Bus duct or cable: where a sandwich busduct is worth the money

A sandwich busduct is worth the money when a low voltage feeder carries high current, would otherwise need several parallel runs of single-core cable, and has to fit a tight route or feed many loads along its length. The classic cases are transformer to main LV panel links, rising mains in multi-storey buildings and production floors where loads move. At low currents or on irregular routes, cable usually stays cheaper. New India Electricals Ltd (NIEL) designs and fabricates LV bus duct systems up to 8000 A and supplies the Power+ B medium voltage bus duct up to 12 kV and 2000 A.

TL;DR

• A sandwich busduct packs insulated conductors tightly together with no air gap. The BEAMA busbar trunking guide calls this construction “high-power, low-impedance”.

• Bus duct pays off at higher current ratings, where cable would need multiple single-core runs, trays and a larger switchboard termination space.

• Cable grouping costs capacity. Under IEC 60364-5-52, nine circuits in a single layer on a tray are derated to 0.70, so the cables need about 43 percent more rated capacity.

• Cable still wins on low current feeders, irregular or changing routes, and where lead time for made-to-measure bus duct pieces is a problem.

• IEC 61439-6 covers busbar trunking up to 1000 V AC. Above that, the choice is metal-enclosed bus duct such as NIEL Power+ B.

• Specify the short-time rating to match the prospective fault level at the feeder, not higher, and ask for design verification evidence.

• Ambient temperature and harmonic neutral current both derate bus duct, so state site conditions in the enquiry.

What is a sandwich busduct, and how is it different from other bus duct?

A sandwich busduct, also called compact busway, holds each phase conductor in its own insulation film and clamps all conductors tightly together inside a metal housing, with no air gap between them. Air-insulated bus duct keeps the bars apart on insulating supports, with air as the main insulation. Both sit under IEC 61439-6 at low voltage.

The IECEE summary of IEC 61439-6:2012 sets the scope as busbar trunking systems (BTS) with a rated voltage not exceeding 1000 V AC or 1500 V DC. It lays down construction, technical characteristics and verification requirements. It replaced IEC 60439-2.

The BEAMA Guide to Low Voltage Busbar Trunking Systems, prepared by manufacturers including Eaton, Legrand, Schneider Electric and Siemens, defines a BTS as “an enclosed electrical distribution system comprising solid conductors separated by insulating materials.” It describes compact or sandwich-type trunking as high-power, low-impedance construction.

That tight packing has three consequences that drive the rest of this guide:

• Magnetic fields from the phases largely cancel, which keeps reactance and external field low.

• Heat leaves through the housing rather than through air gaps, so the enclosure is part of the thermal design.

• The insulation film is the only barrier between phases, so joint quality and moisture control during storage matter.

Above 1000 V, sandwich construction is not an option. Medium voltage links use metal-enclosed bus duct with air clearances, such as the non-phase-segregated Power+ B that NIEL lists for 11 kV class systems.

When does a sandwich busduct beat cable on cost?

A sandwich busduct beats cable once the feeder current forces several parallel single-core cable runs. At that point cable adds trays, fixings, lugs, termination space and jointing labour, and grouping derates every run. The BEAMA guide states that busbar trunking is more economical “particularly for the higher current ratings, where multiple single core cables would be used.”

The grouping penalty is the part most cost comparisons miss. Cable ratings in catalogues assume a single circuit. Schneider Electric’s Electrical Installation Guide chapter on cable sizing reproduces the reduction factors from IEC 60364-5-52:

The last row is simple arithmetic: 1 divided by the factor. A 2500 A feeder run as nine parallel circuits on one tray needs cables whose combined catalogue rating is about 3570 A before ambient temperature is even considered. The same Schneider guide states that for busways, “methods of installation, insulation materials, correction factors for grouping are not relevant parameters,” because the manufacturer has already sized the conductors for its rated current.

The BEAMA guide lists the other savings for a feeder run:

• It replaces multiple cable runs and their supporting metalwork.

• It needs less termination space in switchboards.

• Bends and offsets fit in a smaller area than the equivalent cable.

• No cable jointer is required.

• It can be dismantled and reused elsewhere.

NIEL’s own Power+ B product page makes the same point for medium voltage: “bus duct is often less expensive than equivalent cable installations,” and it is “inherently resistant to many of the forces or conditions which accelerate the deterioration of insulated cable in conduit or trays.”

