With the dog days of summer upon us, I thought I would analyze my own personal situation for voltage drop when the A/C compressor kicks in. My relatives just moved to a new house and have already asked me "our lights flicker when the A/C kicks in, is that normal?" I'm sure everyone has been asked at one time, "how much is normal, is this a sign of a problem?"
For my case, I made a few reasonable estimates and will try to attach a snapshot of the spreadsheet. We have a pad mounted transformer feeding two homes (ours + neighbors) so I assumed 50kVA although it could be 37.5kVA. I assumed the transformer %Z of 0.03 pu and X/R ratio of 1.6 so if someone thinks I'm way off, please let me know. I also quickly looked up 4/0 Al for our 350' service. For simplicity of analysis, I took the transformer X and R, divided by 2 and added to the line parameters. Then I applied the IEEE approx. voltage drop IRcos +IX sin and also did the exact model as well although in this case, there was little difference.
I guess the bottom line is in my particular case, I would expect to see an 8 volt line-line drop... so if the house is sitting at 235V steady-state with whatever normal loads are on, it would drop to 227 on A/C startup which should provide 97% of rated torque so no problem from a starting torque standpoint. I should also mention I'm ignoring the drop from the panel to the A/C as it is right outside the wall where the panel is located. Since the line-neutral drop is 4 V, I would expect to see a momentary flicker on incandescent bulbs. Any comments welcome.
For my case, I made a few reasonable estimates and will try to attach a snapshot of the spreadsheet. We have a pad mounted transformer feeding two homes (ours + neighbors) so I assumed 50kVA although it could be 37.5kVA. I assumed the transformer %Z of 0.03 pu and X/R ratio of 1.6 so if someone thinks I'm way off, please let me know. I also quickly looked up 4/0 Al for our 350' service. For simplicity of analysis, I took the transformer X and R, divided by 2 and added to the line parameters. Then I applied the IEEE approx. voltage drop IRcos +IX sin and also did the exact model as well although in this case, there was little difference.
I guess the bottom line is in my particular case, I would expect to see an 8 volt line-line drop... so if the house is sitting at 235V steady-state with whatever normal loads are on, it would drop to 227 on A/C startup which should provide 97% of rated torque so no problem from a starting torque standpoint. I should also mention I'm ignoring the drop from the panel to the A/C as it is right outside the wall where the panel is located. Since the line-neutral drop is 4 V, I would expect to see a momentary flicker on incandescent bulbs. Any comments welcome.
