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We would like to add a second air-conditioning unit. The original 60 Amp disconnect is grounded via EMT to the panel. The existing EMT will meet code requirements for both conduit fill and derating requirements (this was asked in a separate question). I would like to use the existing EMT to run additional wiring through the existing disconnect to a new disconnect for the new unit. Is this code compliant? Thank you. Elmo

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    So there will be no load on the first disconnect , basically it is now a junction box I do similar regularly but if fused you don't want 2 sets of fuses or for that matter you may not want 2 disconnects bypass fuses and switch with listed splices and it would be legal. – Ed Beal Oct 6 '17 at 3:19
  • @EdBeal -- I think the existing disconnect is staying in service for the existing A/C unit – ThreePhaseEel Oct 6 '17 at 3:58
  • As long as the unit is "in site" of the disconnect it could be still used, I got the impression they were going to install a new disconnect, and wanted to pass the wires through. – Ed Beal Oct 6 '17 at 13:00
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This should be fine, but triple-check the fill if you want to be sure, and make sure it's labeled properly too!

Your usecase (extra wires run through a disconnect box to another disconnect) falls under NEC 312.8:

312.8 Switch and Overcurrent Device Enclosures with Splices, Taps, and Feed-Through Conductors. The wiring space of enclosures for switches or overcurrent devices shall be permitted for conductors feeding through, spliced, or tapping off to other enclosures, switches, or overcurrent devices where all of the following conditions are met:

(1) The total of all conductors installed at any cross section of the wiring space does not exceed 40 percent of the cross-sectional area of that space.

(2) The total area of all conductors, splices, and taps installed at any cross section of the wiring space does not exceed 75 percent of the cross-sectional area of that space.

(3) A warning label complying with 110.21(B) is applied to the enclosure that identifies the closest disconnecting means for any feed-through conductors.

The 40% and 75% fill rules are trivial in this case -- the gutters on even the smallest disconnects are larger than 3/4" EMT in order to provide the Code-required wire-bending space, so you'll be nowhere close to either one. Since these are true feed-through conductors, though, the existing disconnect will need a a label applied to it in accordance with 110.21(B), though:

(B) Field-Applied Hazard Markings. Where caution, warning, or danger signs or labels are required by this Code, the labels shall meet the following requirements:

1) The marking shall adequately warn of the hazard using effective words and/or colors and/or symbols.

Informational Note: ANSI Z535.4-2011, Product Safety Signs and Labels, provides guidelines for suitable font sizes, words, colors, symbols, and location requirements for labels.

(2) The label shall be permanently affixed to the equipment or wiring method and shall not be hand written.

Exception to (2): Portions of labels or markings that are variable, or that could be subject to changes, shall be permitted to be hand written and shall be legible.

(3) The label shall be of sufficient durability to withstand the environment involved.

Informational Note: ANSI Z535.4-2011, Product Safety Signs and Labels, provides guidelines for the design and durability of safety signs and labels for application to electrical equipment.

Given all that, I would simply make up a printed label that says "Caution: Feed-through conductors are disconnected by breaker XYZ in the panel" where XYZ is the label given to the new breaker in the loadcenter's directory, and apply it to the inside of the existing disconnect's cover where it's sheltered from UV, water, etal.

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