Per NEC 250.104(A), metal water piping within a building must be bonded to the electrical system. The bonding conductor must be sized per:

• NEC Table 250.122 based on the circuit breaker or fuse protecting the supply circuit
• NEC Table 250.66 based on the service entrance conductor size
• Always a minimum of 6 AWG copper regardless of circuit size
• The same size as the grounding electrode conductor