A tap conductor is a conductor, other than a service conductor, that has overcurrent protection ahead of its point of supply that exceeds the value of the conductor (see image).

According to 240.21, conductors are to be provided with overcurrent protection at the point where they originate except where permitted in 240.21(A) through (H). These code sections have special allowances for tap conductors.

When it comes to motor taps. Section 430.28 has another set of tap conductor rules that are specific to group motor installations. These rules are similar to the rules in 240.21(B) but more tailored to motor applications and must be used instead of 240.21(B) for group motor installations.

Without using the exceptions, the most common motor feeder taps will be the 10 foot and 25 foot tap rules. The 10 foot tap rule applies when there is no more than 10 foot of tap conductor stealing power from the feeder serving the group of motors. Likewise, the 25 foot tap rule applies when there is more than 10 foot but not more than 25 foot of tap conductor between terminals.

Both tap rules have a few things in common: Feeder tap conductors must have an ampacity not less than what is required by Part II of article 430 which is where the branch circuit conductors are sized. As well, tap conductors shall terminate in a branch-circuit protective device.

For the 25 foot tap rule, tap conductors must be rated at least 1/3 the feeder rating, be suitably protected from physical damage or enclosed in a raceway and they cannot be longer than 25 feet.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to NFPA 70.