Sandwich-type and air-insulated busway are the two dominant constructions for low-voltage busway, and the difference between them explains most of the performance characteristics buyers care about — impedance, voltage drop, heat dissipation, short-circuit withstand, and physical size. Neither is universally better. Sandwich construction wins on electrical performance and compactness; air-insulated wins where cost matters most and where the rating is modest. This guide explains what each construction actually is, what the differences mean numerically, and how to choose.
What the Two Constructions Are
Sandwich-Type Busway
The conductors are flat bars stacked face-to-face with thin insulation between them and between each bar and the housing, so the bars are in intimate contact with the insulation over their entire surface. The housing — typically steel or aluminium — clamps the assembly tightly, forming a composite structure. There is essentially no air gap between conductors.
The consequence is mechanical as much as electrical: the bars, insulation, and housing act together as a single stiff beam. Under short-circuit conditions, that composite action resists the enormous electromagnetic forces between conductors far better than bars standing free in air.
Air-Insulated Busway
The conductors are bars separated by air gaps, supported at intervals by insulators within the housing. Cooling happens partly by convection through the air spaces around the bars. The construction is simpler and uses less insulation material.
The consequence is that conductor spacing is set by clearance and creepage requirements rather than by insulation thickness, so the assembly is generally larger for a given rating, and the bars rely on their supports — not on composite action with the housing — to resist short-circuit forces.
How Construction Drives Performance
| Characteristic | Sandwich-type | Air-insulated | Why |
|---|---|---|---|
| Impedance | Lower | Higher | Closely spaced, face-to-face bars give strong magnetic cancellation |
| Voltage drop | Lower | Higher | Follows from lower impedance |
| Heat dissipation | Better — bars conduct into the housing, which acts as a heat sink | Good, via air convection | Intimate bar-to-housing contact spreads heat; the housing becomes part of the thermal path |
| Size for a given rating | More compact | Larger | Clearance requirements govern air-insulated spacing |
| Short-circuit withstand | Higher | Lower | Composite bar/insulation/housing structure resists electromagnetic forces |
| Mechanical stiffness | Higher | Lower | Same composite action |
| Material cost | Higher | Lower | More insulation material, tighter manufacturing tolerance |
| Typical rating range | Often favoured at higher ratings | Common at lower to moderate ratings | Cost advantage of air-insulated narrows as rating rises |
The Electrical Differences in Detail
Impedance and Voltage Drop
Busway impedance has a resistive and a reactive component. The reactive component depends substantially on the spacing and orientation of the phase conductors — closely spaced, face-to-face bars produce strong magnetic field cancellation, which lowers reactance materially compared with widely spaced bars.
Sandwich construction also exhibits a skin-effect and proximity-effect advantage: the current distributes across the wide face of the bars rather than concentrating, which reduces effective resistance at power frequency relative to a bar of the same cross-sectional area in a less favourable arrangement.
The practical result is lower voltage drop per metre for the same rating and roughly comparable conductor cross-section. On a long run, this can be the difference between meeting and missing the voltage drop limit, or between one rating and the next.
Heat Dissipation
In sandwich construction, the conductors are in contact with insulation that is in contact with the housing. Heat flows from the bars through the insulation to the housing and out to the surrounding air. The housing’s whole surface becomes a heat sink, which is why sandwich busway often achieves a comparable or better rating in a smaller cross-section.
This also means the housing’s ability to dissipate heat matters more for sandwich construction — an enclosed or poorly ventilated installation affects it more directly. The derating guidance in the manufacturer’s data is correspondingly more important.
Short-Circuit Withstand
Under a short circuit, currents of tens of kiloamps produce electromagnetic forces between conductors that attempt to push them apart. IEC 61439-1 and IEC 61439-6 address short-circuit withstand, expressed as rated short-time withstand current (Icw) and rated peak withstand current (Ipk) for the assembly.
Sandwich construction resists these forces through composite action — the bars are clamped against the housing by the insulation stack, so force is transferred into the structure rather than bending the bars. Air-insulated construction relies on the insulators and the bars’ own stiffness. This is why sandwich designs typically carry higher Icw and Ipk ratings, and why they are strongly favoured at higher ratings and where the prospective fault current is high.
Selection Criteria
- Rating. At higher ratings, sandwich construction’s compactness and withstand capability usually make it the better choice. At lower ratings, air-insulated may be adequate and more economical.
- Prospective fault current. Where Icw and Ipk requirements are demanding, sandwich construction has a structural advantage.
- Run length and voltage drop. Long runs benefit from lower impedance. Run the voltage drop calculation for both options and compare.
- Space constraints. Where the route is tight — a crowded shaft or a low ceiling void — the more compact construction may be the deciding factor regardless of other considerations.
- Installation conditions. Both need derating applied for ambient, altitude, and enclosure. Because sandwich construction uses the housing as a heat sink, enclosure effects can be more pronounced — check the derating data carefully.
- Tap-off requirements. Confirm the tap-off range available for each construction and that the tap-off ratings you need are offered. This is a practical constraint that can override an electrical preference.
