SPD grounding at a pole-mounted enclosure fed from a building

Short answer: land the SPD on the equipment grounding conductor that runs with the 120 V supply, and bond that conductor to the pole's concrete-encased electrode at the pole. That gives you one ground point instead of two. NEC 250.54 calls the pole electrode an auxiliary electrode: you may connect it to the EGC, and it never replaces the EGC. Size the conductor from the breaker ahead of the feed, per NEC Table 250.122. Hold the SPD's ground lead under about 12 in [300 mm], 6 in [150 mm] where the layout allows, because a mid-size strike adds roughly 1,000 V for every foot of lead. The sections below give you the wiring order at the pole, the checks that prove the bond, and the two mistakes that kill the camera.

Which ground does the SPD land on?

The SPD lands on the EGC that runs with the supply. Bond it at the pole to the pole's grounding electrode, so the SPD, the enclosure and the camera share one ground point instead of two that can sit kilovolts apart during a strike.

Splitting the two grounds feels safe and is the expensive option. The ufer-only instinct wants the surge to run to earth and stay away from the building. The EGC-only instinct leaves the pole steel floating beside a bonded box. Surge current through soil lifts the electrodes unevenly, so at a 5 kA/µs slope the two nodes differ by roughly 1,000 V per foot of surge path. The camera bridges them, with power referenced to one node and data referenced to the other, and the surge crosses through its ports.

Bonding the EGC to the electrode at the pole collapses the assembly — pole steel, enclosure, ground bus, SPD, camera — into one node. The building keeps its existing neutral-to-ground bond at the service. You carry hot, neutral and EGC out to the pole, and you add no new bond there. The fundamentals behind that bond are in the grounding and bonding guide, and the sizing rules behind the box itself are in the enclosure sizing guide.

Can the pole's earth ground do the job alone?

It can't, for faults. Through the 25 Ω a single driven rod commonly measures to earth, a line-to-case fault draws 120 V ÷ 25 Ω = 4.8 A. A 20 A breaker holds that indefinitely, and NEC 250.4(A)(5) rules the earth out as an effective ground-fault current path.

The copper EGC returns fault current to the source; the electrode only holds the pole's potential steady against lightning and earth. Even a strong ufer, often well below 25 Ω, changes the fault math less than you would hope. At 5 Ω the same fault still draws just 120 ÷ 5 = 24 A, short of the several-times-rating current a thermal-magnetic breaker needs to trip promptly.

The surge case fails the same way. Electrode impedance at kA/µs sits in the tens of ohms, and two electrodes in the same soil rise unequally. An SPD grounded to earth alone clamps against a moving reference. Ground it to the bonded node and the clamping voltage on its label means what it says.

What size equipment grounding conductor should you run?

Size the EGC from the breaker ahead of the feed, per NEC Table 250.122: 14 AWG copper behind 15 A, 12 AWG behind 20 A, 10 AWG behind 60 A.

A camera-and-fiber feed off a lighting panel sits on the 15 A or 20 A row, so you pull that size of insulated copper EGC with the hot and the neutral. Run it green or green with a yellow stripe, as NEC 250.119 requires for insulated EGCs 6 AWG and smaller. Terminate it on the box's ground bus first and bond the enclosure to that bus. NEC 250.96(A) requires metal parts serving as equipment grounding conductors, enclosures included, to be bonded so they can carry the fault current, and NEC 250.12 asks that coatings come off the threaded contact surfaces. A bare stainless box arrives ready to bond.

The neutral takes its own terminal with no connection to ground at the pole. NEC 250.6 exists to keep neutral current off metal parts, and pole steel is exactly that metal. The only neutral-to-ground bond in this system stays at the building service.

How short do the SPD leads need to be?

Keep the SPD's ground lead at or under 12 in [300 mm], and at 6 in [150 mm] where the layout lets you. At the 5 kA/µs rise of a 10 kA 8/20 µs strike, every foot of straight lead adds about 1,000 V on top of the device's clamping voltage.

Worked example — 10 kA through 5 ft of ground lead. Straight 12 AWG wire carries about 0.2 µH of inductance per foot, the standard figure for SPD lead checks, and a 10 kA 8/20 µs wave — the current shape behind SPD discharge-current ratings, per UL 1449 — rises at about 5 kA/µs at its steepest point.

• Inductance: L = 0.2 µH/ft × 5 ft = 1.0 µH
• Added voltage: V = L × di/dt = 1.0 µH × 5,000 A/µs = 5,000 V
• The same lead cut to 6 in: L = 0.1 µH, and V = 0.1 µH × 5,000 A/µs = 500 V

A typical 120 V SPD lists a voltage protection rating near 400 V on its label. So the 5 ft lead delivers about 5,400 V to the camera terminals during that strike, and the 6 in lead delivers about 900 V. A 20 kA strike doubles the slope and doubles both numbers. The instruction sheets for external-mount SPDs state the same thing in words: mount the device as close as possible to the enclosure and to the wiring connection point. Route the SPD's line, neutral and ground leads a couple of inches clear of the protected wiring leaving the box, so the pulse couples into nothing downstream.

