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Estimating Feeders

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July 22, 2020

You probably wouldn't believe me if I told you feeders are one of the easier systems to estimate. I don’t blame you. Staring at a single-line riser diagram can be daunting. But estimating feeders is simply a matter of determining the distance between two points. The difficult part is they are hardly ever a simple, straight line.

There are many different types of feeders, and almost every system has them. With each system comes a different set of considerations. For example, power distribution feeders have several different voltages: high (21 kV and up), medium (600V to 15 kV) and low (120V to 600V). Each voltage size comes with different cable and termination considerations, which for medium and high voltages, are far different from those of the low voltages, both in material costs and labor time. Low-voltage signal feeders have practically the same considerations for conduits, but the cabling and terminations are very different.

Of course, terminations and cables are just the start. There are a few other very important and potentially costly details that occur between points A and B you need to consider.

Wherever you route them, you need to make sure the routing you choose will allow your conduit (or multiple conduits) to be installed. There can be no obstructions or conflicts along the way. If there are, you will have to determine how to get around them. This could require a few large pull-boxes, a bunch of extra fittings, elbows, bends, more supports and possibly some concrete core drilling.

Overhead runs inside the building are not always a straight shot. They often must be concealed above the ceiling or installed in a nonpublic location, such as the attic, basement or a utility tunnel. They also typically have to run down corridors loaded with the conduits of other trades, especially very large HVAC ducts.

Underground and under-slab runs have basically the same considerations, except you typically have to avoid things such as trees, other buildings, utility conduits and piping, roadways, 4-foot-thick concrete footings, walls, etc. You also have to know exactly how deep your conduits must be buried and whether they need to be concrete encased.

Sometimes the best route (or only route) is a combination of underground, under-slab and overhead. This adds another level of complexity, as you have to consider your transitions into and out of the ground or slab and how you will get up and down from the ceiling levels.

Think support

There are many ways to support conduit: one- and two-hole straps, J-hooks, slick-rods, Caddy clips and all sorts of other National Electrical Code-approved methods. Multiple conduit runs can share the same supports, typically “strut” trapeze and wall racks.

What must be determined is where, when, how many, spacing, rod size and length, seismic bracing, and how you will attach them to the structure (beam clamps, welding, anchors). So it is critical to read the specification on hangars and supporting methods.

How many conduits and cables does the feeder have? Your larger amperage sizes (typically 400A+) often are run in parallel or multiple conduit runs. This means a single feeder can have two, three, four or more conduits and sets of cables. This means at least twice as many fittings, elbows, bends, cores, bigger pull-boxes and probably more supports. Trust me, you do not want to miss the little indicator telling you there are eight sets of 4-inch with four 500 mcm and one 2/0 (and neither does your boss).

Wire type

Knowing your wire specification is very important. Is your wire aluminum or copper? This is a huge material and labor cost differential that can win or lose you a bid. Wire insulation type also can be an expensive factor. Is the wire THHN, XHHW or a more expensive type, such as mineral insulated? See NEC Article 695 for fire pump feeder requirements.

Maybe conduit and wire isn’t even a requirement? Is MC cable allowed? If so, how will it be supported, and are there any restrictions to using it?

Because the wire extends beyond the conduit, into the enclosures, you will need more wire than conduit. The standards I have used for years are 10 feet at big boards and 5 feet at panels and J-boxes. Be careful with large vaults and manholes, as they may require a full loop.

There are a few more things to consider when estimating feeders: distance and voltage drop, derating, junction boxes vs. 90s, sizing junction boxes, bends, condulets, connectors, grounding, grounding bushings, terminations (they happen at each end of the run), cable racking in vaults and manholes, and a lot more.

Now, I’ve barely scratched the surface on estimating feeders, I’ve merely given you something to think about. And though I said they were “easy” and “fast” to takeoff, I didn’t say there was anything casual about feeders. So, until you master this critical estimating task, make sure you fully understand everything you are looking at, and learn everything you can about each and every feeder on the job.