Connecting adjacent enclosures in one Type Z purge system

Short answer: run rigid metal conduit between the boxes and keep liquidtight to short whips, with your own equipment grounding conductor pulled through and landed at both ends — 10 AWG copper behind a 60 A breaker. Total the volumes first: NFPA 496 calls for four volume exchanges before anyone energizes. That is about 40 minutes for a 25 ft³ chain at the example unit's rated 2.5 cfm. You then hold 0.1 in. of water (25 Pa) at the last box, where the gauge and the low-pressure switch belong. Liquidtight is an accepted wiring method in Class I locations. What kills a shared purge chain is a leak at a fitting, a missing bond or a throttled vent, and the build below heads off each one.

What you need

• Rigid metal conduit and threaded couplings for the spans between boxes — 1 in. trade size in this example.
• Liquidtight flexible conduit in short whips, about 1 ft (300 mm) at each box, with fittings listed for Class I use. Flexibility is the only reason to fit it.
• Your own equipment grounding conductor (EGC), sized from the breaker ahead per NEC Table 250.122, plus bonding lugs at both enclosures.
• A gauge and a low-pressure switch reading in inches of water, mounted at the last box in the chain.
• A purge timer you set from your own volume math.
• Dry instrument air for the supply, and soapy water for the joint test.

Still choosing the boxes themselves? The enclosure selector sorts candidates by environment, listing and inside size.

Add the volumes, then size the flow

The connecting raceway sits inside the pressurized envelope, so its volume joins the purge math. It adds less than you expect:

These are standard rigid metal conduit dimensions; liquidtight bores differ slightly by maker, and either way the numbers stay small.

Worked example. Assume three boxes with 25.0 ft³ of free interior between them, a purge unit rated at 2.5 cfm, and 7 ft of connection: 5 ft of 1 in. rigid plus a 1 ft whip at each end.

• Raceway volume: 0.864 in² × 84 in = 72.6 in³ = 0.042 ft³. The chain totals 25.04 ft³, so the raceway adds 0.2%.
• Purge time: 4 × 25.04 ft³ = 100.2 ft³; at 2.5 cfm that is 40.1 minutes. Set the timer above 40 minutes.
• Flow speed: 2.5 cfm is 0.0417 ft³/s; through 0.0060 ft² of 1 in. bore that is 6.9 ft/s. The same flow pushed through a nominal 1/2 in. tube (0.00136 ft²) runs at 31 ft/s.
• Connection loss at 68 °F (20 °C): each velocity head is 2.7 Pa at 6.9 ft/s. Entry, exit and two bends cost about 5.6 Pa; wall friction over 7 ft adds 4-9 Pa, because a Reynolds number near 3,800 sits in the transition range. Budget the whole run at 14 Pa.

The supply box therefore runs near 40 Pa (25 + 14) while the last box sees the 25 Pa minimum. That is why the switch lives at the far end. Losses climb with speed squared, so a 1/2 in. supply line at 31 ft/s burns roughly 400 Pa over the same 7 ft — sixteen times the set point. Counting exchanges is the same math as a nitrogen tank purge sized by volume exchanges: count exchanges, not minutes.

