Previously, we noticed the NEC vaguely defines "bonded (bonding)." Article 100 doesn't, for example, specify how conductive the bonding jumper must be. But throughout the NEC, and especially in Art. 250, we find the answer embedded in the requirements for conductors used to bond things together
The second of five bonding related terms defined in Art. 100 is "bonding conductor or jumper." The definition is as follows: “A reliable conductor to ensure the required electrical conductivity between metal parts required to be electrically connected.”
But what does this mean? This is where a broader understanding of the NEC helps you understand a specific definition. We know from Art. 310 that the NEC recognizes only aluminum and copper as conductors. So a bonding conductor must be one of these metals.
What makes the conductor reliable, and how can it ensure sufficient conductivity? We know the thicker a conductor is, the more sturdy it is. And we know larger conductors are more conductive than smaller ones, which is why we increase conductor size to reduce voltage drop.
Bigger is better, up to a point; using 2,000kcmil for bonding jumpers would be absurdly expensive. So we must correctly size the conductor. Article 250 provides the requirements for grounding and bonding, and from Figure 250.1 we see the sizing requirements must be in Part V.
