Tap rule for feeding an enclosure AC unit

Tap the load side of the fused door disconnect, keep the tap conductors inside 10 ft [3 m], and the same through-door handle becomes the disconnecting means for the enclosure AC unit. The rest is sizing. The tap rule asks three things of the new conductors: length, ampacity and a protected end point. The AC unit's nameplate settles the ampacity and the device through its MCA and MOCP. The drive keeps the input protection its manual already calls for, so the tap adds nothing on that side. The worked example below runs a 480 V, three-phase panel with a 25 hp drive and an AC unit marked MCA 7.5 A, MOCP 15 A.

Which tap rule applies to feeding an enclosure AC unit?

The NEC's 10 ft tap rule applies. The tap conductors stay within that limit of the disconnect, carry at least the AC unit's nameplate MCA and one-tenth of the upstream fuse rating, and end on overcurrent protection no larger than their ampacity. The rule lets you feed a new load from conductors that are already protected, without adding a breaker at the point you tap. All three conditions hold at once; miss one and the tap needs protection right where it starts. Measure the length along the conductor, from the lug on the load-side distribution block to the terminal of the device at the unit.

The one-tenth floor comes from the NEC's tap provisions for industrial establishments, where trained staff maintain the system. A 60 A upstream fuse sets a 6 A floor, and any cabinet AC unit clears it. The rule buys you the cheapest possible addition: no new protection at the tap point, only the device the unit needed anyway.

Does the AC unit need its own disconnecting means?

No separate disconnect is needed when the tap comes from the load side of the panel's fused disconnect. The door handle then removes power from the AC unit along with the drive; tap the line side and the unit needs its own disconnect at once. The NEC calls for a disconnecting means for air-conditioning equipment, and the door handle is it here, because everything downstream of the fused switch dies together. Padlock the handle in the off position while anyone works on the unit's terminals. The through-door mechanism and its door interlock get their own treatment in the panel disconnect and door interlock rules for a retrofit.

If the unit must run from a separate source, feed it through an auxiliary contact on the main disconnect when the contact is rated for the load, or give the unit its own switch. The line-side option, and why those lugs stay live with the handle off, is the subject of the separate disconnect enclosure: why the line side stays live article.

What size conductors feed the AC unit from the tap?

Size the tap conductors to at least the AC unit's nameplate MCA, taking ampacity from the NEC's conductor table for copper at 75 C: 14 AWG carries 20 A, 12 AWG 25 A, 10 AWG 35 A, 8 AWG 50 A, 6 AWG 65 A and 4 AWG 85 A. The nameplate wins when it prints a size outright. The MCA the maker prints already carries the 125 percent factor that the NEC's air-conditioning rules apply to the largest motor load, so you take it as printed and add nothing. Pick the smallest conductor whose 75 C ampacity clears the MCA.

Most cabinet AC units land on 14 or 12 AWG; the 8 AWG and larger rows matter when a big unit shares the panel. A nameplate that prints a size outright settles the question: some unit nameplates state 10 AWG copper and a 50 A OCPD in a single line, and then that pair is what you install. NEC 312.6 sets the minimum wire bending space at terminals in cabinets and cutout boxes, with one table for conductors that enter or leave through the wall opposite their terminal and another for the rest; the how to size an electrical enclosure guide gathers those tables.

What overcurrent protection does the tap need at the AC unit?

Land the tap on a fused switch or breaker rated between the nameplate MCA and the nameplate MOCP, never above the MOCP: a unit marked MCA 7.5 A and MOCP 15 A takes a device at that ceiling, and that same device protects the tap conductors. The MCA sizes the conductors; the MOCP is the largest fuse or breaker the maker allows on the unit's branch. The device lands anywhere between the two numbers, and its rating stays at or under the tap conductor ampacity.

Use the device type the nameplate allows: most cabinet units accept a fuse or an HACR-rated breaker, because compressor start pulls locked-rotor current, several times the running value, for a fraction of a second. Larger air-conditioning equipment runs on the same MCA-to-MOCP logic, which the rooftop AC feeder sizing starts at the nameplate MCA and MOCP page works through for feeders.

Do you need to add fusing between the distribution blocks and the VFD?

