Ampacity
Current rating depends on diameter, emissivity, sun, wind and allowable temperature. IEEE 605 gives calculation methods; ratings rise noticeably once the surface weathers and its emissivity increases.
Tubular bus conductor guide
6101 Aluminium Tubular Bus Pipe
6101 bus pipe is a round aluminium tube used as a rigid conductor in outdoor substations, switchyards and generator connections. In T6 temper it combines a guaranteed minimum conductivity of 55 % IACS with the strength to span between insulators under wind, ice and short-circuit loads. This guide covers tempers, standard pipe sizes, resistance per metre and the design points that matter to substation engineers.
- Alloy
- 6101 (UNS A96101)
- Tempers
- T6, T61, T63, T64, T65
- Conductivity
- ≥ 55 % IACS in T6
- Standard
- ASTM B317 / B317M
Why substations use tubular aluminium bus
In an air-insulated substation the bus must carry thousands of amperes, span several metres between post insulators and keep its shape in storms and short circuits. A round aluminium tube meets all three needs better than flat bar or stranded cable.
A tube places its metal away from the centre, which gives a high section modulus and moment of inertia for its weight, so it deflects less between supports. For alternating current, skin effect pushes current towards the outer surface of a conductor; a tube has no wasted metal at the centre, so its AC resistance is close to its DC resistance. The smooth, large-diameter surface also lowers the electric-field gradient, which raises the voltage at which corona discharge starts and reduces radio noise and losses at high voltage.
6101 is the alloy designed for this job. ASTM B317 guarantees its conductivity for each temper, which a general structural alloy such as 6063 or 6061 does not. In many markets 6063-T6 pipe is also used for substation bus where a lower, unguaranteed conductivity is acceptable; 6101 is chosen when the current rating must be guaranteed.
Tempers and minimum properties
Bus pipe is normally ordered in T6 for maximum strength. T63 and T61 trade strength for conductivity and are used where spans are short or current is high. Specified minimums for extruded tube and pipe:
| Temper | Thickness | Tensile min | Yield min | Conductivity min (20 °C) |
|---|---|---|---|---|
| T6 | 3.2 – 12.7 mm | 200 MPa (29 ksi) | 172 MPa (25 ksi) | 55.0 % IACS |
| T61 | 3.2 – 19 mm | 138 MPa (20 ksi) | 103 MPa (15 ksi) | 57.0 % IACS |
| T61 | 19 – 38 mm | 124 MPa (18 ksi) | 76 MPa (11 ksi) | 57.0 % IACS |
| T61 | 38 – 50.8 mm | 103 MPa (15 ksi) | 55 MPa (8 ksi) | 57.0 % IACS |
| T63 | 3.2 – 25.4 mm | 186 MPa (27 ksi) | 152 MPa (22 ksi) | 56.0 % IACS |
| T64 | 3.2 – 25.4 mm | 103 MPa (15 ksi) | 55 MPa (8 ksi) | 59.5 % IACS |
| T65 | 3.2 – 19 mm | 172 – 221 MPa (25 – 32 ksi) | 138 – 186 MPa (20 – 27 ksi) | 56.5 % IACS |
| H111 | 6.3 – 50.8 mm | 83 MPa (12 ksi) | 55 MPa (8 ksi) | 59.0 % IACS |
Specified minimums for extruded rod, bar, tube, pipe and profiles for electric conductors, from the Aluminum Association property-limit table that ASTM B317/B317M follows (inch values converted). T65 has both minimum and maximum limits so that bending behaviour is repeatable. Confirm against the current edition of the standard for contract use.
Some suppliers list T4, T5, O or H-tempers for 6101 pipe. These are not conductor tempers in ASTM B317, so their conductivity and strength are not guaranteed by the standard. If they are offered, agree measured minimums in writing.
6101 composition (wt %)
| Element | Minimum | Maximum | Why it is limited |
|---|---|---|---|
| Silicon (Si) | 0.30 | 0.70 | Forms Mg2Si with magnesium |
| Iron (Fe) | — | 0.50 | Impurity; lowers conductivity |
| Copper (Cu) | — | 0.10 | Impurity |
| Manganese (Mn) | — | 0.03 | Very strong conductivity penalty, so held far below normal 6xxx limits |
| Magnesium (Mg) | 0.35 | 0.80 | Main strengthening element |
| Chromium (Cr) | — | 0.03 | Very strong conductivity penalty |
| Zinc (Zn) | — | 0.10 | Impurity |
| Boron (B) | — | 0.06 | Added to tie up dissolved Ti, V and Cr as borides, raising conductivity |
| Others, each / total | — | 0.03 / 0.10 | Tighter than the usual 0.05 / 0.15 |
| Aluminium (Al) | Remainder | About 97.9 – 99.4 % |
Standard bus pipe sizes, weight and resistance
Substation bus is usually specified in ANSI/ASME nominal pipe sizes, schedule 40 or schedule 80. The table gives cross-section, mass per metre (2.70 g/cm³) and calculated DC resistance per metre at 20 °C for 6101-T6 at its 55 % IACS minimum.
