Selecting Bus Trunking Systems: Moeller vs Telemecanique vs GE

Where does cross-brand bus trunking data actually come from?

No manufacturer publishes a table that puts its busway next to a competitor's. The comparison has to be assembled from each vendor's own catalog, and the catalogs are found in three places: the manufacturer's site, the electrical distributor's site, and the type-test certificate the manufacturer supplies on request. Distributors such as Graybar either host the technical data sheets or link to the manufacturer's PDF; searching a distributor catalog for the product family name is often faster than navigating the manufacturer portal. For European product lines sold in North America, distributors on the US east coast carry more of the German and French catalog material than inland branches.

Two practical points before searching. First, brand names move: the Klockner Moeller busbar range is sold today under Eaton, and Telemecanique low-voltage products sit under Schneider Electric, so search the current parent company as well as the legacy name. Second, download the full technical catalog, not the brochure. The brochure gives rated current; the technical catalog gives the impedance, derating and short-circuit tables that decide the comparison.

What is the current path, and which hop sets each rating?

Follow the current. Layer one first: the physical run decides the electrical numbers, and every manufacturer rates the run hop by hop. A trunking system is a chain of discrete elements, and the weakest element governs.

Capture the same fields for every brand at the same rated current, the same conductor material, and the same IP code. A copper sandwich bar at IP55 compared against an aluminium air-insulated bar at IP31 tells you nothing.

Check 1: does the rated current survive the mounting orientation and ambient?

Reading to take: the derating table in each catalog, entered at your mounting orientation (edgewise, flat, vertical riser) and your ambient. Manufacturers quote the headline rated current at one reference orientation and one reference ambient, and the reference conditions differ between catalogs. A flat-mounted run of an air-insulated bar can lose a significant share of its edgewise rating; a sandwich bar loses less because conduction, not convection, carries the heat to the casing.

• If the derated current at your conditions still exceeds the design load with margin, the straight length passes. Go to Check 2.
• If it does not, step up one frame size for that brand only and re-enter the table. Note that the frame step changes the impedance and the tap-off unit family, so re-capture rows 2 and 5.
• If a brand does not publish a derating table for your orientation, treat the headline rating as unverified and request the type-test report through the manufacturer's technical support channel before scoring it.

Check 2: does the short-circuit path hold at the feed?

Reading to take: prospective short-circuit current at the feed unit terminals, from the upstream transformer impedance and cable run, compared with the rated short-time withstand current (Icw, quoted for a stated duration) and the rated peak withstand current (Ipk) of the trunking. The fault current enters at hop 1 and stresses every joint and tap-off it passes through, so the run is only as strong as the lowest Icw in the chain, including the tap-off unit stab contacts.

• If Icw for the quoted duration and Ipk both exceed the prospective values, the run passes with time-delayed upstream protection. Go to Check 3.
• If Ipk is exceeded but the upstream breaker is current-limiting, compare the breaker let-through peak (from the breaker manufacturer's let-through curves) with the trunking Ipk. This is a conditional rating and the trunking catalog will state the breaker types it has been tested with; only those combinations count.
• If neither route closes the gap, the brand is out at that frame size. Record it and move on rather than assuming a larger frame will be tested at higher fault levels; check the table.

Score the tap-off units separately here. Some families permit plugging a tap-off onto a live bar; others require the run to be dead. That decision changes the operating procedure for the life of the installation and belongs in the comparison sheet.

Check 3: does the far tap-off still see its voltage?

Reading to take: per-unit-length resistance and reactance from the catalog, at operating temperature, and the load distribution along the run. For a three-phase run the line-to-line drop over a section of length L carrying line current I at power factor cosφ is:

dU_LL = sqrt(3) * I * L * (R * cos_phi + X * sin_phi)

I       line current, A
L       section length, m
R, X    catalog resistance and reactance, ohm/m per phase
dU_LL   volts, line-to-line

Sum the drop section by section from the feed to the last tap-off, using the current actually flowing in each section, not the feed current for the whole length. Compare the total against the limit set by the project specification.

• If every brand passes, voltage drop is not a discriminator; weight the comparison on Checks 1 and 2, tap-off flexibility and price.
• If a brand fails only at the far end, a centre-feed layout halves the effective length. Re-run the sum before rejecting the brand.
• If the catalog quotes reactance at 50 Hz and the site runs 60 Hz, scale X by 60/50 before entering it; resistance is unchanged.

How do you build the sheet and verify the chosen system?

1. Download the full technical catalog for each brand from the manufacturer site or distributor link. Record the catalog issue date; superseded catalogs carry withdrawn ratings.
2. Create one row per hop from the table above and one column per brand at the same rated current, conductor material and IP code.
3. Enter the derated current at your orientation and ambient (Check 1), Icw and Ipk with the tested breaker combinations (Check 2), and the summed voltage drop to the last tap-off (Check 3).
4. Add operational columns: live tap-off insertion, tap-off spacing, joint torque and retorque requirement, fire-barrier availability, and the mounting hardware supplied.
5. Request the type-test certificate for the frame size you intend to order and confirm the numbers on it match the catalog.
6. On delivery, check that the nameplate rated current, Icw and IP code match the certificate before the run is hung.
7. At commissioning, torque every joint to the catalog value, measure insulation resistance phase-to-phase and phase-to-earth on the dead run, then energise and take a thermal image of every joint and tap-off at the highest load available; retorque any joint the catalog flags for retorque after first load and repeat the thermal scan.

FAQ