How Lights Dim When AC Comes On - Understanding the Mechanism

Nov 5, 2023

In the world of electrical systems, it's not uncommon for people to notice that their lights dim momentarily when their air conditioning (AC) unit turns on. This phenomenon, often experienced in households, is a result of the interaction between the AC unit and the electrical grid.

Understanding the AC Power Load

Before we delve into the specifics of why lights dim when the AC comes on, let's have a closer look at how the AC unit affects the electrical power load in a building. AC units have a significant electrical load due to their motor-driven compressors, fans, and other internal components.

When an AC unit starts, it requires a higher amount of power known as the starting or inrush current. This initial power surge can briefly draw more electricity than the normal operational requirement. This sudden demand for power causes a temporary voltage drop throughout the electrical system, leading to the noticeable dimming of lights.

The Role of Voltage Drop

The voltage drop occurs mainly due to Ohm's Law, which states that the voltage drop across a circuit is directly proportional to the current flowing through it and the resistance in the wires and components. When the AC unit starts, the increased current flowing through the circuit creates a temporary increase in resistance, leading to voltage drop.

Wiring and other components in the electrical system have a certain resistance, and when more current flows through them, the voltage supplied to the lights experiences a momentary decrease. This reduction in voltage causes the lights to dim for a brief moment until the AC unit stabilizes its power consumption.

Implications for Contractors

Contractors often receive inquiries about lights dimming when the AC comes on, especially during peak demand periods. Customers may worry about potential electrical issues or faulty wiring. As a professional contractor, it's important to reassure them that this dimming effect is a normal occurrence caused by the AC unit's electrical load.

However, it is crucial to advise customers to ensure their electrical systems are properly sized and meet the necessary requirements to handle the starting current of their AC unit. Ensuring the electrical system can accommodate the initial power surge helps minimize the dimming effect and potential strain on other electrical devices.

Tackling the Dimming Effect

If customers experience significant lighting dimming when their AC unit starts, there are a few steps that can be taken to minimize the effect:

  • Upgrading the Wiring: In some cases, the existing wiring might be unable to handle the starting current of the AC unit. Contractors can recommend upgrading the wiring gauge to minimize resistance and voltage drop.
  • Installing a Soft Starter: A soft starter can be added to the AC unit to reduce the starting current and ease the initial power surge, eliminating or significantly reducing the dimming effect.
  • Consulting an Electrician: In complex cases, it may be necessary to involve a professional electrician to assess and optimize the electrical system to minimize the dimming effect.


Understanding why lights dim when the AC comes on provides insight into the interaction between the electrical grid and AC units. While this phenomenon can initially cause concern, it is a normal occurrence due to the increased power load during the AC unit's startup. By educating customers and providing appropriate solutions, contractors can successfully address this issue and ensure optimal performance of both the AC unit and the electrical system.

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