Make the connection

1. Lock the chain out and prove it dead before you open or drill a box.
2. Count the free volume in every box, less back plate and gear, and add the raceway from the table. Set the purge timer at four exchanges — over 40 minutes in the example.
3. Thread and make up the rigid span with joint compound rated for the service. Tight joints are the seal; the chain runs on tens of pascals and has none to spare.
4. Fit the liquidtight whips at each box, about 1 ft (300 mm), with fittings listed for Class I use. Whips handle alignment, not distance.
5. Pull the EGC the full length — 10 AWG copper behind a 60 A breaker, per NEC Table 250.122 — and land it on lugs at both enclosures. Add an external bonding jumper across each flexible metal whip. A fault can draw an arc across the convolutions of a flexible raceway; the built-in strip is not the conductor to trust with that job. Sub-panel bonding jumper sizing covers the sizing logic between enclosures, and the grounding and bonding guide covers EGC routing inside boxes.
6. Bond the shells themselves. NEC 250.96(A) calls for the metal parts acting as grounding conductors to be bonded so they can carry any fault likely to be imposed on them. On coated boxes, NEC 250.12 wants paint cleared from the contact surfaces; stainless lugs bite bare.
7. Fit conduit seals where a raceway leaves the pressurized envelope toward unclassified space — within 18 in. (450 mm) of the boundary, or closer when the fitting's instructions call for it. Between two pressurized boxes the raceway carries protective gas at both ends, so made-up joints and bonding handle it instead.
8. Vent the last box only, through an opening at least as large as the supply line, discharging where the exhaust cannot pool. Keep the chain to that single opening: a drain, a louver or a spare gland leaks the maintained pressure away.
9. Feed dry instrument air. Wet air drops condensate into every low point of the run.
10. Set the stages: purge flow until four volumes have passed, then maintained flow holding 0.1 in. of water at the far gauge, with the low-pressure switch there. A Type Z system raises an alarm on pressure loss and keeps the loads running; shutdown on loss belongs to Type X. The Type X, Y and Z breakdown sorts the three schemes.

Prove the chain before you energize

• Soap every joint at full purge flow. Bubbles mean remake the joint.
• Read the far gauge at the maintained stage: 0.1 in. of water (25 Pa) or better, with the supply box near 40 Pa in the example.
• Time the purge against your calc: four exchanges of 25.04 ft³ at 2.5 cfm is 40.1 minutes, so a shorter timer setting means a recount.
• Measure the EGC end to end. Ten feet of 10 AWG reads about 0.01 Ω; a lug reading whole ohms is not made up.
• Dump the pressure and watch the alarm trip at 0.1 in. of water. When that alarm trips in service weeks later, the low-pressure causes article lists what to look at first.
• Watch the doors during the purge stage. A reading in inches of water (250 Pa and up) means a restriction: a gasketed enclosure is not a pressure vessel, and the fix is flow area, not a stiffer door.

Where these connections go wrong

• Trusting the built-in strip in liquidtight as the fault path. The sized EGC plus a jumper across each whip is the arrangement that holds; an arc across flexible convolutions can ignite a flammable atmosphere.
• Throttling the path. A 1/2 in. supply tube, a clogged vent filter or a pinched whip turns 6.9 ft/s into 31 ft/s and burns hundreds of pascals; the first box then flexes at inches of water while the set point needs 25 Pa.
• Wet air and low pockets. Condensate pools at every dip in the run; water backing up a conduit into an enclosure shows what a low point does over a season.
• Extra openings. A drain, a louver or an added gland dumps the maintained pressure; the chain has exactly one vent.
• A pass-through that is not part of the envelope. Connecting a wiring trough to a panel covers the same trap in ordinary ratings work: every added joint and volume either joins the protected space or breaks it.

Chain the boxes or pressurize each one

Chain them when the boxes sit close, their volumes are similar, and the connection loss at rated flow stays small against the 25 Pa set point. The example's 1 in. run costs about 14 Pa; stepping up to 1-1/4 in. drops the speed to 4.0 ft/s and the loss to about 3 Pa. Pressurize each box separately when the boxes are far apart, the volumes differ widely, or the run cannot keep its loss down: each box then gets its own supply, vent and switch, and no box depends on the joint behind it.

When the shells need cutouts for the raceway, the gauge and the vent, custom stainless enclosures are built to your drawing and delivered in 10 business days from your approval of the drawing. They are rated NEMA 4X / IP66 by the manufacturer; not UL listed, and we build no explosion-proof or NEMA 7/9 shells. The pressurization system, applied to NFPA 496, is the protection technique the NEC recognizes for Class I locations.

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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