Electrical Bill of Materials Example: Full BOM for a 630A LT Panel

Everybody knows what a BOM is. Nobody agrees what goes in it A bill of materials is meant to be the one document that says what a panel is made of. Purchasing works off it, costing works off it, and the store issues material against it. Get it wrong and the mistake travels all the way to dispatch. The trouble is that it sits between two departments with different priorities. Design thinks about switchgear: the ACB, the MCCBs, the CTs. Production thinks about all the things design never draws, which is cable lugs, ferrules, gland plates, DIN rail, forty-odd metres of wiring duct and the powder coating. So it ends up being built twice. The estimator builds one from the SLD. Then the store builds a second one during production, once it becomes obvious that half the panel was missing from the first. What a small omission actually costs Take a typical LT panel. Bought-out switchgear runs 55–65% of the cost, sheet metal and busbar another 15–20%, wiring and small material 8–12%, and labour and overheads make up the rest. Miss the small material line completely, which happens constantly because almost none of it is on the drawing, and you have under-quoted by roughly a tenth. On a ₹4.5 lakh panel that is ₹45,000. If you were working on 12% net margin, a single omission has taken a third of the profit on that job. Do it on four quotes a month and the year ends up looking quite different from the projection. The slower version of the same problem is the price list. You quote from one that is four months old because nobody circulated the revision. Switchgear prices in India have moved more than once a year for a while now, and no manufacturer is going to send you a note when your margin disappears. You find out at the purchase stage, by which point the PO is signed. The panel this BOM is for None of this means much without a specific panel, so here is the one everything below is built around. 630A, 3-phase, 4-wire main LT distribution panel Supply: 415V ±10%, 50Hz, 3Ph + N + E Fault level: 50kA for 1 second Form of separation: Form 3b Ingress protection: IP54, indoor, floor-mounted, front access Standard: IS/IEC 61439-1 & 2 Incomer: 1 × 800A 4P ACB, EDO, with microprocessor release Outgoing: 2 × 250A MCCB, 3 × 125A MCCB, 4 × 63A MCCB, all 4-pole Metering: 1 × multifunction meter on incomer, ammeter + voltmeter on each 250A feeder Extras: APFC not included, 20% spare space provision Your panel will be different. The section structure still works, so change the ratings and keep the headings. The BOM, section by section Section 1 – Incomer # Description Make Qty Unit 1.1 ACB 800A, 4P, EDO, 50kA, with microprocessor release (LSIG) L&T / Schneider / Siemens 1 No 1.2 ACB accessories: shunt trip coil, spring charging motor, aux contacts 4NO+4NC Same as 1.1 1 Set 1.3 Current transformer, 800/5A, Class 1, 15VA, resin cast AE / Kappa 3 No 1.4 Multifunction meter, 96×96, RS485 Modbus Selec / Secure / Schneider 1 No 1.5 Control MCB, 6A, 2P, C-curve 2 No 1.6 Indicating lamps, 22.5mm LED, R/Y/B + ON/OFF/TRIP 6 No 1.7 Surge protection device, Type 2, 40kA 1 No Line 1.2 is the one that goes missing. Every manufacturer quotes ACB accessories separately and they add somewhere between 8 and 15% to the breaker cost. If the SLD says "ACB with shunt release" and your BOM just says "ACB", you are short before you have even started. Section 2 – Busbar system # Description Spec Qty Unit 2.1 Aluminium busbar, EC grade, main horizontal 630A rating, per current density 42 kg 2.2 Aluminium busbar, vertical droppers 18 kg 2.3 Neutral busbar, 50% of phase 9 kg 2.4 Earth busbar, GI flat 50×6mm 6 m 2.5 Heat shrink busbar sleeving, colour coded 24 m 2.6 SMC/DMC busbar support insulators 4-way 12 No 2.7 Busbar hardware: bolts, nuts, spring washers, Belleville M10/M12 1 Lot Busbar is priced by weight, and weight is where two estimates start to drift apart. A 630A aluminium bar sized at 0.8 A/mm² is a different section from the same bar at 1.0 A/mm². Two estimators in the same office can quote the same panel fifteen kilos apart, and it is almost always because nobody ever wrote the current density down as a house standard. Pick a figure, put it in writing, and that argument goes away permanently. Section 3 – Outgoing feeders # Description Rating Qty Unit 3.1 MCCB, 4P, thermal-magnetic, 36kA 250A 2 No 3.2 MCCB, 4P, thermal-magnetic, 36kA 125A 3 No 3.3 MCCB, 4P, thermal-magnetic, 25kA 63A 4 No 3.4 Extended rotary handle with door interlock 9 No 3.5 Current transformer, Class 1 250/5A 6 No 3.6 Analogue ammeter, 96×96 + selector switch 2 Set 3.7 Analogue voltmeter, 96×96 + selector switch 2 Set 3.8 Cable lugs, aluminium, tinned Per feeder size 40 No Line 3.4 is the classic miss. Rotary handles never show up on an SLD because they are a mechanical requirement, not an electrical one. Nine of them at current prices is not a rounding error. Section 4 – Enclosure and sheet metal # Description Spec Qty Unit 4.1 CRCA sheet, 2mm, frame and load-bearing members 185 kg 4.2 CRCA sheet, 1.6mm, doors, covers, gland plates 95 kg 4.3 Powder coating, 7-tank pre-treatment, RAL 7032, 60–80 micron 280 kg 4.4 Door hardware: hinges, locks, gaskets, earthing braids 1 Set 4.5 Ventilation louvres with filter 4 No 4.6 Base channel, ISMC 75 6 m 4.7 Danger plate, name plate, mimic, ferrule strip 1 Set Section 5 – Wiring, control and small material # Description Spec Qty Unit 5.1 Control wiring, 2.5 sq mm FRLS copper, multi-colour 