- Budget against whole-life cost. Air-insulated may cost less to buy; sandwich may cost less to run over a long, heavily loaded run through lower losses. On a long run at high load, the loss difference is real and worth calculating.
Application Case: Long Plant Feeder With Voltage Drop Constraint
Scenario Constraints
A plant requires a 1250 A feeder from a substation to a production hall approximately 180 m away. The route is a covered service corridor with restricted ceiling height and limited width. Voltage drop must stay within the project limit. Prospective short-circuit current at the supply end is high. The run will operate near rated load for much of the day.
Selection Approach
Voltage drop is the governing constraint at 180 m, so impedance is the deciding variable. Sandwich construction’s lower impedance gives a materially lower voltage drop over this distance, and on a run this long that difference is likely to be decisive rather than marginal.
The high prospective fault current points the same way: the required Icw and Ipk are more comfortably met by sandwich construction.
Space is tight, which favours the more compact option again.
Load factor is high and continuous, so losses matter over the plant’s life. Lower resistance means lower I²R losses, which on a 180 m run at 1250 A is a genuine operating cost, not a rounding error.
The one caution: the covered corridor with restricted ceiling height suggests restricted ventilation. Because sandwich construction relies partly on the housing for dissipation, the enclosure derating factor needs to be confirmed — and it may be large enough to affect the rating selection. Obtain the derating data for the actual corridor conditions before finalising.
Common Mistakes
- Selecting on purchase price on a long run. Losses over 180 m at high load factor can exceed the purchase price difference over the plant’s life. Calculate it.
- Ignoring impedance when voltage drop is tight. The two constructions are not equivalent in this respect, and voltage drop is often the binding constraint on long runs.
- Assuming the declared Icw applies without checking the actual prospective fault current. Compare the assembly’s Icw and Ipk against the calculated fault level at the point of installation.
- Overlooking enclosure derating on sandwich construction. Its reliance on the housing as a heat sink makes installation conditions more influential, not less.
- Choosing a construction before confirming tap-off availability. If the tap-off ratings you need are not offered for the chosen construction, the electrical advantage is irrelevant.
- Mixing constructions in one run without manufacturer confirmation. Transitions between different products need to be declared as acceptable.
Acceptance Checks
- Construction type confirmed on the delivered product against the specification.
- Rated current, Icw, and Ipk confirmed against the calculated demand and the prospective fault current at the installation point.
- Voltage drop calculated over the actual route length and compared against the project limit, using the product’s declared impedance data.
- Derating applied for ambient, altitude, and installation conditions, with each factor recorded.
- Tap-off units confirmed as compatible with the construction and available in the required ratings.
- Type-test documentation obtained — IEC 61439-6 where applicable, or the standard declared for the product and market.
Frequently Asked Questions
Which construction is better overall?
Neither universally. Sandwich-type has lower impedance, better heat dissipation, higher short-circuit withstand, and a more compact size — favoured at higher ratings and on long runs. Air-insulated is simpler and often less expensive, and may be entirely adequate at lower ratings with short runs and modest fault levels.
Why does sandwich busway have lower voltage drop?
Closely spaced, face-to-face phase conductors produce strong magnetic field cancellation, reducing reactance. Reduced skin and proximity effects also lower effective resistance. Both reduce impedance, and therefore voltage drop, for the same current.
Is sandwich busway always more compact?
Generally yes for a given rating, because conductor spacing in air-insulated construction is governed by clearance and creepage requirements while sandwich construction spacing is governed by insulation thickness. Confirm against the specific products being compared.
Does construction affect short-circuit rating?
Yes. Sandwich construction’s composite bar/insulation/housing structure resists electromagnetic forces better than bars supported on discrete insulators, so it typically achieves higher Icw and Ipk. Always compare declared values against the calculated fault level.
Which dissipates heat better?
Sandwich construction generally, because the bars conduct into the housing, making its whole surface a heat sink. The corollary is that enclosure and ventilation conditions affect sandwich construction more directly — check the derating data for the actual installation.
Can I mix the two in one installation?
Sometimes, where the manufacturer confirms the transition is acceptable and details how it is made. Do not assume it. If not permitted, rate the run on the more restrictive construction throughout.
How do I verify the claimed performance?
Ask for the type-test documentation and the declared values — rated current, Icw, Ipk, impedance or voltage drop per unit length — from the standard the product is declared against, such as IEC 61439-6. Declared values should be traceable to a test report, not taken from a summary brochure.
Select the Right Busway Construction with BANGE Electric
BANGE Electric supplies both sandwich-type and air-insulated busway with declared ratings, impedance data, and Icw/Ipk values, verified to IEC 61439-6. Send us your rating, route length, voltage drop limit, prospective fault current, and installation conditions, and we will compare both constructions against your actual constraints rather than against a general preference.
Review busbar sizing or contact our technical team.
Requirements may vary by application, market, and applicable standard. Figures given here are illustrative examples; always confirm against manufacturer data and your project specification.