Equivalent Resident Connection, or ERC, is a slightly mystic term that used to determine the required amount of source and storage for a public drinking water system. As per rule (https://rules.utah.gov/publicat/code/r309/r309-510.htm) , a system must have approved culinary sources that equal 800 gallons per day, per ERC, to meet peak-day demand (that day your system has the highest water demand from your customers - typically a day, or multiple days during the summer months). When used to determine storage capacity requirements, a system must have 400 gallons per ERC. Bear in mind that upon evaluation, a commercial or industrial connection may (and often does), equal multiple ERC's.
These numbers are solely for indoor usage demands and if a system also supplies irrigation water, as most do to one extent or another, then this must also be factored into the ERC calculation. How much depends on the area of the state in which you’re located; numbers needed for these calculations can also be found within the rule. While this may not be of much interest to an existing, well-established system, it can still be useful if a system determines their actual peak usage demand, or ERC. This can then be used to help calculate what will be required for future growth, impact fees, plans to enlarge the system through capital projects, etc.
Finally, if a system has valid historical data that shows their actual number is different (lower), a reduction can then be requested if desired.