ECN Forum Forums General Topics of the Trade General Discussion Area 3.5 ton A/C condensor

Greg I don't think you sound like that at all. It's good advice.

What the code will allow and what is the most energy efficient way are not one in the same.

Bob

Thanks everyone for making it easier for me to understand what I'm doing, and more importantly, know why I'm doing it.

I checked out article 440.6 and it referred me to table 310.16. Thanks for the tip, Bob.

Oh, and lol @ "copper.org"

--Ron

[This message has been edited by ShockMe77 (edited 10-02-2005).]

I always upsize here in AZ where the air conditioner can just about be a continous load. Besides, my AC guy requests it and he pays for it. There is a 125% requirement for fixed electric heating, well out here our cooling load is much bigger.

BigB that is a good idea.

Just as a point of information there is already 125% figured into the MCA for the start up current on the unit so you are already covered for the continuous load if it is one.

IMO an AC would not be a continuous load unless you leave the window open or it was severely undersized.

Bob



[This message has been edited by iwire (edited 10-02-2005).]

IWIRE

A 12AWG on a 50a ocpd, wouldn`t that be a violation of 240.4(D)?

Maybe I`m misreading your answer or the thread all together.


P.S. Is there a "quote" button anywhere?

No it not, check out the first sentence of 240.4(D)


It happens that for HVAC units it is permitted by 240.4(E) through (G)

No there is not a 'quote' button.

If you follow this link https://www.electrical-contractor.net/ubb/ubbcode.html you will see how to quote or bold etc.

Bob



[This message has been edited by iwire (edited 10-02-2005).]

Bob I know A/C isn't a big deal in Ma and humidity is not an issue in Az but here in Florida a properly designed system is going to be a continuous load. If your A/C cycles too much it won't get the humidity out of the air and you get that clammy cold you feel if you are in a movie theater that is empty.

Morning Greg, you got me wondering what you guys use for cooling equipment.

It seems to me that if the AC is sized to run continuously with an outside temp of say 70 F that it will not cool the place very well when the outside temp reaches 100 F.

And if it sized to run continuously with an outside temp of 100 F it will certainly not run continuously at temperatures below 100 F.

Of course that can be overcome using multiple cooling stages but around here that is only done in commercial work.

All it takes is the compressor to kick off once every three hours and it is not by definition a continuous load.

MCA is 125% of the largest load plus all the other loads and is usually used to size the conductors. It is also the minimum fuse size. Max Fuse/Breaker is a little misleading. It is actually just the Max Fuse size or a non-HACR breaker size. This is 225% of the largest load plus all other loads. If you use a HACR breaker you can go up to 400% of the largest load plus all other loads as the max. Why someone would do that I do not know. Unless it is availability.

Another thing to keep in mind is that some of the loads are locked out when the other ones are running, such as heaters being locked out during the cooling cycle. Whichever load is higher (heat or cool) can throw a kink into the RLA/MCA calculations. But with heaters being resistive the motor load usually trumps on the max fuse size due to inrush on the inductive side.

Another thing my company does is put a note by the disconnect on the drawings saying to use the 60 degree column of 310-16 if the load is less than 100A and the 75 degree if it is over 100A. We use the 90 degree column for all of the wiring internal to our units. Other manufacturers may be different as we do heavy commercial, industrial and large residential and not regular residential units.

They usually use a design temp of 95f <ambient> or so with a 20-25 degree delta.
Larger houses will have 2 or more systems, particularly if they are multi-level. When we have the 95 degree day it will come with 90+ humidity so getting the water out is as important as getting out the heat.