300 m 5.2 CT wiring, 2.5 sq mm 120 m 5.3 PVC wiring duct, 40×60 slotted 45 m 5.4 DIN rail, 35mm 12 m 5.5 Terminal blocks, screw type, 4 sq mm 120 No 5.6 CT shorting terminal blocks 24 No 5.7 Ferrules, ring type crimping lugs, cable ties, spiral binding 1 Lot 5.8 Panel interior light with door switch, 5A socket 1 Set 5.9 Space heater 60W with thermostat 2 Set This section is 8 to 12% of the panel cost and hardly any of it appears on a single line diagram. A BOM that leaves it out is really just a switchgear list with a different heading on top. Section 6 – Non-material costs # Description Basis 6.1 Fabrication labour Hours × shop rate 6.2 Wiring and assembly labour Hours × shop rate 6.3 Routine testing as per IEC 61439-2 Lump sum 6.4 Drawings, GA, wiring schematic, O&M manual Lump sum 6.5 Packing, forwarding, freight % of material 6.6 Factory overhead % of direct cost 6.7 Contingency / price escalation cover 2–3% 6.8 Margin Your call Where the money sits For a panel of this type, expect something close to: Bought-out switchgear: 58% Sheet metal, powder coating, busbar: 19% Wiring and small material: 9% Labour: 8% Testing, documentation, freight, overhead: 6% Two things follow from that split. Switchgear dominates, so an outdated price list will cost you more than a wrong labour estimate ever does. And the 9% small material line is close to pure omission risk, since it is the least visible part of the panel and the easiest thing to leave out of a hurried quote. The 12 items estimators forget most often ACB and MCCB accessories (shunt trip, aux contacts, spring charging motor) Extended rotary handles and door interlocks CT shorting terminal blocks Cable lugs sized per feeder Gland plates, detachable and undrilled Panel interior lighting and utility socket Space heaters and thermostats Earthing braids on every hinged door Neutral link and earth bar hardware Danger plates, mimic diagram, ferrule printing Routine test charges and third-party inspection Freight, unloading and site handling Print it and stick it on the wall near the estimation desk. It costs nothing and it will catch something before the quarter is out. List price is not your price Every brand in the tables above publishes a list price, and nobody pays it. What you actually pay is that number minus a discount, and the discount varies by brand, by product family, by how much you buy in a year and sometimes by the specific project. Which means a BOM with no discount structure sitting behind it is only half an estimate. You need three numbers against every item: the current list price, the discount that applies to you, and the net rate that comes out of those two. Hold only the net rate and you lose track of margin the moment list prices move. Hold only the list price and you are quoting at list, which means losing every bid you send. Doing all this in Excel All of it can be maintained by hand, and plenty of shops do exactly that. The cost is that one person ends up as the keeper of the master file. Prices get updated whenever they remember. The checklist of twelve forgotten items lives in their head rather than anywhere useful, and the week they are on leave, the quotes go out thinner. Switchonomy was built to move that structure out of the spreadsheet. You upload the single line diagram , the software reads it and produces the BOM, including the accessory and small material lines that were never on the drawing. Pricing comes from a library of more than 12,000 switchgear items that is kept current. You review it, change what needs changing, and the quotation comes out the other end. The structure in this article is the same structure the software follows. Build it yourself in Excel using the tables above and you will still be ahead of most of your competition. If you would rather not rebuild it from scratch every time, start a free Switchonomy project and upload your next SLD. Frequently asked questions What is an electrical bill of materials? It is the itemised list of every component, material and consumable needed to build an electrical panel or assembly, with quantities, specifications and preferred makes against each line. Costing, purchasing and production all work off it. What's the difference between a BOM and a panel schedule? A panel schedule lists circuits and their ratings. A BOM lists physical items you have to buy. One describes what the panel does, the other describes what goes into it. How detailed should an electrical BOM be? Detailed enough that purchasing can raise POs without calling you back, and production can build without a trip to the store for one more item. In practice that means make, model, rating and quantity on every line. "MCCBs, 9 nos" is not a BOM line. Can a BOM be generated automatically from an SLD? For the switchgear and feeder content, yes. AI tools read single line diagrams and pull out ratings, poles and breaking capacity reliably enough to work with. Small material and accessories are a separate problem, because they were never drawn in the first place. A good tool adds those from templates and rules instead of pretending to find them on the drawing. Which is better for panel BOMs, Excel or software? Excel is fine at low volume with one estimator. It stops working when you have three estimators, prices that move every few months, and more enquiries than one person can rebuild by hand. Excel doesn't fail because it is slow. It fails because five versions of the master file end up on five different laptops.