Can you use a disconnect switch as a feed through or junction box?

Yes, the NEC allows it, with fill limits. Section 312.8(A) of NEC 2023 permits enclosures for switches and overcurrent devices, which includes safety switches and disconnects, to be used for conductors feeding through and for splices and taps, as long as three conditions are met.

The three conditions of 312.8(A)

First, the total cross sectional area of all conductors at any cross section cannot exceed 40 percent of the wiring space at that point. Second, where splices or taps are present, the total of conductors plus splices and taps cannot exceed 75 percent of the wiring space at that cross section. Third, a warning label identifying the closest disconnecting means for any feed through conductors is required per 312.8(A)(3), so the next person knows the disconnect handle does not kill everything in the can.

Doing the math in the field

Measure the gutter space beside or below the switch mechanism, height times depth, and compare to the sum of conductor areas from Chapter 9 Table 5. In practice, a 60 amp fused disconnect has generous side gutters and passing one extra circuit through rarely gets close to 40 percent. Where it goes wrong is using a small 30 amp switch can as the junction point for three or four circuits with wirenut splices stacked in the gutter; that blows past 75 percent fast.

When not to do it

Service equipment deserves extra care: conductors from another system passing through service equipment can confuse the next tech even with the label. Also check the enclosure listing; some small AC pull out disconnects have essentially no gutter space and the instructions do not contemplate feed through wiring, which brings 110.3(B) into play. If the run needs a real junction point with multiple splices, a proper 4x4 or larger box next to the disconnect is cheaper than a violation.