Troubleshooting guide

Why Do My Lights Flicker or Dim When Equipment Starts?

Starting current causes a voltage dip across the supply impedance. Covers motors, loose connections, weak supply and LED drivers.

Electrical & Power · Last reviewed October 17, 2026

Equipment such as compressors, pumps and air conditioners draws several times its running current for a moment when it starts. That current flows through the wiring and the supply, and the voltage drops by current times impedance. Every light on the same supply sees the dip. The first question is how widely the flicker is seen.

Flicker can be a sign of a loose connection, and loose connections make heat. Investigation of panels, meters, service conductors and connections belongs with a licensed electrician or the utility. Do not open energized equipment.

What you see	Likely cause	What to check
One fixture flickers, all others steady	The lamp or driver, a loose connection at the fixture, or a faulty dimmer	Swap the lamp, check the dimmer compatibility, then the connections at that fixture
Several lights on one circuit flicker when a load starts on that circuit	Loose neutral or hot connection on the circuit, or an overloaded circuit	Connections along the circuit by a licensed electrician. Load on the circuit
Lights across the building dip when a large motor or compressor starts	Voltage dip from the starting current across a weak supply (transformer, service, long feeder)	Voltage recording during a start. Supply impedance and the motor starting method
The dip is large and lasts for seconds	Very high starting current, a long feeder, undersized conductors or a generator supply	Recorded voltage versus time, and the starting method (soft starter or drive)
LED lights flicker or turn off briefly while incandescent lights do not	Some LED drivers drop out or are sensitive to small dips and to dimmer behavior	Driver data sheet, dimmer type, and a voltage recording at the fixture
Flicker at random with no equipment starting	Loose connections, a failing service connection, or a utility disturbance	Utility and electrician inspection, thermal imaging of connections under load

How big is the dip?

A rough estimate of the voltage dip is the ratio of the starting apparent power to the short-circuit apparent power at the point where the lights are fed. A motor drawing 600 A at 480 V when starting is:

S(start) = √3 × 480 × 600 = 499 kVA

If the available fault current at that point is 12,000 A, then S(short-circuit) = √3 × 480 × 12,000 = 9,977 kVA, and the dip is about 499 ÷ 9,977 = 5%. A 5% dip is easily visible in many lamps. A stronger supply (higher fault current) or a gentler start (soft starter, drive, star-delta) reduces it directly.

One fixture or the whole building

One fixture: a local component or connection. The supply is not the issue.
One circuit: shared wiring or a connection on that circuit.
The whole building at one time: a supply-side effect, which means the service, the transformer or the utility network.
Other buildings also affected: the utility supply, which is a call to the utility.

Write a useful problem statement

Which lights flicker, how many, and whether they share a circuit or panel.
What equipment starts at the same time and its size and starting method.
How long the dip lasts and how deep it looks, and any voltage recording.
Lamp and dimmer types, and whether the problem started after a change.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

More troubleshooting

Questions

Is a flicker always a supply problem?

No. A single fixture flickering is usually a lamp, driver or connection. A pattern across many fixtures that follows equipment starting points to the supply.

Can a dip damage equipment?

Small dips usually do not, but deep dips can drop out contactors, reset drives and computers. The sensitive equipment is often hurt first.

How do I reduce a motor-starting dip?

Use a reduced-voltage starter, a soft starter or a drive. A larger transformer or shorter feeder also lowers the supply impedance.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

← All troubleshooting