HVAC feeder size

[Strathead]

Well then I must be wrong, cause we all know only the moderators provide correct answers ; and I should probably surrender my engineering seal because the minimum safety requirements of the NEC certainly provide the best design basis. hmy:

That is uncalled for sarcasm. I was pointing that out so that you could review their positions and state a counter argument, if you have one. Looking at their answers and rereading the code book, I feel they gave valid points. Tell me, what does your Engineering seal have to do with it. The Electrical Engineers I know have always stated there was little classroom instruction on the NEC in pursuit of their degrees.

 

[kingpb]

That is uncalled for sarcasm. I was pointing that out so that you could review their positions and state a counter argument, if you have one. Looking at their answers and rereading the code book, I feel they gave valid points. Tell me, what does your Engineering seal have to do with it. The Electrical Engineers I know have always stated there was little classroom instruction on the NEC in pursuit of their degrees.

Your taking yourself wayyyyyyy to seriously...

As we ALL know, NFPA 70 (NEC) is nothing more than a guide developed by the National Fire Protection Association to give a minimal set of installation requirements intended to create a safe environment. There is nothing to teach to engineers in the NEC because it is not a design guide. Engineering was done in order to develop the practices within the NEC, and the engineering is what is taught to future engineers.

BTW: Sarcasm is just one more service I offer :thumbsup:

 

[jumper]

Your taking yourself wayyyyyyy to seriously...

As we ALL know, NFPA 70 (NEC) is nothing more than a guide developed by the National Fire Protection Association to give a minimal set of installation requirements intended to create a safe environment. There is nothing to teach to engineers in the NEC because it is not a design guide. Engineering was done in order to develop the practices within the NEC, and the engineering is what is taught to future engineers.

BTW: Sarcasm is just one more service I offer :thumbsup:

Easy king, we are all here to learn and explore. Strat and others, including me, are merely curious why you think the nameplate MCA is not good enough.

David L, who is also an EE, wrote:

The MCA=360.5, so the conductor need only be sized to be capable of 360.5A.

in response to this:

MCA = 360.5; therefore size conductor to be capable of 451A.

Why the upsize of the conductors? Past experience? Personal preference?

 

[iwire]

Your taking yourself wayyyyyyy to seriously...

Pot meet kettle.

There is nothing to teach to engineers in the NEC because it is not a design guide.

:lol:

Yeah, all you do is add a boiler plate note telling the installer to conform to the NEC.

BTW, I really hope that was a joke as EEs should know the rules

 

[kingpb]

Upsizing is based on multiple factors, such as:
- The MCA is a minimum size; it's always OK to go bigger
- MCA is based on actual running load, not motor/load because mca is a UL driven calculation, not NEC calculation.
- I have never met a mechanical/HVAC engineer that supplies/buys what they initially told me; typical goes up
- During performance testing the HVAC may not supply the amount of air required and mechanical will change fans to increase airflow, thus MCA goes up.
- Installation by Contractor may be such that cable derating is required due to elevated temperature; e.g. roof install
- Don't have to worry about short circuit damage

From experience; it does provide a margin to cover the unknowns during detail design, especially since the electrical is last to finish, and we are often times having to make guesstimates to cover the indecisiveness of other disciplines, and thus helps keep me from having to explain to the Owner why he needs to give the Contractor a change order.

And nope, I don't say the NEC has to be met. The job has to be permitted, in doing so the AHJ/state takes care of the minimum safety requirements. If I want more than minimum, I provide the details.

 

[augie47]

Upsizing is based on multiple factors, such as:
- The MCA is a minimum size; it's always OK to go bigger
- MCA is based on actual running load, not motor/load because mca is a UL driven calculation, not NEC calculation.
- I have never met a mechanical/HVAC engineer that supplies/buys what they initially told me; typical goes up
- During performance testing the HVAC may not supply the amount of air required and mechanical will change fans to increase airflow, thus MCA goes up.
- Installation by Contractor may be such that cable derating is required due to elevated temperature; e.g. roof install
- Don't have to worry about short circuit damage

From experience; it does provide a margin to cover the unknowns during detail design, especially since the electrical is last to finish, and we are often times having to make guesstimates to cover the indecisiveness of other disciplines, and thus helps keep me from having to explain to the Owner why he needs to give the Contractor a change order.

