Monday, May 07, 2007

Are you getting the power you're paying for?

This is going to be a rant.

The National Electric Code is "the bible" electricians and inspectors abide by. The purpose of the code is to provide the rules for "safe" installations.
Article 90.1(A) Practical Safeguarding. The purpose of this Code is the practical safeguarding of persons and property from hazards arising from the use of electricity.
Sounds good so far right? Everyone wants safe electrical installations. HOWEVER, that's all the code guarantees in many cases - safety. One can have a perfectly code compliant installation, that an inspector MUST PASS, that is neither efficient, nor practically usable, nor allows for any future expansions.
Article 90.1(B) Adequacy. This Code contains provisions that are necessary for safety. Compliance therewith and proper maintenance results in an installation that is essentilly free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion of electrical use[...]
This is why you'll see things like switches for a room light that require you to walk across a dark room to turn it on -- convenience isn't part of the code, so a sleazy contractor will do what is most convenient and cheapest for them. In my house, when you walk in the front door, you have to walk 20 feet across a dark living room to turn the living room ceiling light on. One of these days, its going to piss me off enough that I'll put in a bunch of 4-ways and do it the way it should have been done ;->

On to EFFICIENCY -- the main topic of this rant. The code specifies minimum wire sizes for particular motor loads and such. One of the things that gets cheaped out on all the time is the wire for A/C compressors. My whole development was wired with #10 wire on the A/C compressors. The code allows it, its done all the time in millions of houses, its considered "safe". However, its not efficient.

Typical residential A/C compressors are drawing in the range of 20 amps of 240V load while running -- well within the stated range of that 30A #10 wire commonly used. However, at a continuous 20A load, that #10 wire is going to get warm, quite warm, warm enough that it gets uncomfortably hot to the touch. The #10 that was in my attic got so hot I didn't want to keep my hand on it within about 5 seconds of the compressor kicking in.

When the A/C is running, its already hot in the attic right?

What is all that heat the #10 wire is throwing off doing? Heating an already hot attic, that's what its doing.

What's causing that "heat"? The intrinsic resistance within the wire itself The wire is acting like an inefficient form of space heater.

That resistance is lowering the voltage the A/C compressor motor is seeing causing it to draw more current to do the same amount of work. The motor starts operating at the lower limits of its practical voltage range.

All that POWER that's going towards heating up the minimum sized wire, is power that NOT making its way to the compressor to do something useful, and its robbing the compressor of the ability to operate in its most efficient voltage range.

Here's the bottom line: I up sized the wire going to my outside A/C compressor disconnect to #8 and now it only gets luke warm when the compressor is running rather than noticeably hot. I also fed the compressor from a panel that's only about 10' away rather than 60'. That panel is being fed by even larger #4 wire and all the circuits in it are normally lightly loaded, so the compressor is the heaviest thing it sees normally.

This upsizing of the wire on the path that feeds the A/C compressor made a VISIBLE DIFFERENCE that you could feel in the performance of the A/C compressor.

If you ever have a house built, demand that the electricians upsize the wire going to the A/C unit. The cost difference is probably going to be less than $20 even at today's insane copper prices, but you're going to get that back very quickly in power saved by not heating your attic with skinny higher resistance code minimum wire. Your compressor will last longer because it'll be operating in its happy voltage range too.

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