By Sarah Xiao 
A switchgear auxiliary material takeoff is built from per-panel quotas × cabinet count × a loss factor, then rounded up to pack size. The rules below turn that sentence into 25 line items: wiring duct from cabinet geometry, lugs from circuit count, ferrules and ferrules markers from control-wire count, and fasteners from device count. Applied to the 25-panel project compiled in the source workbook, the method totals CNY 14,539, or CNY 581.56 per panel in auxiliary materials alone.
Key Takeaways
- Separate geometric items (duct, rail, lugs, ferrules, terminal blocks, markers, ties) from empirical items (bolts, insulators, barriers, PE bar). Only the first group can be derived from drawings.
- Net quantity → purchase quantity → suggested order quantity are three different numbers. Collapsing them is the most common estimating error.
- Loss factors are not a safety margin you can invent per project — use fixed values by material class (1.05 to 1.12 below).
- Cost per panel is a function of cabinet mix, not project size. In the dataset behind this article, a 24-panel project costs more per panel than a 94-panel project.
- Five line items carry about 60% of auxiliary material spend; the visible consumables (ties, tape, markers) are almost irrelevant to cost.
1. Scope: What Counts as Auxiliary Material
In LV switchgear estimating, auxiliary material means the installation consumables inside the enclosure — wiring duct, DIN rail, terminals, lugs, ferrules, markers, ties, conduit, earth bar, insulators, fasteners, labels, grommets.
It explicitly excludes:
- primary and secondary devices (breakers, contactors, meters, CTs, capacitors)
- cable trays, conduits and cables outside the enclosure
- main earthing conductors and busbars
That boundary matters because the same word (“BOM”) is used loosely. A switchgear BOM in the procurement sense includes devices; this takeoff covers only what a panel shop consumes to build the enclosure, which is typically the number estimators underestimate.
| Standard | What it governs in this takeoff |
|---|---|
| GB/T 7251.1-2013 (aligned with IEC 61439-1) | Temperature rise, clearance and creepage, internal conductor cross-section, protective circuit continuity |
| GB 50171-2012 | Installation and acceptance of panels; secondary wiring, terminal connection, wire marking, earthing |
| GB 50303-2015 | Site acceptance of electrical installations |
| GB 50054-2011 | LV distribution design; conductor selection and protective coordination |
| GB 50052-2009 | Power supply system design |
| JB/T 2436.1 | Copper crimp terminals for conductors |
| GB/T 14315 | Compression-type copper and aluminium terminals and connectors |
| IEC 60715 | 35 mm standard mounting rail |
Editions above follow the source workbook. Confirm the currently effective edition before quoting them in a tender document.
2. The Five-Step Calculation Model
- Per-panel theoretical quantity — derived from enclosure geometry and circuit count: wiring duct, copper lugs, ferrules, terminal blocks, markers, ties.
- Per-panel empirical quota — items that cannot be derived geometrically (bolts, insulators, barriers, earth bar, grommets) are set by process experience.
- Project net quantity = Σ (cabinet quantity × per-panel quota).
- Purchase quantity = net quantity × loss factor.
- Suggested order quantity = ROUNDUP (purchase quantity ÷ pack size) × pack size, so the order matches how material is actually sold — by length, pack or reel.
Step 5 is where paper estimates meet reality. A requirement of 118.5 m of 40×40 duct against a 2 m pack becomes 120 m; 2,310 ferrules against a 1,000-piece box becomes 3,000. Skip this and the purchase order bounces.
3. Per-Panel Quotas by Cabinet Type
Eight cabinet types cover most LV work: GGD incoming (C1), GGD feeder (C2), GCK drawer (C3), XL-21 distribution box (C4), GGJ capacitor (C5), dual-source (C6), terminal lighting box (C7), VFD/PLC (C8).
