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Re: Lights flicker when AC unit cycles on

From: info@builderswebsource.com
Category: Electrical/Lighting
Remote Name: 63.198.181.43
Date: 26 Jun 2005
Time: 11:26 AM

Comments

According to Trane's website,

"Light flicker associated with the starting of appliances usually does not pose any safety problems and does not damage appliances. Proper wire connections and grounding are necessary to reduce flicker. Contractors and homeowners should check with their local utility regarding the appropriate wire size and transformer servicing the home. In cases where light dimming is related to the starting of a central air conditioner, and the previously discussed steps have not resolved the situation, manufacturers offer start assist kits that can be purchased which may help reduce flicker. Please contact your local independent Trane dealer for assistance."

Builders Websource advises caution with this statement if taken out of context. Although some flicker may be tolerable and safe, excessive and prolonged voltage sag (or spikes caused by sudden current surges) can damage some appliances and other sensitive electronic equipment. In your case, you've already had initial consultations with the builder, the electrician, and the power company, each of whom deny any responsibility for your problem. In addition, you've already had the soft-start kit installed, which has helped but not eliminated your flicker problem.

Power-hungry motorized appliances such as microwave ovens, spa pumps, central vacuums, air compressors, or in this case, your 5-ton Trane air conditioning unit, draw significant inrush current, particularly during the turn-on cycle. Typical appliance inrush durations range from 0.2 - 1 second, depending on the type of load. Despite this short duration, the inrush current is often as much as 3-7 times higher than the steady-state operating current. This results from the extra power required to overcome the initial friction and inertia of mechanical or motorized appliances. As a result, any resistance in your electrical system (such as wiring resistance, long runs between the AC unit and the breaker and/or transformer, or a poor ground or hot connection) can result in substantial instantaneous voltage drop on that branch circuit as well as the entire incoming service. Shared appliances or lighting circuits will experience the same voltage drop, resulting in flicker or sluggish motor operation.

Soft start kits such as the one you installed help to reduce peak inrush currents by slowing down how voltage is applied to the HVAC unit at turn-on. By ramping the voltage gradually and compensating for phasing issues caused by inductive loads (like motors), large spikes are mitigated, which should help to reduce flickering. Generally, the voltage drop is approximately equal to the resistance times the inrush current. Phasing factors and signal shape also impact the instantaneous voltage drop. However, using simplifying assumptions, if your electrical supply has a resistance of 0.25 ohm and you have an inrush current of 100 amps, this would result in a voltage sag of approximately 25V or nearly 20% of your 120V line, or 10% of your 240V service. This is the math behind your flickering light problem.

However flickering may not be entirely eliminated. In your case, given that other homes in your neighborhood with similar HVAC units do not experience this phenomenon, it's possible that you have either a poor terminal connection somewhere (such as a loose screw on a breaker or service panel), or that the branch line supplying your AC unit or the breaker box to which it is attached is undersized. Sometimes aluminum wiring is used on the main service to connect the step-down transformer to the house. Have your power company verify this connection and make sure that the terminal screws are tight. To avoid excessive voltage drop, we always recommend heavy-duty copper service and branch lines which, although more expensive than aluminum which is commonly used for service drops, can help to reduce voltage drop considerably.

Although unlikely, it is possible that the wiring feeding your branch circuit is faulty. We've seen all kinds of situations such as damaged wiring from cutting nics to nails, screws or bolts which were driven through branch lines during the construction process. Also, if there are any junction boxes in the path, it's always a good idea to check the wire nuts (with the power off) to make sure that any splices are properly secured.

If you remain concerned that there is a unique problem in your home, the power company may be able to assist with a power measurement device that can monitor the inrush current as well as instantaneous voltage drop during the power cycling process. Based on this information, it is possible to determine the effective resistance of the circuit to determine whether the overall resistance is within expected range. An excessive voltage drop would indicate excessive resistance, due to excessively long wiring or too small a gauge for the connected appliance inrush curret, a defective transformer, circuit breaker, or loose connection somewhere along the way, including all the way back at the drop from the utility pole/transformer.

One other point of note is that many power companies recommend two smaller AC units under 3 tons each, specifically due to known power quality issues that can result from larger units at turn-on. While this won't be a practical retrofit in your case, it's possible your Trane model is fitted with dual compressors due to its size. Keep us posted and let us know what you find.

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