Electrical Services Measurement
Electrical measurement is repetitive but exacting: the same circuit generates conduit, cable, boxes and accessories, and each is a separate item. This domain tests whether you measure cable by core length and whether you carry accessories across circuits without losing count.
Units
- Conduit and trunking in m by size and material; cable tray and ladder in m; cable in m by conductor size and number of cores.
- Wiring accessories, luminaires, distribution boards, switches, sockets and control gear are enumerated.
- Earthing and bonding in m and nr; testing and commissioning as items.
Measurement rules
- Cable is measured as fixed along the route, with slack and loops included in the rate; no waste is added to the quantity.
- Switch drops are short vertical runs; they are measured once per point and must not be double counted with the horizontal circuit.
- Builders' work - chases, boxes, fire seals, penetrations and supports - is measured or expressly included, and the preamble must say which.
Worked example
30 luminaires, each with a switch drop of 3.000 m of 20 mm conduit and 3-core 1.500 mm² cable, plus a 25.000 m submain of 4-core 25 mm² cable. Conduit = 30 × 3.000 = 90.000 m of 20 mm. Cable for the drops = 30 × 3 cores × 3.000 m = 270.000 m of 1.500 mm². Submain = 25.000 m of 25 mm² 4-core cable. Luminaires = 30 nr; switches = 30 nr. Each core counts toward the cable length, so a 3-core drop of 30 m produces 90 m of cable; the earth conductor is not omitted.
Traps that cost marks
- Counting a multicore cable as a single length regardless of cores.
- Omitting the earth conductor from the cable measurement.
- Forgetting testing and commissioning.
- Mixing conduit and trunking sizes into one item.
- Missing accessories at each point, or counting them twice through the switch drop.
Bills of Quantities, Preliminaries, Preambles and Measurement-Related Contract Administration
This domain ties measurement to money. It tests whether you know how a bill is structured, what belongs in preliminaries rather than measured work, and how measured quantities become certified payments at valuation.
Structure of a bill of quantities
- Preliminaries, preambles, measured works by trade, daywork schedule, provisional sums, prime cost sums and contingency.
- Preliminaries are time-related and site-related: site establishment, supervision, insurance, bonds, temporary works, traffic management, cleaning and handover. They are priced as a section, not spread silently through trades.
- Preambles state the method of measurement, any deviations, description conventions and what rates are deemed to include.
Contract administration
- Interim valuations are measured on site against the bill; retention is typically 5%, often capped, and certificates are issued within the period stated in the contract.
- Variations are valued in a fixed order: bill rates first, then rates pro-rated from bill rates, then dayworks or a fair valuation.
- Provisional sums are expendable and valued on instruction. Prime cost sums relate to nominated suppliers and are adjusted by the difference between the PC sum and the actual invoice.
Worked example
Contract sum €1,000,000. Valuation 1 is €200,000; retention at 5% = €10,000, giving a net certificate of €190,000. A variation adds 15.000 m³ of concrete at the bill rate of €120/m³ = €1,800. The variation is added to the gross valuation before retention, so it is also subject to the 5% retention. Retention applies to the gross value including variations, and the order in which valuation methods are applied is fixed by the contract, not chosen by the surveyor.
Traps that cost marks
- Confusing provisional sums with prime cost sums; they are adjusted differently.
- Omitting preliminaries from a tender comparison so the cheapest tender looks different from the cheapest outturn.
- Applying retention after variations instead of to the gross valuation.
- Failing to state the method of measurement in the preambles, leaving rates open to dispute.
- Valuing a variation by negotiation when bill rates or pro-rata rates exist.