Lamp Post Cross Arm Guide: Types, Dimensions, Tolerances and Spec Tips

Two pallets of 8 m conical poles arrive on site with 76 mm tenons, while the road luminaires in the same container were built with 60 mm slip fitters. Nothing is defective. The order simply treated the cross arm as a spare part instead of an interface. On any street, park or car park lighting project, the lamp post cross arm fixes the mounting height, the setback of the light from the pole, the wind load path into the foundation, and whether the luminaire can be fitted at all.

What a Lamp Post Cross Arm Actually Does

A cross arm, also written crossarm and often called a bracket arm, davit arm or mast arm, is the horizontal member that carries the luminaire away from the pole shaft. On a residential street it may be a 0.6 m straight tube bolted to the shaft. On a trunk road it can be a 2.5 m curved davit welded to a reinforcement collar. It fixes four things at once: mounting height, setback, lamp aim and the transfer of wind and dead load into the pole.

Two joints matter, arm to pole and arm to luminaire. Most warranty disputes on outdoor lighting start at one of them rather than inside the LED module.

• Mounting height. An 8 m pole with a 0.9 m rise puts the light center near 8.9 m, which changes the pole spacing needed for the same road class.
• Setback. Span is measured from the pole face to the luminaire mounting point. Too short and light lands on the kerb, too long and the weld takes a larger moment.
• Load path. Every kilogram at the end of the arm becomes a moment at the pole face and again at the base plate.

Common Cross Arm Types and Where Each One Fits

Nearly every outdoor lighting layout is served by one of five configurations. The choice is a question of road class, pole height and how much load the pole top can carry, not of appearance.

In practice the arm is rarely bought alone. It is ordered with the pole as one assembly, which is why the pole drawing should carry arm span, rise, tenon size and finish in a single table.

Dimensions and Interfaces to Lock Down First

Agree on these seven values before discussing finish or price. They cause more delivery problems than any other part of the specification.

1. Tenon outside diameter. 60 mm (2-3/8 inch) and 76 mm (3 inch) dominate the market, and the slip fitter bore is normally 2 to 3 mm larger.
2. Tenon engagement. 100 to 150 mm of insertion keeps the luminaire steady, while shorter engagement leaves it rocking on the setscrews.
3. Arm span, from pole face to luminaire mounting point. Stock sizes run 0.5, 0.75, 1.0, 1.5, 2.0 and 2.5 m.
4. Rise, the vertical offset between the two ends of the arm. Davit arms usually lift the luminaire 0.5 to 1.0 m.
5. Bolt pattern for direct mounts: hole spacing, hole diameter (often 13 to 14 mm for M12 bolts) and shaft radius at the fixing point.
6. Pole taper. A 1:12 taper is standard on conical poles, so a bracket made for a straight shaft only touches on two lines and will loosen.
7. Wall thickness. 3 mm is a sensible minimum up to 1.5 m of outreach, and 4 or 5 mm beyond that.

Write the tenon size on the purchase order as well as the drawing. Buyers who quote only luminaire wattage are the ones who receive a 76 mm tenon against a 60 mm fitter.

Materials, Galvanizing and Coating Durability

Steel remains the default because it is weldable, stiff and inexpensive. Q235B and Q355B are the grades that appear most often on export orders from China, but in most climates the finish matters more than the grade.

Hot dip galvanizing to ISO 1461 gives an average coating of 70 microns on steel between 3 and 6 mm thick, with a minimum local reading of 55 microns. ASTM A123 calls for an average of 86 microns on members 3.2 mm and thicker. Where color is required, polyester powder goes over the galvanizing at 60 to 100 microns and is normally tested to 500 or 1000 hours of salt spray. Two production details get missed repeatedly: galvanizing adds roughly 70 to 100 microns per surface, so holes and tapped threads need allowance or re-tapping, and slag or pinholes left in a weld become coating defects that show up as rust streaks two winters later.

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Buying pole, arm and luminaire from one factory removes the most common failure mode, because the tenon and the slip fitter can be checked against each other before packing. Yuyao Yangming Lighting, a Ningbo area maker of light poles and outdoor luminaires since 2001, sets out its own production floor, testing and export record on its company background page.

Wind Load, Vibration and the Welds That Carry Them

Cross arm design is mostly a fatigue and connection problem, not a strength problem. At a design wind speed of 40 m/s the dynamic pressure is roughly 980 pascals. A 60 W road luminaire with an effective projected area of 0.1 square meter and a drag coefficient near 1.2 pulls about 118 newtons, or 12 kg of force, at the end of the arm. Add its own weight of around 6 kg, and a 1.5 m arm generates a bending moment at the pole face in the region of 270 Nm.

