Lights may flicker when the electrical appliances like heating or cooling systems are working under a weak power supply. In some cases the light flickers when the heat-pump compressor is on. To know the reason for the flickering of light, a nearby voltage meter is used to measure the source of the power in the electrical wiring. If there is drop in voltage noticed in the meter when A/C or any other appliance is switched on, then this appliance can be responsible for lowering of voltage supply. The utility of the appliance may have increased the loading. The voltage fluctuation also can be influenced by electric supply connections and size of the service wire. If the problem is observed somewhere between the meter and the light, then it might be due to small size of the wire or any bad connection that can occur in between.
The flickering of light might result from any loose connection that can exist anywhere in the system. The problem also can exist in a public electric supply. The faulty public utility connection should generally be in the service wire connected to your house. The flickering of light might occur due to many reasons. The A/C appliance can have the capacity to interrupt the 200 ampere service a little. Little hindrance in the service by a big electric appliance may not give any problem. But a big electric appliance working in a small service line might create problems in voltage supply to other small bulbs in the line. Hence large load might be the major cause for the lights to flicker.
Loose circuit breakers or loose wires starting from the circuit breakers also can result in improper supply of electricity. The optimum voltage supply will not take place due to the loose contact. When the lights are flickering, if we observe a significant voltage drop between the transformer and the power panel at your house, then there is a fault in the power supply from the public service. Otherwise, the problem might be with any of the heavy load appliances in your house.