A/C Compressor Electrical connections

[LJSMITH1]

I think the "flicker" is more noticeable with the smaller conductors because the voltage dip lasts longer. The smaller wire limits the current and causes the motor to take longer to start resulting in a more noticeable flicker. When you increase the wire size, you increase the current flow and increase the voltage dip on the service conductors but the higher current flow is of a shorter duration.

That's an excellent way of putting it! As far as time, we are only talking about under 1/2 second at most. At startup, the compressor will still be drawing a factor over the running load. Just how much and for how long are caused by the actual compressor, its condition, and the refrigerant charge in the system.

 

[LJSMITH1]

Are you suggesting that, despite the greater voltage drop, the smaller conductors allowed a greater current increase, enough to cause a greater voltage drop on the before-the-panel conductors?

And, that the larger AC conductors reduced the overall voltage drop so much that the current on the entire service was reduced, and not increased, by the lower circuit resistance, because it's a motor load?

If so, okay, I'll buy that. However, I would have loved to take bith voltage and current readings on both the main and the compressor terminals during start-up.



Added: I understand that upsizing helps with start-up voltage sag. What I question is whether that results in a gretaer or lesser current. Does the current increase faster than its own current-caused voltage drop drops?

I would have loved to have a meter fast enough to take those measurements!

If you factor resistance as a load, then yes, there should be a lesser current 'seen' by the main panel bus. I don't think it would be much, but it probably equates to a voltage drop of between 2-4v. The main benefit is what Don stated, that the duration of the flicker would be reduced. That is what my experience indicated.

 

[e57]

I say it also depends on the rest of the load in the house, for that matter in the nieghborhood - thankfully we don't live in a country that needs AC at X-mas time...

 

[mcclary's electrical]

I didn't read that one yet!

I don't even think that this install should even have an exposed UF whip of any size. I have always used NM liquidtight (Type B) in any A/C condenser installation I have done myself, with #10 90C THHN conductors, or #8 THHN 90C for longer runs. The run back from the disconnect to the main panel would be larger than #10 if the run exceeded 15' - this is my arbitrary engineer decision that is designed to overkill any installation...:roll: I actually

reduced the A/C startup flicker in my house lighting by changing out the 55' run of #8/3 NM with 55' of #6/3 NM.

The minimum is always the manufacturer's installation requirements (not just nameplate data), then take into account any local or national code issues - plus additional site requirements.

I do think that with moderate inductive loads like a compressor motor, a larger conductor is always better for the compressor and reduction of any objectionable flicker or voltage drop.

In YOUR mind it reduced flicker, it reality, there's no way it did. The voltage of seperate circuits is not affected by each other.

 

[e57]

In YOUR mind it reduced flicker, it reality, there's no way it did. The voltage of seperate circuits is not affected by each other.

I disagree - technically everything on the transformer is on the same circuit in one way or another. I have seen AC units dim the neighbors house...

 

[LJSMITH1]

In YOUR mind it reduced flicker, it reality, there's no way it did. The voltage of seperate circuits is not affected by each other.

Its not in my mind. This is one way of reducing it. Remember, lamp flicker is a response to system voltage drop. Absent of upsizing supply conductors, a hard start kit will also reduce the flicker.

This is not some new circuit theory. It has been discussed in many different areas, and there are many reasons why voltage drop should be considered in an application like this.

Here's a great EC&M article that explains the issue in more detail.

http://ecmweb.com/mag/electric_calculating_voltage_drop/

Here's an excerpt from that article:

Low voltage on a system increases maintenance costs and decreases safety and performance. Operating electrical equipment below its acceptable voltage rating can lead to premature failure and hazardous situations. Inductive loads, such as motors and ballasts can overheat, shortening equipment life and increasing energy consumption. If operated below their rated voltage, resistive loads, such as heaters, simply won't provide the desired output. A 10% reduction of voltage reduces the power output by 19%, because power output reduces as the square of voltage. Reduced circuit voltage can cause lights to flicker when other appliances or equipment cycle on.

It then goes on to explain more about conductor sizing to reduce voltage drop. In my case, I had a 55' (one way) run back to the main panel. That's not a short run for this kind of circuit. There are many voltage drop calculators online that can help illustrate that by upsizing a conductor from say a #10 to #8, your voltage drop at a defined voltage and amperage will be less - just in the conductors alone.

 

[LJSMITH1]

I say it also depends on the rest of the load in the house, for that matter in the nieghborhood - thankfully we don't live in a country that needs AC at X-mas time...

Some parts of the country still do!

 

[LJSMITH1]

As an add on to my previous post, and from the same EC&M article:

Circuit current. Use the expected steady-state load demand current to calculate the voltage drop. But don't just add up all the loads and call that your total load. Determine the total load by considering the maximum load occurring at any time due to simultaneous operation. Use this demand load for calculating the voltage drop. You may need to allow for future demand as well. If you size your conductors to just barely accommodate the present load, adding load later is going to be expensive. It's much less costly to install slightly oversized wire now than to rip it out and install a new conductor later. Pay special attention to lighting and appliance circuits and to mains that feed several loads.

This is where the LRA on the nameplate could come in handy to help determine the appropriate conductor size for an A/C compressor.