Safety guide for wiring a feeder tap

Feeder Tap Basics and When the 10/25/100 Foot Rules Apply

A feeder tap is a conductor that connects to a feeder without the overcurrent protection sized to the tap conductor's ampacity. NEC 240.21(B) governs them, and it is the only place in the Code where you can legally run a smaller conductor off a larger feeder without protecting it at its own ampacity. Get the rule wrong and you have an unprotected conductor in the wall.

The four common tap rules sit in 240.21(B)(1) through (B)(5). Pick the one that fits the run before you pull wire, not after. The tap length, ampacity ratio, termination, and physical protection all change depending on which subsection you are working under.

• 240.21(B)(1) ... 10 foot tap, tap ampacity at least 10 percent of feeder OCPD.
• 240.21(B)(2) ... 25 foot tap, tap ampacity at least one third of feeder OCPD.
• 240.21(B)(3) ... taps supplying a transformer, primary plus secondary not over 25 feet.
• 240.21(B)(4) ... outside taps of unlimited length, terminating in a single OCPD.
• 240.21(B)(5) ... 25 foot horizontal plus unlimited vertical for high bay industrial occupancies only.

Sizing the Tap Conductor

Start with the feeder OCPD, not the load. The tap conductor's ampacity is the qualifying number, and that is the 75 degree column in Table 310.16 unless your terminations are rated otherwise per 110.14(C). For a 400 amp feeder under the 10 foot rule, the tap must carry at least 40 amps. Under the 25 foot rule, at least 134 amps, which puts you at 1/0 copper minimum.

The tap also has to terminate in a single circuit breaker or set of fuses sized at or below the tap's ampacity. You cannot land a 1/0 tap on a 200 amp panel main and call it good if the calculation says 134. Round up to the next standard size only if 240.4(B) lets you, and only when the load is below the next standard.

Field tip: Mark the tap length on the conduit with a Sharpie at rough-in. The inspector will measure, and so will the next electrician who extends it five years from now.

Physical Protection and Routing

Tap conductors must be protected from physical damage by being enclosed in a raceway, cable, or approved means per 240.21(B)(1)(2) and (B)(2)(2). EMT, RMC, IMC, or a listed cable assembly all work. Free air taps inside a panelboard or wireway are fine within the equipment, but the moment they leave, they need a raceway.

Keep the tap as short and direct as the rule allows. The 10 foot measurement runs from the point of tap to the point where the tap terminates at the OCPD, including any vertical drops or horizontal offsets. Slack inside the cabinet counts. Do not assume the inspector measures only the conduit run.

• Measure tap length from the centerline of the splice or tap point to the line side of the OCPD.
• Include the conductor inside the cabinet, not just the raceway.
• Verify bending space at terminals per Table 312.6(B).
• Confirm the raceway fill against Chapter 9, Table 1 before pulling.

Tap Devices, Splices, and Terminations

The tap connection itself has to be made with listed equipment. Polaris-style insulated multi-tap connectors, Burndy or Ilsco mechanical lugs, or compression connectors with proper dies are all acceptable. Whatever you use, it must be listed for the conductor material and size, and torqued to the manufacturer's spec. NEC 110.14(D) requires a torque tool for any termination with a published value, full stop.

Cover the tap with insulating tape or a listed insulating cover rated for the system voltage. Bare connectors in a wireway are a callback waiting to happen. If you are tapping inside a junction box, use a box sized per 314.28(A) for conductors 4 AWG and larger, and check that the wire bending space at the connector lets you dress the conductors without stress.

Common Mistakes That Fail Inspection

The fastest way to get red-tagged on a feeder tap is to exceed the length. A tap that runs 10 feet 6 inches under the 10 foot rule is not a tap, it is an unprotected conductor. There is no tolerance and no rounding. The same goes for ampacity ratios... a 39 amp tap on a 400 amp feeder fails the 10 percent test even though it looks close.

Other repeat offenders: tapping outside the equipment without a raceway, landing the tap on a panel with a main breaker larger than the tap ampacity, missing the single OCPD requirement on the load end, and using the 25 foot rule to feed multiple panels through a wireway. Each tap needs its own qualifying termination.

Field tip: If you find yourself drawing a tap on a napkin to make the math work, you are probably outside the rule. Pull the calc, then pull the wire.

1. Confirm which 240.21(B) subsection applies before sizing.
2. Calculate tap ampacity from the feeder OCPD, not the connected load.
3. Measure the full conductor path, including cabinet slack.
4. Verify the load-end OCPD is sized at or below tap ampacity.
5. Torque every termination and document it.

Documentation and Handoff

Mark the tap on the as-built. Note the rule used, the tap length, the conductor size, and the OCPD at each end. Future troubleshooting depends on the next electrician knowing the tap is there and why it was sized the way it was. A label inside the panel cover saying "10 ft tap per 240.21(B)(1)" takes thirty seconds and saves an hour later.

Take a photo before you close the cover. Conductor routing, lug torque marks, and connector type all matter if the tap ever gets questioned. The Code is on your side when the work is documented.