Cutting and machining
Tubes are saw-cut square, deburred and can be drilled or slotted for fittings and drain holes. Ends can be machined for welded couplers.
Guide · Tubular bus conductors
Electrical Aluminium Tubular Bus Pipe
Tubular bus pipe is an aluminium tube used as a rigid conductor, mainly for outdoor substation bus, generator connections and high-current indoor runs. A tube puts the metal where AC current actually flows and where bending stiffness is greatest, so it spans further between insulators and runs cooler than a flat bar of the same weight. This guide covers alloy choice, standard sizes, electrical and structural data, fabrication and ordering.
- Main grades
- 6101-T6 · 6063-T6 · 1350
- Sizes
- NPS 1 – 6 in and metric OD
- Schedules
- Sch 40 and Sch 80
- Standards
- ASTM B317 · B241 · IEEE 605
Why bus conductors are made as tubes
In an alternating-current conductor, current crowds towards the surface (skin effect). In a thick flat bar or solid rod the centre carries little current, so part of the metal is wasted. A tube with a wall of sensible thickness uses nearly all its cross-section, which keeps AC resistance close to DC resistance.
The same geometry is ideal structurally. For a given weight, a round tube has a far higher moment of inertia than a flat bar or solid rod, and it is equally stiff in every direction. That lets substation designers use longer spans between post insulators, reducing the number of supports, foundations and connectors. The round surface has no sharp edges, which lowers the surface electric field and the risk of corona at high voltage, and the large surface area dissipates heat well.
Tubular bus is therefore the standard rigid conductor for air-insulated substations from medium voltage up to extra-high voltage, and is also used for generator and transformer connections, rectifier and furnace bus, and indoor high-current runs in industrial plants.
Alloys used for aluminium bus pipe
| Alloy and temper | Conductivity (% IACS) | Strength | Where it fits |
|---|---|---|---|
| 6101-T6 / T61 / T63 | About 55 – 59 (55 min. in T6) | Medium | The standard bus-conductor alloy: good conductivity with enough strength for long spans and short-circuit forces |
| 6063-T6 | About 53 | Medium | Widely used substation bus where structural and electrical demands are balanced; easy to extrude and weld |
| 1350 / 1060 / 1070 | About 61 – 62 | Low | Indoor or well-supported high-current runs where conductivity outweighs stiffness |
| 6061-T6 | About 40 – 43 | High | Structural bus and support members where stiffness and strength matter most |
| 3003 | About 40 – 50 (by temper) | Low – medium | Light-duty conductors and earthing applications needing good corrosion resistance |
Most utility specifications call for 6101-T6 or 6063-T6. 6101 gives roughly 3 – 4 percentage points more conductivity than 6063 for a similar strength level; 6063 is often easier to source in large diameters. 1350 conducts best but is too soft for long outdoor spans and is mainly chosen for indoor bus that is supported at short intervals.
Mechanical minimums for 6101 tubes are set by ASTM B317/B317M; for 6063 and 6061 pipe by ASTM B241/B241M or B429/B429M. The mill certificate states the measured values for the lot.
Schedule 40 and Schedule 80 bus pipe: dimensions and calculated data
Bus pipe is usually ordered in American nominal pipe sizes (NPS) with Schedule 40 or Schedule 80 wall thickness to ASME B36.10 dimensions, or in metric outside diameter and wall. The tables convert the standard dimensions to metric and add calculated cross-section, weight, DC resistance in 6101-T6 at its 55 % IACS minimum, and moment of inertia for deflection checks.