| Type | W × H (m) | Outgoing circuits | Control wires | 40×40 duct (m) | 35 mm rail (m) | Cu lugs (pcs) | Ferrules (pcs) | Terminals (pcs) |
|---|---|---|---|---|---|---|---|---|
| C1 GGD incoming | 0.8 × 2.2 | 2 | 40 | 5.0 | 1.4 | 14 | 80 | 24 |
| C2 GGD feeder | 0.8 × 2.2 | 6 | 60 | 7.5 | 1.4 | 28 | 120 | 32 |
| C3 GCK drawer | 0.8 × 2.2 | 9 | 110 | 10.7 | 4.6 | 40 | 220 | 60 |
| C4 XL-21 | 0.6 × 1.8 | 8 | 25 | 3.8 | 1.6 | 36 | 50 | 32 |
| C5 GGJ capacitor | 0.8 × 2.2 | 8 | 50 | 7.5 | 1.4 | 36 | 100 | 40 |
| C6 Dual source | 0.8 × 2.2 | 1 | 70 | 5.0 | 1.6 | 16 | 140 | 20 |
| C7 Terminal box | 0.45 × 0.6 | 0 | 10 | 0 | 0.8 | 0 | 20 | 4 |
| C8 VFD / PLC | 0.8 × 2.2 | 6 | 220 | 10.0 | 6.0 | 28 | 440 | 84 |
The derivation rules behind those numbers:
- Wiring duct: horizontal runs = (cabinet width − 0.1 m) each; vertical runs = (cabinet height − 0.4 m) each, allowing for top and bottom mounting plates. Overlaps are not double counted.
- Copper lugs: 4 per outgoing circuit (L1/L2/L3 + PE), plus incoming terminals on incoming and dual-source cabinets. Terminal boxes use busbars or studs, so lugs are zero.
- Ferrules: 2 per control wire (one at each end). The 10% spare lives in the loss factor, not here.
- Terminal blocks: 4 per outgoing circuit, plus a cabinet-type base for external connections, metering and control feed-through.
- Wire markers: 25 mm per wire end.
- Cable ties: 1.6 per control wire (one tie every 0.3–0.5 m of bundle).
- Corrugated conduit: 1–2 m for door-to-enclosure crossing; 0.5 m per drawer unit for moving harnesses.
- Bolts: 4 sets per device, plus busbar joints split evenly between M8 and M10.
A drawer cabinet is the material-heavy outlier: nine outgoing circuits and 110 control wires push it to 176 ties, 220 ferrules, 10.7 m of duct and nine secondary-circuit plug-in sets — roughly 1.9× the tie count of a feeder cabinet, before the plug-ins.
4. Loss Factors and Pack Rounding
| Material class | Loss factor |
|---|---|
| Wiring duct, DIN rail, terminal blocks | 1.08 |
| Copper lugs, ferrules, terminal accessories, bolts, ties, markers | 1.10 |
| Markers/ferrule tubing, earth wire, heat-shrink, spiral wrap, corrugated conduit | 1.12 |
| Copper bar, insulators, barriers, drawer plug-ins | 1.05 |
Loss factors follow cutting waste and handling loss, not commercial buffer. Feeding 1.12 into a copper-bar line inflates cost without improving supply; feeding 1.05 into marker tubing guarantees a shortage.
5. Worked Example: A 25-Panel Mixed Project
Quantities below are the net figures from the workbook’s project summary sheet, carried through loss factors and pack rounding.
| Item | Unit | Net | Loss | Purchase | Pack | Order | Unit price (CNY) | Amount (CNY) |
|---|---|---|---|---|---|---|---|---|
| Wiring duct 40×40 | m | 109.7 | 1.08 | 118.5 | 2 | 120 | 8 | 960 |
| Wiring duct 60×60 | m | 26.4 | 1.08 | 28.5 | 2 | 30 | 14 | 420 |
| 35 mm DIN rail | m | 39.2 | 1.08 | 42.3 | 1 | 43 | 6 | 258 |
| Copper lugs DT/SC | pcs | 462 | 1.10 | 508.2 | 100 | 600 | 4.5 | 2,700 |
| Ferrules | pcs | 2,100 | 1.10 | 2,310 | 1,000 | 3,000 | 0.15 | 450 |
| Terminal blocks | pcs | 600 | 1.08 | 648 | 100 | 700 | 1.2 | 840 |
| Cable ties | pcs | 1,680 | 1.10 | 1,848 | 100 | 1,900 | 0.05 | 95 |
| Galvanized bolts M6–M10 | sets | 1,650 | 1.10 | 1,815 | 50 | 1,850 | 0.8 | 1,480 |
| Insulators / busbar clamps | pcs | 133 | 1.05 | 139.7 | 20 | 140 | 6 | 840 |
| Barriers (DMC/SMC) | ㎡ | 10.0 | 1.05 | 10.5 | 1 | 11 | 120 | 1,320 |
| PE bar TMY 30×4 | m | 32.2 | 1.05 | 33.8 | 3 | 36 | 55 | 1,980 |
| Drawer secondary plug-ins | sets | 27 | 1.05 | 28.4 | 1 | 29 | 45 | 1,305 |
| All 25 items | 14,539 |
That is CNY 581.56 per panel. Note where the money sits: copper lugs (18.6%), PE bar (13.6%), bolts (10.2%), barriers (9.1%) and drawer plug-ins (9.0%) account for roughly 60% of the total. Cable ties — 1,900 pieces — cost CNY 95, or 0.7%.
