What causes a water pressure drop in the shower when someone turns another faucet on?

When someone turns on another faucet in the house, it can cause a drop in water pressure in the shower for a few reasons:

1. Limited Water Supply: Most homes have a single main water line that supplies water to all the faucets and fixtures in the house. When multiple faucets are turned on at the same time, the water flow is divided among them, leading to a reduction in pressure.

2. Water Pressure Regulator: Many homes have a water pressure regulator installed to maintain a consistent water pressure throughout the house. However, if the regulator is set too low, it may not be able to compensate for the increased demand when multiple faucets are turned on, resulting in a drop in pressure.

3. Pipe Size and Design: The size and design of the plumbing pipes in your home can also impact water pressure. If the pipes are too narrow or have too many bends and elbows, it can create resistance to water flow, leading to reduced pressure.

4. Competing Demand for Water: If multiple appliances or fixtures that require water are being used simultaneously, such as a washing machine, dishwasher, and shower, it can strain the water supply and cause a pressure drop.

5. Clogged Pipes or Fixtures: Clogs in pipes or showerheads can restrict water flow, causing a decrease in pressure. Regular cleaning and maintenance of plumbing fixtures can help prevent this issue.

6. High Water Demand in the Neighborhood: During periods of high water usage in your neighborhood, such as during peak morning hours or during hot summer days, water pressure may drop as more households draw water simultaneously.

To address this issue, you may consider upgrading your water pressure regulator to a higher setting or re-piping your home with larger diameter pipes to improve water flow. If the problem persists, it's best to consult a licensed plumber for a proper diagnosis and solution.