Buyer’s Guide: Mast Arm

1. What the Mast Arm Actually Does in a Streetlight Run

A Mast Arm is the horizontal (or slightly upward-curving) steel or aluminum arm that extends out from the side of a wood, concrete, or steel pole to position a roadway luminaire over the lane it's lighting. Without it, the luminaire would sit directly against the pole face — useless for roadway illumination because the light cone wouldn't reach the lane center. The arm length determines the luminaire-to-pole-face offset: a 6 ft arm puts the light over a single residential lane; 8 ft handles a parking strip + one lane; 10 ft reaches the centerline of a two-lane arterial. The arm also acts as a conduit raceway — the luminaire's feeder cable runs from the pole's handhole, through the arm, and out the tenon end where it connects to the luminaire's ballast or LED driver. Pole-line hardware catalogs treat the mast arm as a specialty item separate from cross arms (which carry conductors) and brackets (which mount transformers).

2. Arm Length Selection by Roadway Geometry

Arm length is set by three factors: the pole offset from the curb (typically 2–3 ft), the lane(s) the luminaire must illuminate, and the IES (Illuminating Engineering Society) light distribution pattern of your chosen luminaire. As a rule: residential streets with 24-ft pavement width use 6 ft arms; collector roads with 36-ft pavement use 8 ft arms; 2-lane arterial with parking, 48-ft total width uses 10 ft arms; 4-lane divided arterial uses 12 ft arms on each side OR mid-median 6 ft arms on every other pole. Over-spec'ing arm length wastes steel and increases wind moment on the pole; under-spec'ing leaves dark edges in the roadway photometric pattern. Cross-reference IES RP-8 lighting design tables with your chosen luminaire's IES file before locking in arm length.

3. Steel Sch 40 vs Aluminum 6063-T6 — When Each Wins

Carbon steel Schedule 40 (hot-dip galvanized) is the default mast arm material for ~90% of North American utility specs. It's cheap (~$55/arm for 6 ft), strong enough for any standard luminaire weight, and corrosion-resists for 25+ years in C2–C3 atmospheric environments. The downside is weight (a 10 ft steel arm weighs ~14 kg, requires 2-person install) and visible rust at the galv-bath joints after 20 years — aesthetic, not structural. Aluminum 6063-T6 (mill-anodized) wins in three scenarios: (a) coastal / marine sites where steel rusts in 5–7 years no matter the galv thickness; (b) historic / aesthetic districts where matte-silver aluminum blends with restored cast-iron poles; (c) truck-served retrofit where the 60% weight reduction enables 1-person bucket-truck install. Aluminum costs ~2× the steel equivalent and has 30% lower load rating for the same wall thickness — size up one length class if luminaire weight is borderline.

4. Mount Style: Side-Bolt vs Clamp vs Pole-Top Hub

Three mount styles cover all field conditions. Side-bolt with pole plate (RAX-MA-N-SB) is the workhorse: a flat steel plate bolts to the side of a wood or concrete pole, the arm welds to the plate at the design rise angle. Drilling is required — not a fit for utilities that prohibit pole drilling. Clamp mount (RAX-MA-N-CL) wraps a 2-piece banded clamp around the pole and the arm bolts to the clamp — no drilling, so used on steel and composite poles where drilling voids warranty, also for fast retrofit. Pole-top hub mount places the arm(s) on a hub that slips over the top of the pole; used for streetlight-only poles (no power conductors above the arm) and for dual-arm configurations (RAX-MA-180-2 puts two arms at 180° on a single hub). Choose by pole material first, then by drilling policy.

5. The 2-3/8″ × 4″ Tenon Standard (ANSI C136.13)

Every mast arm in this catalog ends in a tenon: a smaller-diameter pipe stub onto which the luminaire's slip-fitter clamps and tightens. The dimension 2-3/8″ outside diameter × 4″ long isn't arbitrary — it's the ANSI C136.13 standard for North American outdoor luminaire slip-fitters and is universal across Acuity, Cooper, GE, Hubbell, Philips, RAB, and Cree roadway fixtures. The 4″ length gives the luminaire's set-screws (typically 2 screws spaced 1-1/2″ apart) full purchase on the tenon wall; shorter tenons risk fixture rotation under wind. Don't modify the tenon dimensions — cutting it shorter to fit a tight cable bend will void the luminaire warranty AND fail the wind-load certification. Custom luminaires (decorative acorn fixtures, ornamental post-tops) sometimes spec a 3″ or 4-1/2″ tenon — we can supply alternative tenon diameters as a custom build with 5-day tooling.

6. Galvanizing, Powder Coat & the LED-Era Vibration Problem

Standard steel mast arms ship hot-dip galvanized per ASTM A123 at ~50 μm zinc coating — sufficient for 25-year service in C2–C3 environments. For aesthetic districts where the silver-galv finish is objectionable, powder coat over HDG (any RAL color from your spec) is available at +$25/arm. The powder coat itself doesn't add corrosion protection — it's purely visual — but it shields the underlying zinc from UV chalking. There's a newer concern unique to the LED era: LED luminaires weigh 40% less than legacy HPS / Metal Halide fixtures, so the arm's natural vibration frequency shifts upward, often into the 15–25 Hz range where wind-induced oscillation excites resonance. Specify dual-bolt pole plates (RAX-MA-10-SB ships with 4 bolts, not 2) on arms over 8 ft to suppress resonance; if your site has chronic wind (mountain passes, coastal bluffs), add a tuned-mass damper at the arm midpoint — we can quote.