Why would you tap a feeder in the first place?

Figure 01

Note: This article is based on the 2020 NEC.

Understanding the rules for feeder taps could save you time and money without violating the NEC.

Why would you tap a feeder in the first place? One reason might be you need to supply a specific load and there's already a feeder with ample ampacity to also supply this load. Rather than expend the labor, time, and materials to run another feeder circuit, you tap into the nearby feeder.

Overcurrent protection must be provided for each phase conductor at the point where the conductors receive their supply except as permitted by 240.21(A) through (H).

If you think about this for a moment, you see that the feeder overcurrent protective device (OCPD) is oversized for the tap conductor. That is, the conductor has overcurrent protection rated more than the ampacity of the tap conductor because its OCPD is sized for the feeder you tapped.

Instead of being protected at the point where they receive their supply, tap conductors are protected against an overload by the OCPD into which they feed, and against short circuit and ground fault by the feeder protective device from which they originate.

We also know you can't tap a tap for the simple reason the tap you would be tapping does not have its own OCPD. This situation creates other limitations, which the Code addresses and we will look at shortly. Figure 01

Conductors can be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (B)(5). Notice, there is no mention here of tapping to a tap. The reference is always to the feeder. For example, Except at the point of the connection to the feeder. [240.21(B)(1)(3)] and The ampacity of the tap conductors is at least one-third the rating of the overcurrent device protecting the feeder conductors [240.21(B)(2)(1)].

The tap is permitted at any point on the load side of the feeder OCPD. The next size up rule in 240.4(B) is not permitted for feeder tap conductors.
(1) Feeder Tap Not Over 10 Feet. Tap conductors up to 10 ft long are permitted when they comply with the following:

(1) Tap conductors have an ampacity equal to or greater than:

a. The calculated load per Article 220, and
b. The rating of the OCPD or the equipment supplied by the tap conductors.

(2) The tap conductors are not permitted to extend beyond the equipment they supply.

(3) The tap conductors are installed within a raceway.

(4) Tap conductors that leave the enclosure where the tap is made must have an ampacity of at least 10 percent of the rating of the OCPD that protects the feeder.

Informational Note: If a tap supplies a panelboard, the tap conductors must terminate in an OCPD in accordance with 408.36.

10-foot tap rule
The length of your tap determines which rules to apply to it. When a tap is not over 10 ft, you determine the tap conductor size using the 10-foot tap rule. There are actually four rules for 10-foot taps, but it's the first of those four [240.21(B)(1)(1)] you follow when sizing the tap conductors:

The ampacity of the tap conductors must be at least the
1. Combined calculated loads on the circuits supplied by the tap conductors [240.21(B)(1)(a)], and
2. Rating of the equipment containing an OCPD supplied by the tap conductors or (at least the) rating of the OCPD of the tap conductors.

10-Foot Tap Rule Example 1
Question: What size 10-ft tap conductor is needed from a 400A circuit breaker to supply a 200A panelboard if the terminals are rated 75°C?
(a) 1/0 AWG (b) 2/0 AWG (c) 3/0 AWG (d) 4/0 AWG
Solution:
Ten Percent of 400A = 40A minimum conductor ampacity permitted
3/0 AWG is rated 200A at 75°C [110.14(C)(1)(b)(2) and Table 310.16] which is greater than 10 percent of the rating of the 400A OCPD.
Answer: (c) 3/0 AWG

10-Foot Tap Rule Example 2
Question: What size 10-ft tap conductor is needed from a 400A circuit breaker to supply a 150A feeder disconnect if the terminals are rated 75°C?
(a) 1/0 AWG (b) 2/0 AWG (c) 3/0 AWG (d) 4/0 AWG.
Solution:
Ten Percent of 400A = 40A minimum conductor ampacity permitted
1/0 AWG is rated 150A at 75°C [110.14(C)(1)(b)(2) and Table 310.16] which is greater than 10 percent of the rating of the 400A OCPD.
Answer: (a) 1/0 AWG

10-Foot Tap Rule Example 3
Question: What size 10-ft tap conductor is needed from a 400A circuit breaker to supply a 30A feeder disconnect if the terminals are rated 75°C?
(a) 8 AWG (b) 6 AWG (c) 4 AWG (d) 3 AWG
Solution:
Ten Percent of 400A = 40A minimum conductor ampacity permitted
8 AWG is rated 50A at 75°C [110.14(C)(1)(a)(3) and Table 310.16] which is greater than 10 percent of the rating of the 400A OCPD.
Answer: (a) 8 AWG

25-ft tap rule
Tap conductors up to 25 ft long are permitted when they comply with the following:
(1) The tap conductor has an ampacity at least 13 the rating of the OCPD that protects the feeder.
(2) The tap conductors terminate in an OCPD and have an ampacity equal to or greater than the rating of the OCPD.
Notice how this differs from the 10-ft tap rule. It's shifted from being based on the load the tap feeds to being based on the rating of the feeder OCPD.

25-Foot Tap Rule Example 1
Question: What size 25-ft tap conductor is needed from a 400A circuit breaker to supply a 200A panelboard if the terminals are rated 75°C?
(a) 1/0 AWG (b) 2/0 AWG (c) 3/0 AWG (d) 4/0 AWG
Solution:
The tap conductor must have a minimum rating of at least 133A (13 the rating of the 400A OCPD). 3/0 AWG is rated 200A at 75°C [110.14(C)(1)(b)(2) and Table 310.16] which is greater than 133A (13 the rating of the 400A OCPD) and equal to the 200A disconnect.
Answer: (c) 3/0 AWG

25-Foot Tap Example 2
Question: What size 25-ft tap conductor is needed from a 400A circuit breaker to supply a 150A feeder disconnect if the terminals are rated 75°C?
(a) 1/0 AWG (b) 2/0 AWG (c) 3/0 AWG (d) 4/0 AWG
Solution:
The tap conductor must have a minimum rating of at least 133A (13 the rating of the 400A OCPD).
1/0 AWG is rated 150A at 75°C [110.14(C)(1)(b)(2) and Table 310.16] which is greater than 133A (13 the rating of the 400A OCPD) and equal to the 150A disconnect.
Answer: (a) 1/0 AWG

25-Foot Tap Example 3
Question: What size 25-ft tap conductor is needed from a 400A circuit breaker to supply a 30A feeder disconnect if the terminals are rated 75°C?
(a) 3 AWG (b) 2 AWG (c) 1 AWG (d) 1/0 AWG
Solution:
The tap conductor must have a minimum rating of at least 133A (13 the rating of the 400A OCPD). 1/0 AWG is rated 150A at 75°C [110.14(C)(1)(b)(2) and Table 310.16] which is greater than 133A (13 the rating of the 400A OCPD) and greater than the 30A disconnect.
Answer: (d) 1/0 AWG

Outside feeder taps
Outside tap conductors can be of unlimited length if they comply with all the following [240.21(B)(5)]:
(1) The outside tap conductors are protected from physical damage.
(2) The outside tap conductors terminate in a single circuit breaker or single set of fuses that limits the load to the ampacity of the conductors.

The tap's OCPD is part of the building feeder disconnect.

Happy tapping
If you decide to tap a feeder, you can reduce the cost if you can keep the tap to 10 feet or less. Doing so is often not possible, and then you've got to follow the more stringent rules for longer taps.

From a documentation standpoint, taps can add complexity and confusion. In your CAD drawings, use notes, comments, and/or labeling to make it clear which feeder (and thus breaker) supplies a given tap. Adding similar information to the equipment itself and (for maintenance departments) in the CMMS for the affected assets will also help avoid errors with lockout/tagout.