i need help understanding a correction

[Strathead]

Just FYI, the inspector showed his lack of attention to detail when he cited THHN wire, unless this heat pump it the rare beast that is installed inside of a building. The type of wire going to the unit is most likely THWN-2 at 90º but could be THWN at 75º in no case is wire run outside THHN. This doesn't change anything else.

 

[tortuga]

Double check the labeling on the liquidtight or carflex to the unit.

Most have a temp rating of 60 degrees C when used in wet locations

Yeah 356.12(9) and future proofing is why I typically use the 60°C ampacity table.
I am surprised that after all these years someone at Carlon or one of the major manufacturers with access to heat transfer modeling software has not published some calculations and real world tests, or perhaps they did and thats why the restriction is there?

 

[don_resqcapt19]

Double check the labeling on the liquidtight or carflex to the unit.

Most have a temp rating of 60 degrees C when used in wet locations

In this case the "operating current" would not exceed the 60°C ampacity. I do not see the fictitious 25% of the compressor current that is added to obtain the MCA as "operating current".

 

[Eddie702]

I think it is great that the inspector was a reasonable guy. In my 46 years pulling permits most of them are if you approach them right. You just have to know the code.

 

[kwired]

Even though you're right everything I've done with air conditioners and mini splits I do in 10awg for future proofing. Lots of guys do this even though is overkill and he might just be used to it.

The only future proofing to be gained would be if they increase size of the unit. Otherwise if it fails or simply wears out chances are the replacement will be higher efficiency and presumably same BTU rating therefore will likely draw less than the original.

 

[kwired]

Most have a temp rating of 60 degrees C when used in wet locations

I could never understand why being a wet location lowers the temp rating, or why a raceway temp rating would need to match up with conductor temp rating?

Has anyone ever seen any raceway operate much over ambient temperature when there is only three or less current carrying conductors within it, other than maybe if you have inductive effect issues from improper application of ferrous raceways?

 

[Treg the electrician]

The only future proofing to be gained would be if they increase size of the unit. Otherwise if it fails or simply wears out chances are the replacement will be higher efficiency and presumably same BTU rating therefore will likely draw less than the original.

i feel the same way about it

 

[tortuga]

The only future proofing to be gained would be if they increase size of the unit.

In residential lately every new home we wire without fail the HVAC installer either used a different unit than on plans, only provided a BTU rating (meaningless to me), did not provide any specs at all or on the last job the builder had not even chosen a HVAC system at the time of rough in.
But thats all NM cable so I am at 60°C ampacity either way.

Otherwise if it fails or simply wears out chances are the replacement will be higher efficiency and presumably same BTU rating therefore will likely draw less than the original.

Yeah you cant idiot proof everything but with minisplits it has come up more than few times for me in recent years where the HVAC people over sell and under size, they will put in a little one ton minisplt in a living room of 1800 SQFT ranch style home as supplemental heat and AC, however the unit works itself to death trying to cool the entire home to 66 in the hotter summer. Then when they never clean the unit and it fails they upgrade to a three head unit.

 

[tortuga]

In this case the "operating current" would not exceed the 60°C ampacity. I do not see the fictitious 25% of the compressor current that is added to obtain the MCA as "operating current".

So your suggesting 356.12(9) is not referring to the calculated ampacity of the branch circuit required by 440.6?
Thats a interesting take.
If LFNC supplied a different load like continuous lighting or heaters would you also omit the 25%?.

And slightly off topic but doing a article 220 load calculation recently I was reminded trying reconstruct 'total operating current' or Full Load Amps (FLA) information from MCA is impossible. See my recent thread on a common Trane unit that has a MCA of 36.1A and FLA of 14.7 Amps. In that thread @mtnelect reiterated what Trane tech support said that HVAC the compressors are "Hermetic" and cooled by the refrigerant and manufactures take that's into consideration along with Locked Rotor Amps when using their fuzzy logic to make the MCA label on the unit.

 

[wwhitney]

So your suggesting 356.12(9) is not referring to the calculated ampacity of the branch circuit required by 440.6?
Thats a interesting take.
If LFNC supplied a different load like continuous lighting or heaters would you also omit the 25%?.

Certainly it's defensible to ignore the extra 25% ampacity required for continuous loads. As that is solely a termination requirement (the run of the conductor is not required to have the extra 25% ampacity after adjustment and correction), it should not apply to the LFNC.

