7.2 Feeder Tap Rules — 240.21(B)

The General Rule and Why Taps Are an Exception

240.21 requires overcurrent protection at the point where the conductors receive their supply. A tap conductor is smaller than the feeder it connects to and has no overcurrent device at its supply end, so it is unprotected against a short circuit along its length. The Code tolerates this only in narrowly defined situations where the tap is short, protected, and terminates in a device sized to the tap conductor.

Every tap rule shares three ideas:

1. A minimum ampacity ratio related to the upstream device;
2. A length limit (except for the outside tap);
3. Termination in a single overcurrent device that limits the load on the tap.

The 10-Foot Tap Rule — 240.21(B)(1)

Where the length of the tap conductors does not exceed 10 feet (3.0 m) and the tap conductors comply with all of the following:

1. The ampacity of the tap conductors is:
   • not less than the combined calculated loads on the circuits supplied by the tap conductors, and
   • not less than the rating of the device supplied by the tap conductors or not less than the rating of the overcurrent protective device at the termination of the tap conductors;
2. The tap conductors do not extend beyond the switchboard, switchgear, panelboard, disconnecting means, or control devices they supply;
3. Except at the point of connection to the feeder, the tap conductors are enclosed in a raceway, which shall extend from the tap to the enclosure of an enclosed switchboard, switchgear, panelboard, or control devices, or to the back of an open switchboard;
4. For field installations where the tap conductors leave the enclosure or vault in which the tap is made, the rating of the overcurrent device on the line side of the tap conductors shall not exceed 10 times the ampacity of the tap conductor.

Condition 4 is the "one-tenth" rule most people remember, and it applies where the tap leaves the enclosure.

Worked Example — 10-Foot Tap

A feeder is protected by a 400-ampere circuit breaker. A 9-foot tap in a raceway feeds a subpanel with a calculated load of 55 amperes. What is the minimum tap conductor ampacity?

Condition 1: at least the calculated load ............ 55 A
Condition 4: at least 400 / 10 ....................... 40 A
Governing value: the LARGER of the two ............... 55 A

The tap conductor must have an ampacity of at least 55 A, and it must also be at least equal to the rating of the overcurrent device at its termination. If the subpanel main is a 60 A device, the tap conductor must be at least 60 A.

A common trap reverses this: a 200 A feeder with a 15 A calculated load gives 200/10 = 20 A as the one-tenth minimum, which exceeds the 15 A load. The answer is 20 A, because both conditions must be satisfied and the larger governs.

The 25-Foot Tap Rule — 240.21(B)(2)

Where the length of the tap conductors does not exceed 25 feet (7.5 m) and the tap conductors comply with all of the following:

1. The ampacity of the tap conductors is not less than one third of the rating of the overcurrent device protecting the feeder conductors;
2. The tap conductors terminate in a single circuit breaker or a single set of fuses that will limit the load to the ampacity of the tap conductors;
3. The tap conductors are suitably protected from physical damage by being enclosed in an approved raceway or by other approved means.

Worked Example — 25-Foot Tap

A 400-ampere fused feeder supplies a 20-foot tap in rigid metal conduit to a remote panel with a calculated load of 100 amperes. What is the minimum tap conductor ampacity?

One third of 400 A = 133.33 A
Calculated load    = 100 A
Governing value    = 133.33 A

133.33 amperes. The one-third requirement governs because it exceeds the calculated load. A 1/0 AWG copper conductor at 150 A in the 75 degrees C column satisfies it.

Comparing the Two Rules

The distinguishing rule to remember is the ratio: one tenth for the 10-foot rule, one third for the 25-foot rule.

Taps Supplying a Transformer — 240.21(B)(3)

Where the tap conductors supply a transformer and comply with all of the following:

1. The conductors supplying the primary of a transformer have an ampacity at least one third the rating of the overcurrent device protecting the feeder conductors;
2. The conductors supplied by the secondary of the transformer have an ampacity that, when multiplied by the ratio of the secondary-to-primary voltage, is at least one third the rating of the overcurrent device protecting the feeder conductors;
3. The total length of one primary plus one secondary conductor, excluding any portion of the primary conductor that is protected at its ampacity, is not over 25 feet (7.5 m);
4. The primary and secondary conductors are suitably protected from physical damage;
5. The secondary conductors terminate in a single circuit breaker or set of fuses that limits the load current to not more than the conductor ampacity permitted by 310.14.

Taps Over 25 Feet Long — 240.21(B)(4)

In high-bay manufacturing buildings over 35 feet (11 m) high at walls, where conditions of maintenance and supervision ensure that only qualified persons service the systems, tap conductors are permitted to be not over 25 feet (7.5 m) long horizontally and not over 100 feet (30 m) total length, where all of the following conditions are met:

1. Ampacity of the tap conductors is not less than one third of the rating of the overcurrent device protecting the feeder conductors;
2. The tap conductors terminate in a single circuit breaker or single set of fuses limiting the load to the ampacity of the tap conductors;
3. The tap conductors are protected from physical damage by being enclosed in an approved raceway or by other approved means;
4. The tap conductors are continuous from end to end and contain no splices;
5. The tap conductors are sized 6 AWG copper or 4 AWG aluminum or larger;
6. The tap conductors do not penetrate walls, floors, or ceilings;
7. The tap is made no less than 30 feet (9.0 m) from the floor.

Outside Taps of Unlimited Length — 240.21(B)(5)

Where the conductors are located outside of a building or structure, except at the point of load termination, and comply with all of the following:

1. The conductors are suitably protected from physical damage;
2. The conductors terminate at a single circuit breaker or a single set of fuses that limits the load to the ampacity of the conductors;
3. The overcurrent device for the conductors is an integral part of a disconnecting means or shall be located immediately adjacent thereto;
4. The disconnecting means for the conductors is installed at a readily accessible location either outside of a building or structure, inside nearest the point of entrance of the conductors, or in accordance with 230.6 where installed in accordance with the requirements for service conductors.

There is no length limit and no ratio requirement on the outside tap, because the conductors are outside the building where a fault cannot ignite the structure. The trade-off is that they must be outside until the point of termination and the disconnect must be nearest the point of entrance.

Transformer Secondary Conductors — 240.21(C)

240.21(C) is a distinct set of rules for conductors supplied by a transformer secondary, covered in the next section of this chapter. Note the important general statement in 240.21(C)(1): conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio. Every other secondary arrangement must be protected on the secondary side.

No Tap Off a Tap

The closing sentence of 240.21 is short and absolute:

No conductor supplied under the provisions of 240.21(A) through (H) shall supply another conductor under those provisions, except through an overcurrent protective device meeting the requirements of 240.4.

You may not make a 10-foot tap and then, from that tap, make another 10-foot tap. The chain must pass through a properly sized overcurrent device before another tap is taken.