That moment is modest for a 4 mm tube. The real risk sits in three places.

• Weld toes. Fillet legs of 3 to 5 mm with no undercut and no porosity, inspected to a recognized weld quality class, are what stop cracks after a few thousand gust cycles.
• Bolt holes. Oversized or slotted holes reduce bearing area and let the arm work loose over time.
• Deflection. Many specifications cap arm deflection at span divided by 100, which allows roughly 15 mm on a 1.5 m arm under design wind.

Short, thin arms in gusty or coastal locations are also prone to vortex shedding, the low hum that becomes a looseness complaint. A thicker wall or a small damper costs far less than a return visit.

Matching the Arm to the Luminaire and Its Controls

An arm that is strong and well galvanized can still fail to work with the chosen luminaire. Check three interfaces before ordering.

Mounting interface

Slip fitter luminaires slide onto the tenon and lock with two or four setscrews. Direct mount luminaires bolt to a flat bracket plate. Mixing both systems inside one project creates two spare part lines and two installation routines.

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Control gear access

NEMA and Zhaga sockets usually sit on top of the housing. A davit arm that curves over the housing can block access to the socket or the photocell, so check the drawing with the luminaire fitted.

Ingress protection as an assembly

A luminaire tested to IP66 on a bench is not automatically IP66 once it sits on a tenon. If the setscrew hole is left open, or the cable gland faces upward, water finds its way in. Specify a gasket or sealing washer at the joint and route the cable through the shaft into the base compartment.

Receiving Checks and Tolerances

Cross arms and pole brackets are simple enough to inspect at goods in. A caliper, a straight edge and a coating gauge will catch nearly every problem that matters.

Ask for the paperwork as well: steel mill certificates, a galvanizing thickness report, a salt spray report for powder coated parts and packing photographs. On a first order, a trial assembly of one pole, one arm and one luminaire before shipment is worth more than any document.

Installation Notes That Prevent Callbacks

• Torque anchor bolts to the supplier table, usually 150 to 300 Nm for M16 to M20 studs depending on grade and lubrication.
• Check plumb on two axes before grouting, to a tolerance such as 1 mm per 200 mm of height.
• Use anti-seize compound on galvanized threads, because dry galvanized bolts gall and a seized setscrew is a hard repair in the air.
• Tighten tenon setscrews to roughly 20 Nm for M10 stainless screws, then confirm the luminaire cannot rotate by hand.
• Align double or cluster arms so luminaires sit parallel before final tightening, and touch up any drilling or scratch with zinc rich paint.

Specifying a Complete Pole and Arm Package

Once the layout is fixed, one page is enough. List pole height and taper, arm type, outreach and rise, tenon diameter and length, wall thickness, finish, design wind speed, luminaire model and quantity. Add the bolt pattern if the arm bolts directly to the shaft, and note whether the project needs one color across all poles and arms.

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Keep tenon sizes consistent across a project. If two road classes genuinely need different tenons, order arms and luminaires as matched pairs so the spare parts list stays short. Current pole and luminaire documents are available on the downloads page if you want to compare dimensions before sending a request.

Frequently Asked Questions

Can a 60 mm tenon be adapted to a 76 mm slip fitter?

Adaptor sleeves exist, but each one adds a joint that must be sealed and tightened in the air. On a new project it is cheaper and safer to order the luminaire with the matching fitter, or the pole with the matching tenon.

How long does a galvanized cross arm last?

At 70 microns average, hot dip galvanizing to ISO 1461 typically keeps a steel arm in service for decades in rural and suburban atmospheres. Coastal salt spray and industrial sulphur consume zinc faster, so heavier coatings or powder over galvanizing are worth the extra cost there.

Do post top lanterns need a cross arm?

Only when the lantern must sit away from the shaft, for example on a pole that also carries banners. A direct post top mount is simpler, cheaper and gives a cleaner silhouette in parks and plazas.

The lamp post cross arm looks like a length of tube with a bracket on the end. In reality it is the interface between the pole structure, the luminaire optics and the installer working day. Settle tenon size, outreach, rise, wall thickness and finish at the drawing stage, confirm them with a trial assembly, and the rest of the lighting package will fit together the first time it is unpacked on site.