Distribution runs add a second saving. A sandwich busduct with tap-off outlets lets loads connect along its length through plug-in units, so a production line or an office floor can be rearranged without pulling new cable back to the switchboard.

When is cable still the better choice?

Cable is usually the better choice for low and medium current feeders, routes with many obstructions or level changes, runs between buildings, and projects where made-to-measure bus duct pieces cannot be delivered in time. At currents where one multi-core cable does the job, none of the grouping, termination or tray savings apply.

Four practical limits of bus duct deserve a place in any comparison:

• Route geometry. Bus duct changes direction through factory-made elbows and tees, and the BEAMA guide defines a “make-up piece” as a custom-engineered unit made to measure to complete a run. Every offset has to be measured on site before manufacture. Cable bends where it is pulled.

• Structural movement. Where the run crosses a building expansion joint, it needs a building expansion unit, and long straight runs need thermal expansion units.

• Low current runs. The BEAMA guide notes that for BTS rated below 100 A, reactance is treated as negligible, so the impedance advantage of sandwich construction disappears at low ratings.

• Short, simple links. A short, straight feeder at moderate current with one load at the far end gains little from a tap-off capable system.

The table sums up where each option fits:

How does voltage drop compare between bus duct and cable?

Voltage drop in bus duct is usually lower than in the equivalent cable arrangement, and the BEAMA guide says so for the majority of cases. The reason is low reactance. The guide gives the line-to-line voltage drop per metre as the square root of 3, times current, times the sum of resistance times cos phi and reactance times sin phi.

At a typical industrial load power factor, the reactance term carries real weight. Parallel single-core cables laid without attention to phase sequence and spacing can have high and unequal reactance. The BEAMA guide notes that cable installers often pay little attention to conductor spacing or sequence. In a sandwich busduct, the conductors sit close together in a fixed order, so reactance is low and balanced. BEAMA also describes the phase transposition unit, which changes the relative position of phase conductors to balance inductive reactance on long runs.

Three BEAMA details help when checking a supplier’s figures:

• Manufacturers publish voltage drop in mV/A/m, usually at full load and 35 degrees C ambient. That is pessimistic for most real loads.

• IEC 61439-6 requires the manufacturer to provide resistance at 20 degrees C, resistance at rated current, and mean reactance per metre per phase. That allows a precise calculation on long or critical runs.

• On a distribution run with loads tapped off along the length, the voltage drop at the far end is roughly half the full load figure.

What does bus duct change for fire safety, space and magnetic fields?

Bus duct carries less combustible material than a cable installation of the same rating. It also seals building penetrations with factory fire barrier units and produces a lower external magnetic field. The BEAMA guide states that busbar trunking systems “provide a better resistance to the spread of fire” than cable.

On fire load, Siemens’ SIVACON 8PS busbar trunking systems brochure rates busbar fire load as “very low” against “very high, dependent on cable type” for cable. That is a manufacturer’s comparison, so treat it as directional.

The BEAMA guide notes that compact or sandwich-type trunking does not need internal fire barriers, because its construction already acts as one. Air-insulated designs may need factory-fitted internal fire stops at each wall or floor. In every case, the manufacturer must verify performance under fire, and the external seal around the duct must meet local building regulations.

On magnetic fields, BEAMA states that in sandwich construction the induced fields cancel to a large extent, “resulting in an extremely low external magnetic field.” This matters near instrumentation, control rooms and data equipment.

Should a sandwich busduct use copper or aluminium conductors?

Choose copper where space and losses matter most. Choose aluminium where weight and first cost matter most. The BEAMA guide states that performance to IEC 61439-6 is the same for either material at a given rating, but the physical trade-offs differ. It quantifies them for high-power sandwich-type designs.

In a tight riser shaft, copper’s smaller section often decides it; on a long feeder hung from steelwork, aluminium’s lower weight eases supports.

What ratings and evidence should you ask a bus duct supplier for?

Ask for the rated current at your site ambient, the short-time and peak withstand ratings matched to your prospective fault current, the neutral rating for your harmonic load, and design verification evidence to IEC 61439-6. The BEAMA guide is explicit that “there is no advantage in specifying a higher figure for short-circuit rating” than the prospective fault current at the feeder.

