Electrical Takeoff from 2D Drawings: Workflow, Checklist and Quality Checks

Read symbols against the project's legend, not against memory. Standard symbol sets exist: the IEC maintains the IEC 60617 database of symbols for electrotechnical diagrams, and IEEE 315 served the same purpose in North America before being made inactive-reserved in 2019. Building layouts rarely follow either strictly. Design firms keep their own symbol libraries and add suffixes, subscripts and note tags that change what a symbol means on this project.

Four legend checks that catch most misreadings

• Suffixes and subscripts. The same downlight symbol with "EM", a circuit letter or a type tag is a different item. List every suffix found on the plans and match it to the legend.
• Symbols used but not in the legend. Each one is an RFI, logged with the sheet and location.
• Symbols in the legend but not used. Harmless, unless a schedule lists quantities for them: then something is missing from the plans.
• Notes that change quantities. "Typical of 6 units", "mirror of level 2", "provide in all corridors": notes carry quantities that no symbol count will find.

A total without a location cannot be reviewed. Define zones before counting: a floor, a wing, a grid range, or the zones the client's BOQ uses if it has them. Then count each symbol type per sheet and per zone, and keep those sub-totals on the takeoff sheet. When a reviewer disagrees with a total, the zone sub-totals show where to look.

• Count one system at a time (lighting, then power, then containment) so a device is never counted on two layers.
• Handle typical floors explicitly: count the typical floor once, record the multiplier and the sheets it applies to, and count the exceptions separately.
• Mark counted symbols on the drawing (a layer, a colour, a stamp) so the reviewer sees what was counted and what was not.
• Keep a sheet log: sheet number, revision, scale verified, systems counted, date, estimator.

Lengths are where 2D takeoffs are weakest, because electrical drawings often show a device and a destination but not the path between them. Split every length into what was measured and what was assumed, and write the assumptions down in words the client can check.

• Measured. Containment and conduit routes that are actually drawn, measured at a verified scale along the drawn path.
• Derived. Vertical drops from ceiling height (from a section or reflected ceiling plan) to device height (from the specification or a mounting note). The inputs are documented, so the result is checkable.
• Assumed. Home runs drawn as an arrow to a panel, feeders with no route, allowances for terminations and waste. These are your estimate, stated as one: for example, "home run length = measured distance to the panel location along the corridor containment, plus drops at both ends". The percentage you add for waste is a company rule, not a standard.

Home runs are the largest of these assumed lengths on most jobs. The guide to electrical home run calculation works one through, with the voltage drop, conduit fill and derating checks its length drives.

Every takeoff has gaps. What separates a defensible bid from a risky one is whether those gaps are visible. Give each one a status and a decision:

• To confirm. The item is counted but its meaning or type is uncertain. It carries an RFI number and a pricing assumption.
• Missing. The item is required (by a schedule, a single-line or the specification) but not shown on any plan. It carries an allowance, clearly labelled.
• Conflict. Two documents disagree. It carries the question and the assumption you priced on.
• Excluded. Deliberately not priced, with the reason, and listed in the bid's exclusions.

The bid qualifications then list every assumption and allowance that remains open at the submission date, in the same words as the takeoff sheet.

Level 1: the estimator's own checks, before handing over

• Every sheet in the register appears in the sheet log, with revision and verified scale
• Every symbol and suffix found on the plans maps to a legend entry or an RFI
• Zone sub-totals add up to the item totals
• Typical floor multipliers and their sheets are recorded
• Every length line says measured, derived or assumed
• Every open question has a status, an RFI number and a pricing assumption

Level 2: an independent review

• Recount on the drawings, not on the takeoff sheet: the top items by value, every 'to confirm' line, and a random sample of the rest
• Remeasure two or three drawn routes at the recorded scale
• Re-derive one vertical drop and one home run from the stated assumptions
• Check the reconciliation against the specification and the BOQ
• Record differences by zone, agree the correction, and keep the review with the takeoff

Set your tolerance in advance: for example, which differences on a count trigger a full recount of that zone. The threshold is a company decision; having one written down is what makes the review consistent between estimators.

A 2D set supports a sound device count and measured drawn routes. It does not give you coordinated three-dimensional routes, clash-free containment, exact vertical routing in risers, or conditions hidden in existing buildings. When those drive the price, ask for what would remove the uncertainty (a model, a survey, a site visit) or carry it explicitly as an allowance.

The reverse also holds: a model is only as complete as what was modelled. Check whether wiring, small containment and supports were modelled before trusting its quantities over a careful 2D takeoff.

Mirage deploys agents that read a plan set, extract quantities and link each one to the sheet and location it came from, for the estimator to check before anything reaches a bid; the electrical takeoff page describes that workflow. The published deployments are on other trades: at OCBM, a Moroccan general contractor, on formwork and structural drawings with every quantity traceable to its source; at John Cockerill, on piping, boilermaking and structural steel drawings. Mirage has not published an electrical deployment. The fair test for an electrical lot is the protocol above, run on one of your own closed bids.