6. Why Cost Per Panel Is Not a Benchmark
The same workbook models four project scales. Auxiliary cost per panel does not fall as projects get bigger:
| Scenario | Panels | Cabinet mix | Total (CNY) | Per panel (CNY) |
|---|---|---|---|---|
| A Single sample | 1 | 1 feeder | 672 | 672.38 |
| B Small switch room | 11 | 1 in, 4 feeder, 1 cap, 1 dual, 4 terminal | 5,126 | 465.98 |
| C Mid-size plant | 24 | 2 in, 8 feeder, 5 drawer, 3 XL-21, 2 cap, 1 dual, 3 terminal | 17,849 | 743.72 |
| D Multi-building | 94 | 4 in, 18 feeder, 12 drawer, 10 XL-21, 4 cap, 3 dual, 40 terminal, 3 control | 50,070 | 532.66 |
Scenario C costs nearly 40% more per panel than scenario D, despite being a quarter of the size, because five drawer cabinets and three XL-21 boxes carry far more auxiliary material than terminal boxes. Normalize by cabinet mix before comparing quotes — otherwise a lean bid and a padded bid look identical per panel.
7. Common Errors
- Quoting net quantities. No loss factor, no pack rounding, and the site runs short in week three.
- Deriving everything geometrically. Bolts and insulators are not on the drawing; they are process experience. Both methods belong in the same sheet.
- Using one quota for all cabinets. A drawer cabinet and a terminal box differ by a factor of ten in consumables.
- Ignoring bar ownership. Tooling, drawings and fixtures need an ownership clause, otherwise switching suppliers later means rebuilding tooling.
- Treating the sheet as a design document. It is an estimating tool; short-circuit withstand and temperature rise must be verified separately.
8. FAQ
What is the difference between auxiliary material and main components in a switchgear BOM?
Auxiliary material is what the panel shop consumes to build the enclosure: duct, rail, terminals, lugs, ferrules, markers, ties, conduit, earth bar, insulators, fasteners, labels. Main components are the devices that carry or switch current — breakers, contactors, meters, CTs. Auxiliary material typically runs 5–10% of a panel’s material cost and is the line most often under-estimated.
How do you calculate wiring duct length per panel?
For each horizontal run, take cabinet width minus 0.1 m; for each vertical run, cabinet height minus 0.4 m (top and bottom mounting plates). Sum the runs and do not count overlaps twice. A 0.8 × 2.2 m cabinet with two horizontal and two vertical runs comes to 5.0 m.
Why round the order quantity up to pack size?
Because material is sold by length, pack or reel. A purchase quantity of 2,310 ferrules becomes a 3,000-piece order; 118.5 m of duct becomes 120 m against a 2 m pack. Skipping this step produces orders suppliers cannot fill as written, and site teams that improvise.
Why does auxiliary cost per panel vary so much between projects?
Cabinet mix, not project size, drives the number. In the dataset here, a 24-panel project with five drawer cabinets costs CNY 743.72 per panel, while a 94-panel project dominated by terminal boxes costs CNY 532.66. Benchmarking requires normalizing for mix.
Are these quotas valid for IEC-only assemblies outside China?
The geometry-based rules (duct, lugs, ferrules, terminals, markers) travel well because they follow circuit count and enclosure dimensions. The empirical quotas and the standards table are tied to GB practice and the source workbook; for IEC 61439 assemblies, re-verify temperature rise, clearance and creepage requirements, and localize unit prices and pack sizes.
Conclusion
A reliable switchgear auxiliary material takeoff is not a price list — it is three rules applied consistently: derive what geometry allows, apply process quotas to what it does not, then multiply by a class-specific loss factor and round to pack size. Run it once properly and the per-panel number stops moving between estimators, which is the real value: comparable bids, no mid-project shortages, and a defensible cost baseline.
This method is an estimating tool, not a design document. Explosion-proof, fire-rated, marine, high-altitude and corrosive environments, plus short-circuit thermal and dynamic withstand verification, require review by suitably qualified designers. Busbar and main conductor sizing must follow GB/T 7251.1 temperature-rise testing and GB 50054 short-circuit verification, and is outside this scope. Unit prices are 2026 East China reference rates — replace them with local quotations before issuing a bid.