power consumption, SEER, etc.

[Bill]

I'm hoping I can get an explanation about SEER that makes sense. Here's my situation. I am going to have a Trane XL19i compressor added to my existing Trane XV90 heating system (the brand and particulars don't really matter...). I am going to have the HVAC work done by a pro, but I want to run the electric myself. I am trying to understand the electric load in some detail and it just doesn't add up.

I am considering a 36kBTU (3 ton) compressor. The spec sheet says that it needs a 30A 220V dedicated circuit (which is fine...), but I am trying to figure out what the *true* load on the subpanel will be. The spec sheet says that the max compressor draw is 15.1A and that the max fan draw is 2.8A. So, I figure that when the unit is running wide open it can be drawing 17.9A.

Here's my confusion. 36000BTU/19SEER = 1895watts. At 220V this implies 8.6A. What's up with this?

I came up with a number of possibilities:

*the 15.1A on the compressor load is the inrush current at startup *the compressor never runs at 100% duty cycle (this makes no sense to me).

What I want to know is when the compressor and fan are already running (that is neglecting any startup transients) how much current will this unit (compressor plus fan) be drawing from my subpanel.

Any help much appreciated.

[gofish]

The manufacturer of your XL19i states on the equipment label that the MCA (minimum circuit ampacity) for a 3 ton condenser is 22 amps and the MOP is 35 amps.

If warranty means anything to you, I would recommend you install the electrical as per the manufacturers specifications.

[Bill]

Yes, I understand this. I intend, in fact, to use 8 gauge wire to minimize the IR drop. I also intend to use a 30A breaker to protect the circuit, and I have enough capacity from my subpanel to supply 30A++. I merely wanted to

*understand* (a concept apparently foreign to old Paul, here) how much current is actually flowing.

I also stated that I intend to have a pro ( a licensed Trane dealer ) install the unit, but I prefer to do my own electrical. I prefer to do my own work not to cheap out, but instead because on occasion (not often but on occasion) I have had poor quality work and damage done by Cro-magnon subs like Paul.

So, back to my original question. If the RLA is 17.8amps, how does this reconcile with a 36kBTU 19 SEER unit?

[<kjpro]

How long is this electrical run?

[Noon-Air]

You must be an EE.

Lets keep it real simple.... all you need to know is what is called for on the data plate or the manufacturers installation instructions and size the circuit accordingly.. The manufacturer specifies Min and Max circuit fuse or breaker. The manufacturers design engineers get paid a lot of money to figure this stuff out. Maybe you should get a job with an equipment manufacturer??

[Don Keener]

[gofish]

suggest you take up your reconciliations with the equipment manufacturer. their engineers can talk to your engineers and ya&#39;all can over-engineer things &#39;til you&#39;re blue in the face.

also suggest you learn every single thing you can about MOP as it applies to ac units in particular. reconciliation will be much easier for you ..........

[Bill]

30 feet. I over-engineer everything.

[Bill]

touchdown

[Bubba]

Gee. Who&#39;d a thunk? Always trying to re-invent the wheel. Bubba

[<kjpro]

Must have more money than brains.

[<kjpro]

No further explaination required.

[Bill]

You win some, you lose some. An interesting experiment.

You know, I&#39;m not trying to re-invent *anything*. Running a branch circuit w/ a 30A breaker per the manufacturer. Running wire with greater than 22A ampacity per the manufacturer.

I just wanted to understand *why* on some of this stuff.

Sorry being a little inquisitive offends you guys.

Have fun reading your tables.

[Noon-Air]

You still don&#39;t get it..... The breaker and wire sizes are called out by the manufacturer and the NEC..... If you want to know *WHY*........ Your an EE, you should already have a pretty good idea of not only what, but why and the supporting theory behind it.

[Jeffrey Lebowski]

It will be appx LRA--which might just as well be coinsidered as being almost a dead f****ng shot least for some small fraction of a second.

Now go ahead an continue to figger

-=-

[Bubba]

Then ponder this........ The world is round..........Why? The solar system is infinite. WHY isnt there an end to it? Bubba

[Noon-Air]

You forgot one......

genius is finite....... Why??

[Sparks Johanson]

Well, it&#39;s just not that simple. The instantaneous current draw is dependant on the conditions the unit is operating under at a particular moment. The heat load on the house, the temperature and humidity of the outside air, etc, etc, all factor in to how much electricity the compressor will be drawing _right now_.

You can&#39;t just divide by the SEER, because the SEER is a synthetic number. It&#39;s derived by looking at the cooling profile over an entire year for an "average" location, and making some calculations as to how efficient it might be at that location. In short, it&#39;s useful for comparing the efficiency of units, but useless for figuring any actual power usage.

What you need is to look at the manufacturers cooling performance data. Here&#39;s some for the XL19i:

That&#39;ll tell you how much the compressor will draw for a wide variety of particular environmental conditions. You can then divide the kbtuh by the KW and get an EER (Not a SEER, an EER) for that unit under those conditions, if you&#39;re curious. Then, use the performance specs along with actual climate data for your location for an entire year, and you can calculate your own SEER based on your location. Not a trivial task, but interesting. Were you to do this, you could answer the question "how efficient is this unit _for me_." Which, of course, is what everyone wants to know and nobody can tell you.

Of course, all of this is subject to the usual caveat that actual efficiency is highly dependant on installation factors like proper ductwork design and installation, proper installation of the unit, clean filters, proper airflow, etc, etc.

That&#39;s should be the peak operating load. The inrush (startup) load will be quite a bit more than that (3-4 times) but only for a second or two.

The manufacturer will also state the LRA (Locked Rotor Amps) which is the max the unit will draw if the compressor is prevented from turning for some reason.

Depends on conditions! Refer to the performance data to be sure. It will generally be drawing less that the 15.1 Amps you cite, but when it gets really hot out, watch out!

(Oh, and be sure to change the thermostat!)

[Bill]

Thank you very much for this information. It is very useful, and answers my original question.

[Tekkie®]

And that was? Oh, I remember; why fundie idiots come here without doing basic research first. Google is broken?