I'm hoping I can get an explanation about SEER that makes sense.
my situation. I am going to have a Trane XL19i compressor added to
existing Trane XV90 heating system (the brand and particulars don't
matter...). I am going to have the HVAC work done by a pro, but I
to run the electric myself. I am trying to understand the electric
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
trying to figure out what the *true* load on the subpanel will be.
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
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
What I want to know is when the compressor and fan are already
(that is neglecting any startup transients) how much current will this
(compressor plus fan) be drawing from my subpanel.
Any help much appreciated.
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.
On Apr 10, 3:50 pm, email@example.com wrote:
Yes, I understand this. I intend, in fact, to use 8 gauge wire to
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
(a concept apparently foreign to old Paul, here) how much current is
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
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
with a 36kBTU 19 SEER unit?
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
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??
You win some, you lose some. An interesting experiment.
You know, I'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
I just wanted to understand *why* on some of this stuff.
Sorry being a little inquisitive offends you guys.
Have fun reading your tables.
You still don'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.
SEER isnt an absolute or constant ratio... its a 'point on the curve'.
Lots of things effect SEER... indoor and outdoor temp, duct restriction,
dirt in the coils, charge, etc etc etc... Most likely... the SEER
rating, like EPA Highway Mileage, is measured at ideal conditions which the
unit may not see all that often.
suggest you take up your reconciliations with the equipment
manufacturer. their engineers can talk to your engineers and ya'all
can over-engineer things 'til you'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 ..........
Well, it'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't just divide by the SEER, because the SEER is a synthetic
number. It'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's useful for
comparing the efficiency of units, but useless for figuring any actual
What you need is to look at the manufacturers cooling performance
data. Here's some for the XL19i:
That'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'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'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
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!)
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