March 21, 2024 – Making a mistake when estimating the length, size, and/or type of wire can cause significant cost overruns and delays, require additional labour and, potentially, eliminate any profit—or even cause a financial loss—on the job.

You want to get this right.

Start by reviewing the civil and mechanical drawings for any structures or equipment that we will need to work around. Only then can we takeoff the installation of conduit and wire to connect the myriad of boxes, panels, and equipment indicated on the drawings.

To put together a takeoff that ends up being close to the actual installation onsite, you must use all the documents provided—in combination with your own common sense and experience.

Follow the specs with regard to the size and type of wire or cable (if provided) and ensure it complies with the electrical code for your jurisdiction. If you are undertaking design-build work, and the wire or cable is not specified, then you need to perform the necessary code-compliant calculations to determine wire and cable requirements.

The most common mistakes when estimating wire and cable include:

• Inaccurate measurements when taking off runs from a drawing.
• Not setting the scale according to the drawing.
• Not allowing drops from the ceiling or rises from the slab to panels and boxes.
• Not allowing sufficient length to make connections in boxes, panels, or equipment.
• Not having the right number of wires—including neutrals and ground wires—where required.
• Missing runs during the takeoff.
• Not following the specs (especially when they show the maximum length of wire for voltage drop before the wire needs to be upsized).

When ordering wire and cable for a job, you need to work closely with your suppliers to get accurate pricing, availability, and timely delivery. Should the specs identify a particular manufacturer, then you need to follow that requirement for pricing.

Remember, an estimate is only a budget: your job is to make it as accurate as possible.

John F. Wiesel is the president of Suderman Estimating Systems Inc., and has been estimating and teaching estimating since the early 1980s. Dan Beresford served as an electrician in the Canadian Navy, then worked in various roles in the electrical sector before joining Suderman.

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