Short-circuit ratings. BEAMA gives typical short-time withstand values from about 10 kA to 50 kA or more, usually for 1 or 3 seconds. The peak withstand rating matters too, because the first asymmetric peak of a fault can reach up to 2.2 times the steady-state fault current. That peak is what stresses the supports. A conditional rating is only valid with the specific upstream fuse or breaker named by the manufacturer.

Ambient temperature. Bus duct ratings are normally stated at 35 degrees C. The correction factors Schneider Electric publishes for its medium and high power range fall to 0.97 at 40 degrees C, 0.90 at 50 degrees C and 0.86 at 55 degrees C. A switch room or cable gallery in a hot climate needs the derated figure, not the catalogue figure. NIEL covers the wider principle in its guide to derating electrical equipment for African installations.

Harmonics and the neutral. Triplen harmonic currents add in the neutral. BEAMA’s worked case for single-phase electronic loads shows neutral current reaching 173 percent of phase current. Its rating factors for a system carrying neutral current equal to 100 percent of rated current are 0.87 with a 100 percent neutral and 0.93 with a 200 percent neutral. Specify a double neutral where drives, UPS and IT loads dominate. NIEL’s article on VFD panels and what drives do to your supply explains where that current comes from.

Tap-off devices. BEAMA warns that tap-off units are verified with specific protective devices, and “substituting devices not verified by the BTS manufacturer invalidates any testing/verification and warranty.”

Commissioning. Joint torque must follow the manufacturer’s setting, because current capacity depends on contact pressure. BEAMA suggests recording an external surface temperature profile at commissioning and repeating it periodically. It notes that a surface temperature rise of up to 55 K may be expected at full load. Joints are unprotected until assembled, so store components dry and run an insulation resistance test before installation.

Where does NIEL’s bus duct range fit?

NIEL covers both sides of the voltage line. At low voltage, NIEL designs and fabricates bus duct systems up to 8000 A. It offers 2, 3 or 4 bar configurations per phase, with entries at top, bottom, back, front or side to suit the switchboard. At medium voltage, the Power+ B range covers metal-enclosed bus duct up to 12 kV and 2000 A.

The published Power+ B features are specific:

• Housings in galvanised sheet steel, stainless steel or aluminium

• Busbars of annealed electrical grade copper, tin or silver plated at contact surfaces

• Breather drains at regular intervals to prevent condensation

• Self-extinguishing, crack-resistant busbar supports

• Ground pads for connection to the earth grid

• Type tested for BIL and temperature rise as per IS at CPRI, Bangalore

NIEL’s MV brochure lists the construction as non-phase-segregated and metal enclosed, with IP4X for indoor and IP52 for outdoor installation. NIEL’s guide to specifying medium voltage panels names Power+ B for high-current transformer to switchgear connections.

The transformer end matters too. NIEL’s distribution transformers list an LV side cable box or bus duct as a feature, so the bus duct decision should be made before the transformer is ordered, not after. If the transformer is still being sized, NIEL’s guide to sizing a distribution transformer sets the current the link has to carry.

What should a bus duct enquiry include?

Send these details with every bus duct or cable comparison, so that quotations stay comparable.

• System voltage, frequency and earthing system

• Rated current, and load current at each tap-off if it is a distribution run

• Prospective short-circuit current at the feeder and the upstream protective device

• Maximum ambient temperature along the route, including shafts and roof spaces

• Harmonic profile or load type, and required neutral size (100 or 200 percent)

• Route drawing with lengths, elbows, offsets, wall and floor crossings, and expansion joints

• Indoor or outdoor, and required degree of protection

• Conductor preference: copper or aluminium

• Termination details at the transformer and switchboard ends

• Fire barrier requirement at each wall or floor penetration

• Voltage drop limit for the run

• Design verification or type test evidence required

Weighing bus duct against cable for your feeder?

NIEL designs and fabricates low voltage bus duct systems up to 8000 A and Power+ B metal-enclosed bus duct up to 12 kV and 2000 A, alongside the transformers and switchboards they connect. Send us your route drawing, rated current, fault level and site ambient temperature, and our engineers will review the rating and termination arrangement before quotation.

Find the right bus duct for your voltage:

• Low voltage panels: LV bus duct systems up to 8000 A, with 2, 3 or 4 bars per phase.

• Power+ B bus duct: non-phase-segregated, metal-enclosed bus duct up to 12 kV and 2000 A.

• Distribution transformers: LV side cable box or bus duct termination.

Call +91 80 42434343, email support@newindiaelectricals.com, or contact our team with your route details.

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