This excerpt taken from the PCG 10-K filed Feb 18, 2005.
UCLs, LCLs, UWLs and LWLs for Index 3 When Number of Lines is > 125
According to standard procedures for proportion control charts for voltage classes where the median number of lines in service is greater than 125 for any given year, the upper and lower control chart limits (UCL, LCL, UWL, and LWL) for the kth year are determined using the normal approximation to the binomial distribution. The formulas are:
UCL = CLPvc + 3SPvc,k LCL = CLPvc - 3SPvc,k
UWL and LWL are calculated by replacing the "3" above with a "2".