By SupplycZar editorial team · Published 21 September 2026 · 7 min read
Electrical purchasing for a fit-out often goes wrong for a dull reason: the RFQ says “MCB 16 A, 20 nos” and nothing else. Every vendor then prices a different product. Start from the load, and most of the specification writes itself.
Start from the load schedule and circuit list
Before ordering anything, get a load schedule: every circuit, what it feeds, its connected load, whether it is single- or three-phase, and the run length from the distribution board (DB). Your electrical engineer or MEP consultant normally owns this. MCBs, cables and the DB are all sized from it, not from what the last office used.
A circuit list ties procurement to the drawing: DB name, circuit number, load type, MCB rating and curve, cable size and length. If a line in your RFQ cannot be traced to a circuit on that list, question it before it goes out.
MCBs: rating, curve and breaking capacity
An MCB is defined by three numbers that people routinely confuse. Rated current (commonly 6, 10, 16 and 32 A) protects the cable downstream from overload, so it must not exceed what the cable can carry. Curve (B, C or D) sets how much short surge the MCB tolerates before it trips instantly. Breaking capacity (commonly 6 kA or 10 kA in building DBs) is the fault current the MCB can safely interrupt.
- B curve: trips at roughly three to five times rated current. Suits lighting and general socket circuits with little inrush.
- C curve: trips at roughly five to ten times rated current. The usual choice for mixed loads, air conditioners and equipment with a switching surge.
- D curve: trips at roughly ten to twenty times rated current. For loads with heavy inrush, such as some motors and transformers.
Breaking capacity must be at least the prospective short-circuit current where the MCB is installed. That figure comes from your engineer or the supply authority, not from a catalogue. MCBs are covered by IS/IEC 60898-1; check the current edition, and look for conformity on the product marking.
| Circuit type | Typical MCB rating | Typical curve | Typical copper size |
|---|---|---|---|
| Lighting | 6 A or 10 A | B | 1.5 mm² |
| General socket outlets | 16 A | B or C | 2.5 mm² |
| Split or cassette AC (single-phase) | 16 A or 20 A | C | 2.5 mm² or larger |
| Water heater or pantry equipment | 16 A | B or C, with RCBO | 2.5 mm² |
| IT and server-room circuits | 16 A or 32 A | C | 2.5 mm² to 4 mm² |
| Motors and pumps | As per motor rating | C or D, as advised | As per motor and run length |
Earth-leakage protection and DB sizing
An MCB protects against overload and short circuit. It does not protect a person from earth leakage. That is the job of an RCCB (residual current circuit breaker) or an RCBO, which combines an MCB and residual-current protection in one unit. A 30 mA rating is the conventional choice for personal protection on socket and wet-area circuits.
An RCCB usually protects a group of circuits and needs overcurrent protection upstream. An RCBO protects a single circuit, so a leakage trip takes out one circuit rather than a whole group. The trade-off is cost and DB space. For circuits you cannot afford to lose, such as server rooms, ask your engineer whether individual RCBOs make sense.
On the DB itself, count the ways: a single-pole MCB takes one module, a three-pole unit takes three, and the incomer, RCCBs, surge protection and neutral and earth bars all take space. Check that the busbar rating matches the incomer and that the enclosure suits the location, for example a suitable IP rating in a plant room or basement.
Leave spare ways, covered with blanking plates, for circuits added after handover. How many is a design decision for your engineer. A DB filled to the last module cannot take a new circuit without replacing the board, which is far more disruptive than buying a larger one at the start.
Cables: copper, FR/FRLS, size and voltage drop
For final circuits in offices, copper conductors are the usual choice. PVC-insulated cables commonly follow IS 694, and heavy-duty PVC-insulated cables, including armoured types, follow IS 1554 (Part 1). Check the current editions. In occupied buildings, ask your engineer whether flame-retardant (FR) or flame-retardant low-smoke (FRLS) cables are required by the project’s fire design; National Building Code of India 2016, Part 4, is the reference, and your fire consultant should confirm.