| NPS (in) | OD (mm) | Wall (mm) | Area (mm²) | Weight (kg/m) | DC resistance 6101-T6 (µΩ/m) | Moment of inertia (cm⁴) |
|---|---|---|---|---|---|---|
| 1 | 33.4 | 3.38 | 319 | 0.86 | 98.4 | 3.6 |
| 1.25 | 42.2 | 3.56 | 431 | 1.16 | 72.7 | 8.1 |
| 1.5 | 48.3 | 3.68 | 516 | 1.39 | 60.8 | 12.9 |
| 2 | 60.3 | 3.91 | 693 | 1.87 | 45.2 | 27.7 |
| 2.5 | 73.0 | 5.16 | 1099 | 2.97 | 28.5 | 63.7 |
| 3 | 88.9 | 5.49 | 1438 | 3.88 | 21.8 | 125.6 |
| 3.5 | 101.6 | 5.74 | 1729 | 4.67 | 18.1 | 199.3 |
| 4 | 114.3 | 6.02 | 2048 | 5.53 | 15.3 | 301.0 |
| 5 | 141.3 | 6.55 | 2774 | 7.49 | 11.3 | 631.1 |
| 6 | 168.3 | 7.11 | 3601 | 9.72 | 8.7 | 1171.4 |
Schedule 80 walls are thicker, giving more area and stiffness for the same outside diameter, which suits heavier short-circuit duties and longer spans:
| NPS (in) | OD (mm) | Wall (mm) | Area (mm²) | Weight (kg/m) | DC resistance 6101-T6 (µΩ/m) | Moment of inertia (cm⁴) |
|---|---|---|---|---|---|---|
| 1 | 33.4 | 4.55 | 412 | 1.11 | 76.1 | 4.4 |
| 1.25 | 42.2 | 4.85 | 569 | 1.54 | 55.1 | 10.1 |
| 1.5 | 48.3 | 5.08 | 689 | 1.86 | 45.5 | 16.3 |
| 2 | 60.3 | 5.54 | 953 | 2.57 | 32.9 | 36.1 |
| 2.5 | 73.0 | 7.01 | 1454 | 3.93 | 21.6 | 80.1 |
| 3 | 88.9 | 7.62 | 1946 | 5.25 | 16.1 | 162.1 |
| 3.5 | 101.6 | 8.08 | 2373 | 6.41 | 13.2 | 261.4 |
| 4 | 114.3 | 8.56 | 2844 | 7.68 | 11.0 | 400.0 |
| 5 | 141.3 | 9.52 | 3943 | 10.65 | 7.9 | 860.4 |
| 6 | 168.3 | 10.97 | 5423 | 14.64 | 5.8 | 1685.3 |
Calculated from nominal dimensions, density 2.70 g/cm³ and resistivity 0.0313 Ω·mm²/m at 20 °C. For 1350 multiply resistance by 0.90; for 6063-T6 by about 1.04. AC resistance is slightly higher.
Typical supply range
- Outside diameter
- About 10 – 250 mm; NPS ½ – 8 in; larger by special extrusion
- Wall thickness
- About 1 – 40 mm depending on diameter
- Length
- Standard mill lengths of about 6 m; longer lengths for spans by arrangement
- Manufacture
- Seamless (ASTM B241) or extruded structural pipe; seamless is preferred where tubes will be bent
- Finish
- Mill finish; tin- or nickel-plated contact ends; insulating sleeves on request
Design checks: current, spans, vibration and expansion
- Current ratingAmpacity depends on ambient temperature, solar radiation, wind, emissivity and permitted conductor temperature. IEEE 605 gives a method for outdoor tubular bus; manufacturers' tables based on it are a starting point.
- DeflectionGravity sag is checked against a limit, commonly a fraction of the span or of the tube diameter set by the utility. Deflection of a simply supported tube is proportional to the fourth power of span and inversely to the moment of inertia, so a modest diameter increase allows a much longer span.
- Short-circuit forceElectromagnetic forces between phases during a fault load the tube and insulators; tube strength (yield of the chosen temper) and insulator cantilever rating must both be checked.
- Wind and iceOutdoor bus must carry wind pressure and, in cold regions, radial ice; these often govern span in exposed sites.
- Aeolian vibrationLight, steady winds can make long spans vibrate. A common remedy is a length of stranded conductor placed loosely inside the tube as a damper, or purpose-made dampers.
- Thermal expansionAluminium grows about 2.3 mm per 10 m for every 10 °C of temperature change. Use sliding clamps and flexible expansion connectors so movement does not load the insulators.
Cutting, bending, welding and finishing bus pipe
Bending
Bending success depends on ductility, the ratio of wall to diameter and the bending method. Seamless tube in a ductile temper bends more reliably than extruded structural pipe with seam welds from a porthole die.
Welding
MIG and TIG with argon or argon-helium shielding. 4043 filler is common for 6xxx bus; joints are solvent-cleaned and wire-brushed, and parts may be pre-dried at low temperature to remove moisture. Tack welding controls alignment.
Weld strength
Welding locally reduces the strength of T6 tempers, so couplers or sleeves are often used at joints, and joints are placed away from the point of maximum bending moment.
