Somewhere in the building you are sitting in right now, there is very likely a conductor with no overcurrent device between it and the transformer on the pole. Not a code violation. Not a hack job left by the last guy. A perfectly legal, inspector-approved, engineer-stamped conductor, smaller than the feeder it hangs off of, running naked through a wireway with nothing upstream sized to protect it from overload.

That sentence tends to make careful electricians uncomfortable, and it should. We are trained from the first week that the breaker protects the wire — that the number on the handle exists to keep the copper from cooking. Then you open a switchboard, find a 400-amp feeder with a set of #3s tapped off the lugs feeding a 100-amp panel, and the whole mental model wobbles. What's protecting those #3s? Nothing at that end. The Code knows. The Code wrote it down.

The rule everyone learns, and the exception nobody explains

NEC 240.4 says conductors shall be protected against overcurrent in accordance with their ampacity. NEC 240.21 says that protection has to sit at the point where the conductor receives its supply. Together they form the sentence every apprentice can recite: the breaker goes at the source, and it's sized to the wire.

Then 240.21 immediately spends several hundred words on the exceptions — the tap rules — and this is where the trade gets made.

To understand why the exceptions exist, you have to separate two things that get mashed together in everyday talk. An overcurrent device does two different jobs. It clears short circuits and ground faults, which are violent, thousands of amps, over in a few cycles. And it clears overloads, which are slow — a conductor carrying 130 amps when it's rated for 100, warming quietly for twenty minutes until the insulation gives up.

A tap conductor still has short-circuit protection. That 400-amp breaker upstream will absolutely see a bolted fault on a #3 tap and clear it, because a bolted fault draws far more than 400 amps. What the tap lacks is overload protection. If something downstream draws 150 amps through those #3s, the 400-amp breaker is perfectly content. It sees a third of its rating. The copper heats. Nobody trips.

So the Code did not remove protection. It removed one kind of protection, and then spent the rest of 240.21 building fences around the risk that creates.

The fences: length, ratio, and a single point of termination

The two rules you'll meet most often on a feeder are the 10-foot tap and the 25-foot tap. Read them not as arbitrary numbers but as three levers the Code pulls to make an unprotected conductor safe enough.

Lever one: keep the exposure short. Ten feet. Twenty-five feet. The logic is blunt and honest — the shorter the unprotected run, the less conductor there is to damage, and the less of it lives in the part of the building where a forklift or a falling pipe can find it. Beyond the enclosure, a 10-foot tap has to be in a raceway. A 25-foot tap has to be protected from physical damage — raceway, cable tray, an approved means.

Lever two: don't let the tap be pathetically small relative to what's ahead of it. For a 25-foot tap under 240.21(B)(2), the tap conductor's ampacity must be at least one-third the rating of the upstream overcurrent device. For a field-installed 10-foot tap under 240.21(B)(1), it's at least one-tenth. The ratio isn't a magic safety factor; it's a bound on how far the upstream device can be from the tap's real thermal limit. Off a 400-amp feeder, a 25-foot tap needs at least 133 amps of ampacity — 1/0 copper at the 75°C column gives you 150. A 10-foot tap needs at least 40 amps, which #8 clears easily, though the third lever usually forces you bigger.

Lever three — the one people forget — is that the tap must terminate in a single overcurrent device, and that device has to be sized to the tap. This is where the real protection lives. The tap conductor is unprotected at its source, so the Code puts the protection at its end. Under the 10-foot rule, the tap's ampacity must be no less than the rating of the device it supplies. Under the 25-foot rule, it must terminate in a single circuit breaker or set of fuses rated no more than the tap's ampacity.

Watch how that third lever quietly overrides the second. Take that 400-amp feeder and tap it 10 feet over to a 100-amp panel. The one-tenth ratio only demands 40 amps of ampacity. But the tap has to be at least as large as the 100-amp device it lands on. #4 copper at 75°C is 85 amps — not enough. You need #3, at 100 amps. The ratio was never the binding constraint. The termination was.

What the Code is actually buying

Think about what an unprotected tap really is: a short, deliberately-sized stub between one protected system and another. Protection at the source is impossible without absurdity — you cannot put a 100-amp breaker on the load side of a 400-amp switchboard's lugs without, well, building a whole switchboard's worth of breakers. Sometimes there is no room. Sometimes the tap feeds a transformer, or a motor controller, or a disconnect twelve feet away, and the equipment that would provide source-side protection is the thing at the far end of the tap.

So the Code trades. It says: I will allow a conductor to be thermally undefended at its source, in exchange for a run so short and so bounded that the failure mode I fear — a slow, sustained overload — has nowhere to hide. Everything downstream of the tap's terminating device is protected normally. Everything upstream is protected normally. The naked stretch is measured in feet, sits in a raceway, and ends in a device that can shut it down.

The same architecture repeats elsewhere. Transformer secondary conductors under 240.21(C) live by the same trade, complicated by the fact that the primary breaker sees the secondary current only through the turns ratio and cannot protect the secondary conductors at all in most configurations. Outdoor taps under 240.21(B)(5) can run unlimited length, because the exposure is outside the building and the termination point becomes the first place the conductor comes indoors. Different numbers, one idea.

And 240.21 opens with a line that should be tattooed somewhere: you cannot tap a tap. No stacking exceptions. The Code allowed one unprotected link in the chain. It will not allow two.

Your next moves

  • Open the last panel you worked in and find the tap. Look at the switchboard or wireway lugs. Anything leaving a large overcurrent device with conductors smaller than that device is a tap conductor. Measure it. Trace it to its terminating device. See whether it lands on one device or several.
  • Check your terminations against your ratios in that order. Compute the 1/10 or 1/3 minimum ampacity, then compute the ampacity required by the device the tap lands on. Size to the larger. On short taps to modestly-sized panels, the termination almost always wins — sizing to the ratio alone is the classic failure.
  • Sanity-check the 75°C column, not the 90°C column. Tap conductors get sized by ampacity, and 110.14(C) still governs terminations. That #3 is 100 amps because of the 75°C column, not 115 amps because it's THHN.
  • Walk the physical route of any tap longer than a few feet. The 25-foot rule is contingent on physical protection. A tap conductor running loose through an open joist bay is not a legal tap; it's a violation with paperwork.
  • Verify no one tapped your tap. Where a tap feeds a wireway or gutter that then feeds two panels, ask hard whether that's a single point of termination or two taps in series. Inspectors miss it. It fails anyway.

When you're standing in front of a live switchboard with a tape measure in one hand, this is exactly the arithmetic you don't want to do on the back of a wire label — one-tenth of the upstream device, one-third if it's twenty-five feet, then check it against the panel breaker, then check it against the 75°C column, then check whether derating for the raceway just moved the goalposts. Voltly runs those numbers offline, in the field, where the cell signal isn't: ampacity by column and temperature, derating for bundling and heat, conduit fill on the raceway the tap has to live in, and the voltage drop over the run once you've picked a size. It's the reference you'd otherwise be carrying in a drawer, minus the drawer.

If you'd rather size the tap once than explain it twice, Voltly is waiting on your phone, no service required.