Conductor size is not only about amps. Two checks apply: current-carrying capacity, which depends on conductor size, insulation, installation method (conduit, tray, bunched cables) and ambient temperature, and voltage drop over the length of the run. A long run to a far corner can need a larger conductor than the load alone suggests. Ask the engineer for the size per circuit rather than deriving it from the MCB rating.
Use one colour scheme across the whole building, as specified by your engineer in line with IS 732 (check the current edition), and label circuits at the DB. Earth conductors are conventionally green or green/yellow. The earthing system itself (IS 3043; confirm the current edition), including continuity and testing, is designed, installed and tested by qualified engineers and licensed electricians. Procurement’s part is to make sure the RFQ includes the conductors, lugs, ferrules and earth bars that design calls for.
What to write in the RFQ
Electrical RFQ line items
- Circuit or DB reference each line serves
- MCB: poles (SP, DP, TP or TPN), rated current, curve, breaking capacity, and IS/IEC 60898-1 conformity
- RCCB or RCBO: rated current, poles, sensitivity in mA, and type
- DB: number of ways, single or three-phase, incomer, busbar rating, enclosure and IP rating, spare ways
- Cable: conductor material, cores, size in mm², insulation and sheath (PVC, FR, FRLS), applicable IS standard, quantity in metres or drums
- Accessories: lugs, ferrules, glands, conduit, earth bars, labelling
- Make and series, or “equivalent to” a named make, and whether alternates are allowed with a datasheet for approval
- Delivery location, date window, and whether delivery is phased
If you have a site brand standard, name the make and series. If not, write “equivalent to” and give the performance parameters so the comparison is testable. Our guide on approving substitutes covers the process, and how to write an RFQ vendors can price covers the wider format.
What to check on delivery
Goods-received checks
- MCB markings (rated current, curve letter, breaking capacity, voltage, manufacturer, standard reference) match the purchase order
- ISI mark present on the product or packaging where the product category carries it; check current BIS requirements for the item
- Batch or date code legible and consistent across the lot
- Packaging sealed, undamaged and not re-labelled
- Cable drum or coil label matches size, cores, length and standard; ends sealed; print on the insulation consistent
- Tax invoice with GSTIN and HSN matching the order
Hold anything that does not match rather than let it reach the DB. Installation, testing and energising are for licensed electricians.
How SupplycZar can help
SupplycZar supplies electrical supplies, including protection devices and cables, and can quote against your circuit list and specification. Where a specified make cannot be fulfilled, we propose suitable alternatives against your specification for your approval before ordering. Electrical work itself is available through our electrical services, with scope confirmed against each site. For spare MCBs and cable stock after handover, see MRO procurement, or send your list for a quote.
Frequently asked questions
What MCB rating do I need for an office socket circuit or split AC?
It depends on the connected load, the cable size and the length of the run, because the MCB protects the cable, not just the appliance. A 16 A MCB on 2.5 mm² copper is a common starting point for a general socket circuit or a single split AC, but your electrical engineer should confirm each circuit.
What is the difference between an MCB, an RCCB and an RCBO?
An MCB protects a circuit against overload and short circuit. An RCCB detects earth leakage and trips to protect people, and an RCBO combines both functions for a single circuit.
Do I need FRLS cables for an office?
It depends on the project’s fire design and applicable requirements. Many occupied commercial buildings specify FR or FRLS cables, so confirm the requirement with your MEP or fire consultant against National Building Code of India 2016, Part 4.
How many spare ways should a distribution board have?
There is no universal number. It depends on expected growth and the layout of the floor, so leave meaningful spare ways with blanking plates and have your engineer confirm the allowance.
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This article is general guidance for facility and procurement teams, not engineering, legal or tax advice. Standards, regulations and site conditions vary — confirm requirements with your qualified engineer, consultant or adviser. Brand names mentioned are trademarks of their respective owners; SupplycZar is not an authorised dealer, distributor or partner of these brands.