Contact surfaces
Clamp and terminal areas are cleaned, brushed and coated with oxide-inhibiting compound; tin or nickel plating of ends gives a stable contact to copper equipment terminals.
Protection
Anodising, insulating sleeves or heat-shrink are used where tubes must be touched-safe or protected from aggressive atmospheres.
Fittings used with tubular bus
A tubular bus system needs more than tube. These are the parts buyers usually specify alongside the conductor; Metline supplies the aluminium tube, bar and plate from which they are made and can machine aluminium parts to drawing.
- Couplers and sleeves: join tube lengths and restore strength at welded or bolted joints
- Fixed and sliding support clamps: hold the tube on post insulators while allowing thermal movement at one end
- Expansion connectors: flexible laminated or braided links between tube sections and equipment
- T-connectors and angle fittings: branch the bus to disconnectors, breakers and transformers
- Terminal pads: flat palms drilled to NEMA or IEC hole patterns for bolting to equipment terminals
- End caps and corona balls: close tube ends and reduce electric field at high voltage
Applications of aluminium bus pipe
Air-insulated substations
Main and transfer bus, equipment interconnections and jumpers from 11 kV to extra-high voltage.
Power stations
Generator and transformer connections carrying very high currents at short spans.
Industrial plants
Furnace, rectifier and electrolysis bus, plus indoor high-current distribution.
Renewable plants
Collector substations for wind and solar farms, where weight and corrosion resistance help on remote sites.
Railway and traction
Substation bus feeding traction networks.
Earthing
Ground conductors and risers where a rigid, corrosion-resistant conductor is required.
How to order bus pipe
- SpecifyAlloy and temper, OD and wall or NPS and schedule, length, quantity, minimum conductivity and standard (ASTM B317, B241, IS 5082 or EN 755).
- ProcessingCut lengths, bends, drilled holes, plated ends, insulating sleeves, with drawings where needed.
- Quote and termsWe confirm availability, certification and terms (EXW, FOB, CIF or DAP).
- DocumentsMill test certificate to EN 10204 3.1 with chemistry, mechanical properties and conductivity; third-party inspection on request.
- PackingTubes are bundled with end protection and separators in export crates suitable for long lengths.
References and standards
- ASTM B317/B317M – Aluminum-Alloy Extruded Bar, Rod, Tube, Pipe and Profiles for Electrical Purposes (Bus Conductor)
- ASTM B241/B241M – Aluminum and Aluminum-Alloy Seamless Pipe and Seamless Extruded Tube
- IEEE 605 – Guide for Bus Design in Air Insulated Substations
- ASME B36.10M – Welded and Seamless Wrought Steel Pipe (dimensions used for aluminium pipe schedules)
- The Aluminum Association – Aluminum Electrical Conductor Handbook
Aluminium (aluminum) bus pipe: frequently asked questions
Which alloy is best for tubular bus pipe?
6101-T6 is the dedicated bus-conductor alloy, with at least 55 % IACS conductivity and good strength. 6063-T6 is a close alternative with slightly lower conductivity. 1350 conducts best but is too soft for long outdoor spans.
Why use a tube instead of a flat busbar outdoors?
A tube carries AC current evenly, is equally stiff in every direction and has no sharp edges. It spans further between insulators, resists wind and fault forces, and limits corona at high voltage.
What is the weight of 4 inch Schedule 40 aluminium bus pipe?
NPS 4 Schedule 40 has a 114.3 mm OD and 6.02 mm wall, a cross-section of about 2,050 mm² and a weight of about 5.5 kg per metre (3.7 lb/ft).
How do you stop long tubular bus spans from vibrating?
Light steady winds can cause aeolian vibration. Designers usually insert a loose length of stranded conductor inside the tube as a damper, or fit purpose-made dampers, following IEEE 605 guidance.
Should I order seamless or extruded structural pipe?
Seamless pipe to ASTM B241 is preferred if tubes will be bent or highly stressed. Extruded structural pipe is acceptable for straight runs where the specification allows it.
Can you supply bus pipe with plated ends and cut lengths?
Yes. Metline supplies bus pipe cut to length, drilled, bent and with tin- or nickel-plated contact ends on request, with mill test certificates and export packing, worldwide.
Planning a substation or high-current bus run?
Send the alloy, size, schedule, lengths and quantity. We will confirm conductivity, certification and a delivered price.
