The smallest size copper wire permitted as a main bonding jumper in a 200-A main breaker panelboard is ___ AWG.

The smallest size copper wire permitted as a main bonding jumper in a 200-A main breaker panelboard is 4 AWG. This is established by the National Electrical Code (NEC) requirements, which specify the minimum sizes for bonding jumpers based on the size of the service or feeder.

Specifically, the NEC outlines that for a 200 amp service, the main bonding jumper must be at least 4 AWG copper. This ensures that the bonding jumper has adequate capacity to conduct any fault current and maintain safety in the electrical system. In doing so, it helps to ensure that the grounding system is effective, minimizing the risk of electrical shock or fire.

Choosing a larger size, such as 1/0 or 3/0, is not necessary for a 200-A panel and results in an unnecessarily large conductor, which can complicate installation and increase costs without providing additional safety benefits in this context.