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Sometimes, it makes sense to "tap into" a feeder conductor to power another conductor. From Art. 100, we know that a feeder conductor runs between service equipment (or a similar source) and the branch-circuit overcurrent protective device (OCPD). The conductor that's tapped into a feeder conductor is called the tap conductor.
Think about this arrangement for a second. The OCPD supplies the feeder. You tap the feeder, making the feeder OCPD also the OCPD for the tap conductor. The OCPD is situated ahead of the point of supply to the tap conductor because it's situated ahead of the feeder. Normally, you have to size the OCPD for the size and insulation of a given conductor, per 240.4. But in this arrangement, the OCPD is sized for the feeder conductor and is thus oversized for that tap conductor (not sized per 240.4). In such a situation, you have a feeder tap [240.2].
How do you keep your feeder tap from burning up? Getting an answer to that question is why we have 240.21(B), which provides the requirements for feeder taps. Unfortunately, 240.21(B) can easily confuse anyone trying to apply it. The first step in avoiding that problem is to understand two basic rules that apply to all feeder taps.
You can't tap a tap; that is, don't use a tapped conductor to supply another conductor. This rule isn't explicitly stated, but:
You can infer this rule from the first paragraph of 240.21(B).
By definition, a feeder tap is to a feeder
, not to a tap.
No upsizing. This rule is explicitly stated in the first paragraph of 240.21(B): "The provisions of 240.4(B) shall not be permitted for tap conductors."
The "next-size-up protection rule" [240.4(B]), which you can use for nontapped conductors, allows you to use the next highest OCPD above the ampacity of the conductors being protected. If you were trying to protect a 1 AWG feeder conductor, you'd go to Table 310.16 and see the conductor is rated at 130A. You could use a 150A circuit...