Conduit and wire calculator: homerun footage

How conduit and wire are actually taken off

The device count on the floor plan is not the quantity. Conduit and wire come off the homerun lengths from each device group back to the panel, and the plan does not dimension those routes, so the estimator measures or scales them.

The order of work on a real bid is: read the panel schedules to get the circuit count, because that is the number of homeruns you owe; group the devices on the power plan against those circuits; then measure the route from each group back to its panel. The route is not the straight line on the plan. It goes up the wall, into the joist space or above the ceiling, around whatever is already in that space, across to the electrical room, and down into the panel. Vertical rise at both ends is real footage and it is invisible in plan view.

That is why two estimators can agree on every symbol on the sheet and still be far apart on pipe and wire. The device count is the easy half. The footage is the half that decides whether the bid holds.

The formulas

Once you have a circuit count and an average homerun length, the arithmetic is short.

Conduit runs = ceiling(circuits / circuits per raceway) Conduit length = conduit runs x average homerun length Conductor length = circuits x conductors per circuit x average homerun length Order quantity = length x (1 + waste and slack / 100)

Conductors per circuit is an input because it changes with the circuit. A 20 A branch circuit commonly carries two current-carrying conductors plus an equipment grounding conductor, so three, but a multiwire branch circuit, a switched leg with travelers, or a 3-phase circuit all give a different number. Set it to what your circuits actually carry, or run the calculator once per circuit type and add the results.

Circuits per raceway is the other input that moves the answer a long way. Give every circuit its own pipe and conduit footage tracks circuit footage. Combine four homeruns into one raceway and the conduit footage drops to a quarter while the conductor footage does not move at all. That trade is where the fill and derating rules bite.

Conduit fill and derating

Conduit fill limits how many conductors fit in a given trade size. The tables live in NEC Chapter 9, which carries the interior dimensions of each conduit and tubing type, the cross-sectional area of each conductor by insulation type, and the fill percentages. The widely published limit is 40 percent of the raceway area where more than two conductors share it. Chapter 9 is what you size against. Do not size pipe off a rule of thumb, because two conductors of the same gauge with different insulation take up different amounts of the raceway, so the gauge alone does not answer the question.

Derating is the other half of the same decision. Once a raceway carries more than three current-carrying conductors, the NEC ampacity adjustment factors in Article 310 apply, and the allowable ampacity of every conductor in that raceway drops. If that drop takes a conductor below the load it has to serve, the conductor goes up a size, and the larger conductor then needs a larger trade size on every foot of the affected run. Note that the equipment grounding conductor is not current-carrying, and the neutral is not always counted either, so count current-carrying conductors from the circuit configuration rather than counting wires in the pipe.

The practical consequence: decide the routing strategy during the takeoff, not in the field. Combining homeruns is the cheaper answer on paper, and it can reverse once fill and derating are applied. It reverses on material and labor together, because a larger trade size is slower to bend, support, and pull.

What the clean footage misses

The calculator gives the geometric footage plus your own percentage. Several things sit outside it.

• The average hides the tails. One average homerun length across a whole panel is a fair check figure. It understates a job where a handful of circuits run to the far corner of the building, and those long runs are also the ones most likely to get upsized for voltage drop. Where the spread is wide, split the panel into near and far groups and run the tool twice.
• Conduit waste and wire slack are different drivers. Pipe waste is cut waste and the short pieces you cannot use again. Wire slack is the tail you leave at both ends, in the panel gutter, at the box, and at every junction. They do not have to be the same percentage, and a single figure covering both is a simplification.
• Fittings, supports, and boxes are separate line items. Couplings, connectors, straps, hangers, boxes, plaster rings, and firestopping are quantified per run or per foot, not folded into the footage.
• Voltage drop upsizing. A long run can need a larger conductor than the breaker size suggests, which changes the wire cost and the pipe size together.
• Copper is a moving price. Building wire is priced off the copper market. Quote it, note the date, and put a validity period on the number.

Worked example

Take the calculator defaults: 24 circuits off a panel schedule, an average homerun of 90 ft, 3 conductors per circuit, one circuit per raceway, and 10 percent for waste and slack.

Conduit runs = ceiling(24 / 1) = 24. Net conduit = 24 x 90 = 2,160 lf. Net conductor = 24 x 3 x 90 = 6,480 lf. Apply the 10 percent: conduit 2,160 x 1.10 = 2,376 lf, and conductor 6,480 x 1.10 = 7,128 lf. That is the homerun order quantity for one panel, before fittings, boxes, and supports, and before the device-to-device branch runs inside each circuit.

Now change one input to price the routing decision. Put 3 circuits in each raceway: conduit runs = ceiling(24 / 3) = 8, net conduit drops to 8 x 90 = 720 lf, and the conductor total does not change at all. Then check that raceway. It holds 9 conductors, 6 of them current-carrying if each circuit carries its own ground, which is past the three-conductor threshold, so the adjustment factors apply and the conductor may have to go up a size before you can price the pipe.

From footage to a priced line

Footage is the quantity half. The priced line multiplies conduit and conductor footage by your own installed unit costs, which carry material, fittings, and labor per foot at the height and finish you are working in. That is the step Ruh automates: it reads the drawing set, measures the scope, and prices the measured quantities against your own price book, with every line traceable to the sheet it came from and your estimator reviewing the result.

For the wider trade workflow see the electrical estimating software page, for the bid mechanics see how to bid electrical as a subcontractor, and for the unit itself see the linear foot glossary page.