I had a guy I work with tell me that 12,000 British thermal units (1 ton of cooling) equals 7 amps of alternating current. Is this accurate or is this just hearsay? I was taught to always refer to the manufacturers specs per article 110.3 (B) and will continue to do it that way but I'm interested in finding out if there is any truth to this.

--Ron

Sounds like 12 apples = 7 oranges to me. I spose you could work backwards using your efficiency to find the power required and calculate the voltage where 7 amps would be the required current. Easier to just say it's apples to oranges as stated.
Joe

I think he is actually exactly right, the only variable he left out is the voltage
I'm sure one ton is equal to 7 amps at some voltage.
Back in the olden days we used 1.25kw per ton when SWAGing a computer room HVAC calc but we really wanted the numbers from the manufacturer. I think that 1.25kw assumed some duty cycle adjustment.

this may be related to the idea that 1HP = 1 ton- which is not necessarily true, but 1 HP = 760 watts = ~7 amps @ 115v? But 1 HP does not correlate to 1 ton of refrigeration, sometimes it takes a 2 HP motor to drive a 1 ton load.

This was/is just an old timer's saying but when we have to do a quick approximation we often "ball park" the load at 10A/ton (208).

I should have mentioned that the application here are split-system central A/C's rated at 240 volts. All I have to hear is "this is what the old-timers used to do" and that's good enough for me. It's not until you start listening to the old-timers (and I mean that respectively) that the serious learning can begin.

Thank You.

I looked up how they compute SEER which would affect the current and I am even more confused. Wikpedia makes it sound like a 10 SEER machine would give you 12000 BTU/hs with 1200w but that doesn't sound right (5a per ton at 240v) but it is close to the old number we used (1250w). I do think SEER is an average too. I tried looking up some installation info from Trane but my eyes started glazing over before I found any real specs.

1 Ton of cooling is output.
BTU/HR is output.
746W/HP is also output.

Amps is input.

If you don't know the efficiency and the power factor then you can't figure out the input.

For a more conservative rule of thumb I use 1000W/HP to estimate input.

12000= a ton BTU's It's hard to convert into motor amperage because most motor are 60% - 70% efficient.

Single phase A/C units need about 1 HP per Ton. One Horse Power is 746 watts.

So, 746 watts x 160% eff = 1230.9 watts needed to produce 1 ton of A/C.

1230.9 / 120 volts = 10.5 amps for the motor




[This message has been edited by Minuteman (edited 09-18-2006).]

We do seem to be bouncing around that 12xxw per BTU number, but if that is true for a 10SEER machine a 15 SEER machine should be 66.6% of that.