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#1752 - 05/25/01 01:52 PM determing the load of heat pump compressors
C.Urch Offline
Member

Registered: 05/23/01
Posts: 36
Loc: Kill Devil Hills, NC USA
How would one go about determining the load of a heat pump compressor in Volt Amperes? Is there a formula to convert tons to V.A.? Let's say that we have 3 - 3.5 ton units that has a R.L.A.(Running Load Amps) of 21.2, a minimum supply circuit of 28 amps, a max. fuse size of 45 amps and a minimum fuse of 35 amps. What would the load calculation or actual draw of these units be?

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#1753 - 05/25/01 03:00 PM Re: determing the load of heat pump compressors
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
test

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#1754 - 05/25/01 04:32 PM Re: determing the load of heat pump compressors
Bill Addiss Offline
Member

Registered: 10/07/00
Posts: 4196
Loc: NY, USA
Wouldn't you use the RLA figure?

Bill

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#1755 - 05/25/01 07:24 PM Re: determing the load of heat pump compressors
Redsy Offline
Member

Registered: 03/28/01
Posts: 2138
Loc: Bucks County PA
Why do you want the load in VA? It seems that you have all you need already. At the risk of going in circles--12,000 BTU/Hr. per ton. .293xBTU/hr.=watts. watts divided by (volts x power factor) or (volts x pf x 1.73 for 3phase)= amps. There is probably efficiency to consider also. I think this is right.

[This message has been edited by Redsy (edited 05-25-2001).]

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#1756 - 05/26/01 04:35 AM Re: determing the load of heat pump compressors
C.Urch Offline
Member

Registered: 05/23/01
Posts: 36
Loc: Kill Devil Hills, NC USA
In other words, one must "shoot from the hip" whether to upgrade a service from say 200 to 400 Amp when doing a re-model that is adding an additional heat and air system. If you think it will be close, then cover yourself. How about determing service size for new construction? Would it be safe to take the R.L.A. X 125% (for start up) per unit and and use this figure in determing your load calculation?

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#1757 - 05/28/01 06:32 AM Re: determing the load of heat pump compressors
Scott35 Offline

Broom Pusher and
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Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
Like Bill said, use the RLA [Running Load Amperes] to figure your KVA on each unit.

On large and/or multistage heatpumps, figure them to be LCLs [Long-Continuous Loads], so figure the RLA at 120% [or derate the circuit by 80% - but no need to do both]. If you know that they will not run for 3 hours + continuously, then LCL is not required. Heatpumps normally run at least 4 hours continuously at some time of the year, so best to figure LCL in your calcs.

List this on your Panel Schedule [with LCL added] for each heatpump.

List the heatpumps on a Load Calc as the NEC shows in the examples given at the end of the book, or in Article 220.

For LRA [Locked Rotor Amps], or starting amperes, there should be a rating on the nameplate stating what it should be calc'd at - but if that's not available, figure 600% of FLA for a somewhat close "Hip Shot"
Use the rated FLA of all motors per HP, not the RLA, when doing the 600% thingee.

Apply the LRA to your SCA [Short Circuit Amperes] calcs for motor contribution.

Sorry to throw so many TLA's at 'ya!!
[TLA = Three - Letter Acronyms].

Nothing like being bombarded with abbreviations!!


Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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