And nope, I don't say the NEC has to be met. The job has to be permitted, in doing so the AHJ/state takes care of the minimum safety requirements. If I want more than minimum, I provide the details.

I understand your reasoning and often remind folks that NEC is a minimum requirement.
I would like to ask how upsziing the conductor reduces the concern about short circuit damage...

 

[jumper]

Upsizing is based on multiple factors, such as:
- The MCA is a minimum size; it's always OK to go bigger
- MCA is based on actual running load, not motor/load because mca is a UL driven calculation, not NEC calculation.
- I have never met a mechanical/HVAC engineer that supplies/buys what they initially told me; typical goes up
- During performance testing the HVAC may not supply the amount of air required and mechanical will change fans to increase airflow, thus MCA goes up.
- Installation by Contractor may be such that cable derating is required due to elevated temperature; e.g. roof install
- Don't have to worry about short circuit damage

From experience; it does provide a margin to cover the unknowns during detail design, especially since the electrical is last to finish, and we are often times having to make guesstimates to cover the indecisiveness of other disciplines, and thus helps keep me from having to explain to the Owner why he needs to give the Contractor a change order.

And nope, I don't say the NEC has to be met. The job has to be permitted, in doing so the AHJ/state takes care of the minimum safety requirements. If I want more than minimum, I provide the details.

Those reasons are understandable and I can easily see your point.

Well, except this:

- Don't have to worry about short circuit damage

I am a little unclear as to why.

But the questioned asked about conductor size and the nameplate MCA satisfies the NEC.
No other conditions were posited by the OP.

Arguably, other factors such as you have presented may well come into play, but I can only answer the question asked.

 

[Strathead]

Those reasons are understandable and I can easily see your point.


But the questioned asked about conductor size and the nameplate MCA satisfies the NEC.
No other conditions were posited by the OP.

Arguably, other factors such as you have presented may well come into play, but I can only answer the question asked.

As the OP, I agree with you. I am glad for the explanation by King and agree totally with the wisdom of sizing larger, but at the same time, if you had multiple compressors, it may be wise to check the actual required feed before using the pumped up MCA. An example I used earlier would be a multifamily dwelling unit. If you had 150 3 ton AC units on a single feed, it may be wise to use the actual allowed amperage in the calculation.

And, I didn't know this stuff before the question, so it has opened my eyes to some new issues.

 

[kwired]

​replies to some comments within the quoted material:

Upsizing is based on multiple factors, such as:
- The MCA is a minimum size; it's always OK to go bigger
True, but you are not required to go bigger that is a design choice.
- MCA is based on actual running load, not motor/load because mca is a UL driven calculation, not NEC calculation. Is it not based on maximum rated load of the compressor (comparable to what they call full load amps for motor circuits) plus full load amps of other motors, heat strips, other accessories that may be a part of the unit?
- I have never met a mechanical/HVAC engineer that supplies/buys what they initially told me; typical goes up
- During performance testing the HVAC may not supply the amount of air required and mechanical will change fans to increase airflow, thus MCA goes up.If you want to design it to cover that, then that is your choice. If I am asked to install a circuit to a unit it is usually getting MCA supplied to it, and maybe is upsized for voltage drop if deemed necessary. If they change their mind and want a bigger unit - why is that my problem? I get to sell them larger conductors and maybe raceway and do the job twice because of their mistake - not my mistake.
- Installation by Contractor may be such that cable derating is required due to elevated temperature; e.g. roof install
MCA needs to include any necessary adjustments per NEC.
- Don't have to worry about short circuit damage
Others have asked just what this is about as well.

From experience; it does provide a margin to cover the unknowns during detail design, especially since the electrical is last to finish, and we are often times having to make guesstimates to cover the indecisiveness of other disciplines, and thus helps keep me from having to explain to the Owner why he needs to give the Contractor a change order.

And nope, I don't say the NEC has to be met. The job has to be permitted, in doing so the AHJ/state takes care of the minimum safety requirements. If I want more than minimum, I provide the details.

I have run oversized conductors before, just in case. It usually is a case by case decision and I don't see it as necessary all the time. It is a design choice not a code requirement

I must also add that the topic if this thread is about sizing feeders, not the circuit to an individual unit.

 

[kingpb]

Cable damage criteria - View attachment Cable Damage.pdf

You can see from the graph that depending on time it takes for the protection to trip; the cable can be damaged. Unfortunately, this is a topic that is often overlooked.