What do you need at the pole?

You need the feed with its EGC, a listed bond to the ufer conductor, a DIN-rail SPD with a ground lead under a foot, and a gasketed stainless box entered through the bottom wall with IP66-rated glands.

• The feed: hot, neutral and EGC on a 15 A or 20 A breaker, conductor size per the table above
• A listed bonding lug or an exothermic weld for the ufer conductor, landing on the same EGC
• A ground bus or bonding stud inside the box, with star washers under the bonding hardware
• A DIN-rail SPD for the supply voltage with line-neutral and line-ground modes, plus the dedicated breaker or fuse its instruction sheet calls for ahead of it
• A data SPD at each end of any copper Ethernet run; the fiber span itself carries no surge between buildings and needs none
• The enclosure: a 16 x 12 x 6 in stainless box gives you 15.88 x 11.88 x 5.88 in inside — enough for the breaker, the SPD and a terminal strip with room for the short lead bends above. We build it empty or cut to your layout and deliver in 10 business days, rated NEMA 4X / IP66 by the manufacturer; not UL listed. In most outdoor air the 304 wall-mount box holds; on salt air or road-spray routes order the same size in 316L
• Bottom-wall entry with glands sized to your cable outside diameters, a drip loop outside, and a spare few inches of conductor at every terminal

For the weather side of the spec — orientation, sun, hose-down — the outdoor enclosure guide fills in what this article leaves out, and the mounting heights and clearances live in the enclosure mounting guide.

What are the steps, in order?

Lock the building breaker off, verify absence of voltage at the pole end, and work the eight steps with the box dead throughout; power comes back only after the checks in the next section.

1. Lock the feed off at the building panel and verify absence of voltage at the pole end before you strip a conductor.
2. Land the EGC on the box's ground bus and bond the enclosure wall to the same bus, hardware tight on bare stainless.
3. Bond the ufer conductor to that bus through the listed lug or exothermic weld, so EGC, electrode, enclosure and bus form one node.
4. Mount the SPD closest to the power-entry glands and connect its ground terminal first, with the lead at or under a foot to the bond point.
5. Wire line and neutral to the SPD across its line-neutral and line-ground modes, with the dedicated breaker or fuse its sheet calls for ahead of it.
6. Feed the DIN breaker and the terminal strip from the SPD's protected side, and land the neutral on its own terminal; the neutral-to-ground bond stays at the building service, nowhere else.
7. Ground the camera's power terminals and any data SPD to the same bus, with power wiring at one end of the panel and low-voltage wiring at the other.
8. Fit the IP66-rated glands in the bottom wall, set the drip loop outside, and close the door on a clean gasket; the box holds its NEMA 4X / IP66 rating only while every opening is a rated gland or a sealed plug.

Gland selection, hole finish and the entry hardware are covered in the cutouts, glands and windows guide.

What checks prove the bond when you're done?

Meter from the box's ground bus back toward the building. A 100 ft run of that EGC reads about 0.16 Ω, because copper runs 1.6 Ω per 1,000 ft at 20 C, and the neutral-to-ground reading at the pole must stay open.

• Ground bus to the building panel's ground: about 0.16 Ω for the 100 ft run; an open reading means a missed termination somewhere in the line
• Ground bus to the enclosure wall: a firm near-zero reading at the bonding point
• Ground bus to the ufer conductor: near zero — this is the reading that proves the pole is one node
• Neutral to ground bus at the pole: open. A continuity reading here means a bond got added that belongs at the service only
• SPD status indicators healthy once the feed is restored, per the device sheet
• Lead lengths recorded: a photo of the SPD ground lead and its bond point keeps the next tech under the 12 in ceiling

What mistakes cost the rating or the camera?

Two failures reach you months later: two separate grounds and one long ground lead. Both cost roughly 1,000 V per foot to create at a 5 kA/µs strike.

• Two grounds: an SPD on the ufer alone and equipment on the building EGC leaves the camera bridging two nodes about 1,000 V per foot apart at that slope. Bond the EGC to the electrode first; the SPD lands after
• One long ground lead: the worked example's 5 ft lead turns a 400 V clamp into a 5,400 V event at the terminals. Re-mount the SPD until the lead sits at or under a foot
• A drain hole or open vent punched into the pole box: any opening rated below the box drags the whole assembly down to that opening's rating, so a Type 4X / IP66 build stays sealed with rated glands or plugs
• A neutral-to-ground bond at the pole: it puts load current on the pole steel, the failure NEC 250.6 exists to prevent
• A copper Ethernet span with protection at one end only: the far end's equipment becomes the return path, so protect both ends and ground each to its local bus

One ground point, one short lead, one sealed box — that is the whole job. When the layout outgrows a catalog size, our custom enclosures take your dimensions and cutouts from one piece.

Enclosures in this article

Questions