit on conductors which if you started with UF or NM is not a big deal, but if you started with say EMT the OP could have in theory used 14 instead of 12 (unless some other code comes into play like the energy code).
And If recall correctly Liquidtight Flexible Metal Conduit Type (LFMC) does not impose such a limit on the conductors so if you spend $8 more on your flex whip (counting connectors) you can save 1.50 on your wire.
Residential ropers just go by the MCA and the 60C column and call it a day its very easy that way.

 

[don_resqcapt19]

The issue at hand is Liquidtight Flexible Nonmetallic Conduit (LFNC) appears to impose a 60C ampacity limit on conductors which if you started with UF or NM is not a big deal, but if you started with say EMT the OP could have in theory used 14 instead of 12 (unless some other code comes into play like the energy code).
And If recall correctly Liquidtight Flexible Metal Conduit Type (LFMC) does not impose such a limit on the conductors so if you spend $8 more on your flex whip (counting connectors) you can save 1.50 on your wire.
Residential ropers just go by the MCA and the 60C column and call it a day its very easy that way.

You have to look at the marking on the LFMC that you are using. The requirement in 350.10(4) and 356.10(9) are identical.

From the UL Guide Information for LFMC (DXHR)

Liquid-tight flexible metal conduit not marked with a temperature designation or marked "60 C" is intended for use at temperatures not in excess of 60°C (140°F).

Conduit intended for use in dry or oily locations at a temperature higher than 60°C (140°F) is marked "____ C dry, 60 C wet, 70 C oil res" (or "____ C dry, 60 C wet, 70 C oil resistant") with "80" or "105" inserted as the dry-locations temperature.

Conduit marked "80 C dry, 60 C wet, 60 C oil res" or "80 C dry, 60 C oil resistant" is intended for use at 80°C (176°F) and lower temperatures in air, and at 60°C (140°F) and lower temperatures where exposed to water, oil or coolants.

 

[Little Bill]

How about this scenario. The MCA is 59A. You run 6-2 NM to the disconnect, then run #6 THWN in LFNMC to the unit. Is this permissible?

 

[wwhitney]

I have zero idea of how this relates to the operating current of the equipment.

The question is whether a limit on operating current means that the ampacity is also so limited.That would mean a violation of the MCA, not a violation of 356.10(9).

Or whether a conductor's "maximum operating current" is separate from its ampacity, and the former can be lower than the latter. Are there any other examples of such a distinction?

Cheers, Wayne

 

[wwhitney]

How about this scenario. The MCA is 59A. You run 6-2 NM to the disconnect

The ampacity of 6-2 NM is definitely limited to 55A, so the above is a violation.

Cheers, Wayne

 

[Little Bill]

The ampacity of 6-2 NM is definitely limited to 55A, so the above is a violation.

Cheers, Wayne

What if the disconnect is fused?

 

[wwhitney]

What if the disconnect is fused?

The minimum feeder ampacity is also 59A (if the feeder has no other loads, or more if there are other loads).

Cheers, Wayne

 

[electrofelon]

. Why do we have to struggle with something so simple because they don't know how to write the code?

Unfortunately NFPA refuses to address their chronic problem of poor and ambiguous wording. What they need to do is after accepting a code change in principle, is have a language and clarity committee to write the final wording and get rid of all these word salads the CMP's keep pumping out.

 

[infinity]

Unfortunately NFPA refuses to address their chronic problem of poor and ambiguous wording. What they need to do is after accepting a code change in principle, is have a language and clarity committee to write the final wording and get rid of all these word salads the CMP's keep pumping out.

That would be nice but don't hold your breath.