As for the extra 25% for motors, I'm unclear on why it's included in the NEC. If it's again a purely fictional factor like the extra 25% for terminations of continuous loads, then I've come around to Don's point of view that it does not count as operating current. If, however, it's included in the NEC because, say, a mechanically overloaded motor may well draw above 100% of the table FLA for an extended period of time before the overloads trip, then I would say that you have to include it in the operating current, as that would be a contemplated operating mode.

So some greater clarity on why motor FLA gets a 125% factor for conductor ampacity is required.

And slightly off topic but doing a article 220 load calculation recently I was reminded trying reconstruct 'total operating current' or Full Load Amps (FLA) information from MCA is impossible.

It's not if you have a correct nameplate and it identifies the largest motor and any heater loads (which IIRC also get a 125% factor in the MCA). The the operating currrent is MCA minus 25% times (largest motor FLA or RLA plus any heaters).

See my recent thread on a common Trane unit[/URL] that has a MCA of 36.1A and FLA of 14.7 Amps.

I'm convinced that that nameplate is incorrect, and that any reassurances you got from Trane technical support to the contrary did not come from anyone knowledgeable enough to say that with authority. Either the FLA is higher than 14.7A, or there's an undisclosed heater load. There's no other way for the arithmetic to work out.

Cheers, Wayne

 

[Eddie702]

AC and refrigeration compressors are subjected to a wide range of loads. The compressor pumps gas and does rely on the refrigerant for motor cooling. If they don't get cooled by suction gas they die a slow painful death.

Compressor reliability is VG. All compressors have a "safe operating envelope" If the compressor is forced to run outside of this envelope either by dirty air filters, dirty evaporators or condenser coil or low on charge or overcharged etc it will die prematurely.

The system usually (for AC) run with an evaporator temp of about 40 degrees and will discharge 55-60 degree air depending on the humidity load. More humidity load makes the unit work harder because you are condensing water but you get less sensible temp change. Les humidity load = more sensible change.

As far as protecting the compressor they have internal overloads but picture this.

If the compressor is running with low charge and low load (no compressor cooling) it will short cycle on low pressure control (if the unit has one) and the compressor will run hot and constantly trip overload. When this happens too often the windings burn. They are very difficult (I think impossible) to protect from overcurrent. The motor amps will constantly change depending on outdoor and indoor air temp and other factors. The OL in the compressor has to be high enough to keep it running during extreame weather so it cant protect in mild weather.

Running at 66 degrees as @tortuga mentioned is wrong. This forces the compressor out of its design envelop and the evaporator will have to run at below 32 deg and will frost and Ice and possibly slug the compressor with liquid.

When I did AC service you would go on calls and the stats are buried to 70 or as low as they will go. Wrong.

If the unit is designed and sized properly it will remove humidity and the normal AC design indoor temp is 74-78 degrees.

For residential a rough guestimate is 600 sq feet/ton depending on insulation etc

For commercial office buildings with computers and printers that give off heat 250 sq feet/ton.

Maintenance is very important. If it short cycles find out why. Keep the coils and filters clean. Contactor condtion (the contacts) is very important. Short cycling kills contactors. Maintenance is the key these units can last a long time. We had jobs (commercial) where we changed contactors every year or two.

 

[don_resqcapt19]

If the unit is designed and sized properly it will remove humidity and the normal AC design indoor temp is 74-78 degrees.

Then mine should never work as the night time temperature setting for me is always 65 or lower.

 

[tortuga]

Certainly it's defensible to ignore the extra 25% ampacity required for continuous loads. As that is solely a termination requirement (the run of the conductor is not required to have the extra 25% ampacity after adjustment and correction), it should not apply to the LFNC.

So then if we can agree on how to calculate RLA and that RLA does not exceed the 60°C wet rating of LFNC we're good to go.

I'm convinced that that nameplate is incorrect, and that any reassurances you got from Trane technical support to the contrary did not come from anyone knowledgeable enough to say that with authority.

And here is my example I got the rated load current per the code (440.4), 440.4 does not prescribe a disassembly of MCA to get rated load current it clearly states rated load current shall be marked by the manufacturer and I called them to confirm it and they did.
My understanding is they need to factor in the LRA (440.22) to the MCA and that can be be as much as 175% of RLA or more with exceptions.