| NPS (in) | Schedule | OD (mm) | Wall (mm) | Area (mm²) | Mass (kg/m) | DC resistance (µΩ/m) |
|---|---|---|---|---|---|---|
| 1 1/2 | 40 | 48.26 | 3.68 | 515 | 1.39 | 60.8 |
| 1 1/2 | 80 | 48.26 | 5.08 | 689 | 1.86 | 45.5 |
| 2 | 40 | 60.33 | 3.91 | 693 | 1.87 | 45.2 |
| 2 | 80 | 60.33 | 5.54 | 954 | 2.57 | 32.9 |
| 2 1/2 | 40 | 73.03 | 5.16 | 1100 | 2.97 | 28.5 |
| 2 1/2 | 80 | 73.03 | 7.01 | 1454 | 3.93 | 21.6 |
| 3 | 40 | 88.90 | 5.49 | 1439 | 3.88 | 21.8 |
| 3 | 80 | 88.90 | 7.62 | 1946 | 5.25 | 16.1 |
| 3 1/2 | 40 | 101.60 | 5.74 | 1729 | 4.67 | 18.1 |
| 3 1/2 | 80 | 101.60 | 8.08 | 2374 | 6.41 | 13.2 |
| 4 | 40 | 114.30 | 6.02 | 2048 | 5.53 | 15.3 |
| 4 | 80 | 114.30 | 8.56 | 2844 | 7.68 | 11.0 |
| 5 | 40 | 141.30 | 6.55 | 2773 | 7.49 | 11.3 |
| 5 | 80 | 141.30 | 9.53 | 3945 | 10.65 | 7.9 |
| 6 | 40 | 168.28 | 7.11 | 3600 | 9.72 | 8.7 |
| 6 | 80 | 168.28 | 10.97 | 5421 | 14.64 | 5.8 |
Metric sizes are also common, for example outside diameters from about 60 to 250 mm with walls of 4 – 15 mm, and larger tubes for generator and high-current buses. Lengths of 6 – 12 m are typical, with longer lengths available by arrangement and limited mainly by transport.
Design considerations for rigid tubular bus
Deflection
Span length is often limited by gravity sag rather than stress. A common criterion is sag no more than about 1/150 to 1/200 of the span; check your utility standard.
Short-circuit forces
Peak fault current produces large electromagnetic forces between phases. The tube, insulators and fittings must resist them without permanent set.
Aeolian vibration
Long tubular spans can vibrate in steady wind. A damping conductor (old stranded cable) placed inside the tube or proprietary dampers suppress it.
Thermal expansion
Aluminium expands about 23 mm per 10 m per 100 °C. Use sliding supports and expansion fittings on long runs.
Corona
At EHV, choose a diameter large enough to keep surface gradient below the corona onset; end caps or corona balls finish open tube ends.
Joining, finishing and fittings
- Welding: sections and fittings are commonly joined by TIG or MIG welding with ER4043 filler. The weld zone loses part of its T6 strength, so place welds away from mid-span where bending stress peaks.
- Bolted and compression fittings: clamps and couplers in aluminium alloy avoid galvanic problems; clean contact areas and use jointing compound.
- Plating: tin or silver plating at connection pads improves contact stability; most tube bodies are left bare.
- Insulation: heat-shrink or other insulating covers are used in compact indoor installations and for animal protection.
- Bending: large tubes are bent on mandrel benders; T6 needs generous radii, so plan bends in the layout or use cast or welded elbows.
- Inspection: dimensional, visual, conductivity and tensile tests per lot, with straightness checked on long lengths.
6101 vs 6063 bus pipe
| Aspect | 6101-T6 / T63 | 6063-T6 |
|---|---|---|
| Product standard | ASTM B317 (conductor) | ASTM B241 / B429 (general pipe) |
| Conductivity | Guaranteed: 55 % (T6), 56 % (T63) IACS minimum | Not specified by the standard; about 53 % IACS typical |
| Minimum tensile strength | 200 MPa (T6), 186 MPa (T63) | 205 MPa (T6, ASTM B221) |
| Typical use | Utility substations needing guaranteed ampacity | Lower-cost bus where ratings include margin |
Both alloys perform well mechanically. The decision usually comes down to whether the purchaser's specification calls for guaranteed conductivity; if it does, 6101 is the safe choice.
How to order 6101 bus pipe
- SpecifyAlloy, temper and standard ("6101-T6 to ASTM B317"), NPS and schedule or OD and wall, length and quantity.
- State testsConductivity, tensile and dimensional checks; any third-party inspection.
- Add processingCut lengths, end preparation for welding, plating of pads, insulation.
- DocumentsEN 10204 3.1 mill certificate with chemistry, mechanical and conductivity results.
- ShipBundles or crates with end protection; EXW, FOB, CIF or DAP terms.
References and standards
- ASTM B317/B317M – Aluminum-Alloy Extruded Bar, Rod, Tube, Pipe, Structural Profiles, and Profiles for Electrical Purposes (Bus Conductor)
- The Aluminum Association – Property Limits for Electric Conductors (Table 16.3)
- IEEE 605 – Guide for Bus Design in Air Insulated Substations
- ASME B36.10M – Welded and Seamless Wrought Steel Pipe (dimensions used for nominal pipe sizes)
- IS 5082 – Wrought aluminium and aluminium alloy bars, rods, tubes, sections, plates and sheets for electrical purposes
6101 aluminium (aluminum) bus pipe: FAQ
What temper is used for substation bus pipe?
T6 is standard because it gives the highest strength (200 MPa tensile minimum) with 55 % IACS minimum conductivity. T63 is used where slightly higher conductivity is wanted.
Why is tubular bus better than flat bar outdoors?
A tube is stiffer for its weight, has lower AC resistance because of skin effect, and its smooth round surface resists corona at high voltage.
What sizes of bus pipe are common?
Nominal pipe sizes from 1 1/2 in to 6 in in schedule 40 or 80 are most common, plus metric ODs of about 60 – 250 mm.
How is aeolian vibration prevented?
By inserting a length of damping cable inside the tube or fitting dampers, and by limiting span lengths.
Can 6101 bus pipe be welded on site?
Yes, with TIG or MIG and ER4043 filler by qualified welders. Design for the reduced strength of the heat-affected zone.
Planning a substation bus?
Send the sizes, temper and lengths. Metline will quote 6101 bus pipe with certificates and export packing.