No, the tap changes nothing on the drive side. The VFD keeps the input protection its manual calls for, and you add fusing between the distribution blocks and the drive only when that manual names it and the ampacity it prints. The tap's only new conductors feed the AC unit; the drive branch keeps its existing protection, whether that is the disconnect fuses or the pair the drive manual names. Drive manuals print input fusing by class and ampacity, often fast semiconductor classes, and those fuses stay in place. Never assume the disconnect fuses cover the drive when the manual names its own smaller pair.

The case where one set of supply fuses serves several protected devices gets a fuller treatment in the fusing a power supply and motor drivers inside an enclosure article.

How do you wire the tap step by step?

Run the tap from the load-side distribution blocks to a fused switch beside the AC unit, keep every conductor inside the 10 ft [3 m] limit, and work through the steps below after you lock the handle off and verify absence of voltage at the load lugs.

1. Read the AC unit nameplate and write down four values: voltage and phase, MCA, MOCP.
2. Land three tap conductors and an equipment grounding conductor on the load side of the distribution blocks; size the grounding conductor from the tap device's rating with NEC Table 250.122.
3. Route the conductors to the AC unit's fused switch or breaker, holding the run within 10 ft [3 m] and leaving the wire bending space NEC 312.6 sets at terminals.
4. Set the tap device between the MCA and the MOCP, which on the example unit is the nameplate ceiling.
5. Land the conductors at the unit's terminal box and torque every lug to the value its maker prints.
6. Bond the AC unit chassis to the ground bus and clean paint or gasket residue off any bonding contact surface, as NEC 250.12 requires at grounding connections.
7. Walk the drive branch and confirm it is untouched: disconnect fuses unchanged, any drive-manual fuses in place.
8. Close up, remove the lock and run the unit through a full compressor cycle before you call the panel done.

What does the arithmetic look like on a 60 A drive panel?

A 480 V, three-phase drive panel with a door-mounted fused disconnect carrying 60 A class J fuses takes an AC unit with MCA 7.5 A and a nameplate MOCP as the device ceiling. The tap floor lands at 6 A, the MCA governs, and 14 AWG copper finishes the job. The arithmetic is four lines long, so swap in your own nameplate and repeat it:

• Tap floor: one-tenth of the 60 A fuses = 6 A.
• Load: the MCA of 7.5 A beats the 6 A floor, so the load sets the minimum ampacity.
• Conductor: 14 AWG copper carries 20 A at 75 C (the NEC ampacity table); 20 A clears 7.5 A and clears the 6 A floor.
• Device: the MOCP sets the ceiling; a device at that ceiling sits at the MCA-to-MOCP top, stays under the conductor's 20 A, and matches the small-conductor ceiling that pairs 14 AWG with a 15 A device.
• Length: 6 ft [1.8 m] of routed conductor against the 10 ft [3 m] limit.

The same four lines work for a 100 A panel: the floor rises to 10 A, and a unit with an MCA under 10 A takes conductors sized to that floor instead.

What do you check before you close the door?

Before you close the door, check the four numbers the tap lives on: length against 10 ft [3 m], conductor ampacity against the MCA, the device against the MOCP, and running current against the nameplate. The checks that prove each one:

• Clamp each phase at full compressor load: running current sits under the nameplate RLA and the three legs read within a few percent of each other.
• Confirm the device marking against the nameplate: the MOCP ceiling on the example, in the fuse or breaker type the unit's manual allows.
• Look at the wall penetration: the AC unit's gasketed flange carries the seal, so the opening follows the unit's template, the edges are deburred and the flange screws get a re-torque after the first heat cycle; the rating consequences of cutting are covered in does drilling an enclosure affect its rating.
• Set the mounting the unit's manual calls for; side and roof mounting trade airflow and condensate handling, as the cabinet air conditioner mounting: side vs roof article lays out.

One more check protects the box itself. Our enclosures come rated NEMA 4X / IP66 by the manufacturer; not UL listed, and the gasketed flange is what keeps that rating where the unit sits in the wall. When the panel is a custom build, draw the opening before anything is cut: the enclosure builder puts climate options and cutouts on the drawing you approve, so the unit lands on a ready, sealed opening.

Catalog dimensions to compare

Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.

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