House lights go dim when Air/Cond. is on

[Joe Cool]

Older ranch type house. Looks like it might have been
upgraded to 100 amp service. Square D type QO breakers.
But a lot of the old braided jacket 2 conductor house wiring
is still in operation. No other major loads are on at the same with the Air Cond.
except for maybe the fridge. The hot water heater and the cook stove
are LP gas. Is it common for the lights to dim when the air conditioner goes on?
Is it a dangerous situation? Should something be done to fix the problem?

 

[bphgravity]

This common and should be expected. The a/c creates a voltage drop on the service is noticed as the lights dim a bit. It's probably not a serious issue. You may want to check terminations of the service conductors especially the grounded (neutral) conductor.

 

[77401]

Has this always happened or is this new. Its common, but you can always call POCO & report a power flucuation or partial power. Let them check their connection

 

[W6SJK]

Dimming lights

Dimming lights

To say it a different way, the lights are dimming because the voltage is lower at the panelboard. The voltage is lower because there is more current flowing through the power company transformer, and through the service conductors from the power company transformer to the panel and through all connections in between. A small amount of dip is normal due to the resistance of all the above. But if there is a bad connection the resistance will be higher and the connection will heat up = bad situation. So it's a hard call to make without being there...

 

[infinity]

To say it a different way, the lights are dimming because the voltage is lower at the panelboard. The voltage is lower because there is more current flowing through the power company transformer, and through the service conductors from the power company transformer to the panel and through all connections in between. A small amount of dip is normal due to the resistance of all the above. But if there is a bad connection the resistance will be higher and the connection will heat up = bad situation. So it's a hard call to make without being there...

I agree with Sparkie. The voltage at the panel in my basement is about 246 volts in January when the demand from the POCO is low. Today when it will be 100 degrees the voltage is barely 230 volts. Dimming lights are common when the AC kicks in.

 

[robbietan]

it could either be a bad connection at the panel or wires are under-capacity. increased resistance due to the two above could cause light dimming.

a very long shot would be the transformer serving the house is at its load limit....

 

[don_resqcapt19]

a very long shot would be the transformer serving the house is at its load limit....

Really? The 25kVA transformer on the pole outside of my house serves 7 houses. I expect on hot summer days the transformer is operating well above 25kVA.
Don

 

[Minuteman]

Has this always happened or is this new. Its common, but you can always call POCO & report a power flucuation or partial power. Let them check their connection

I agree, call your power company. They will check out their stuff and not bill you. If the connections and equipment are okay, then contact a local electrical contractor to tighten the neutral connection in your panel.

Also, ever notice the wire from POCO is like a #6 aluminum (underground or overhead) for a 100 amp service? That's partly because of the demand factor that POCO is allowed to adjust for. I believe their demand factor is a little conservative for summer.