I have covered upsized branch before. I wrote about the changes to electrical plans, and how they no longer showed size and wire fill for branch conduits. This article is a review of the information given in past articles, and an update on my most recent problems with upsized branch.

How engineers require electrical estimators to deal with branch sizing has been changing since at least the 1980s. The first change I saw was a requirement in the specifications that limited the voltage drop on branch circuits to a certain percentage, such as 5%. This had the potential to be very time-consuming, requiring voltage drop calculations on all of the branch circuits. Also, some engineers stopped showing the conduit, wire counts and wire sizes on the floor plans.

The next part of the evolution was again shown in the specifications. An example of the new text is: “The wire in 20A, 120V one-pole circuits over 100 feet in length will be increased in wire size to #10 AWG.” The specifications went on to cover similar requirements for longer circuits and those with different voltages. 

This is better for the estimator, since we do not need to do voltage drop calculations. However, we must pay attention to the length of the branch circuits. Variations of this requirement include the size being increased only to the first device or the end of the circuit. Make sure you read the specifications carefully. I have also seen these kinds of requirements in notes on the plans.

Harmonic feedback

Somewhere around this time, the industry realized that harmonic feedback (see “Bad Harmony” in the May 2021 issue of ELECTRICAL CONTRACTOR) was becoming a problem. Up until this time, the practice of combining three circuits with one neutral in a three-phase system was standard. Since the harmonic feedback was creating too much current on the neutral, this is no longer acceptable.

I am aware of three ways engineers responded to this problem. The first was to increase the size of the neutral. The second was to require a separate neutral for each circuit. The third was to only allow single-­circuit branch conduit runs. I still see all three solutions, but most often see the requirement for a separate neutral for each circuit.

Another evolution in upsized branch was showing wire sizes in tables, such as a schedule for mechanical equipment. These tables usually show requirements for starters and disconnects as well. In just a few cases, I have been referred to the mechanical drawings for wire size requirements. A couple of those projects made the estimator responsible for sizing the wire to mechanical loads.

The version of tables I see most often is conduit and wire requirements shown in panelboard schedules, which led me to write this article. Recently, I completed an estimate for a project that had panel schedules that included the conduit and wire sizes for each circuit. For a 20A, 120V circuit, some of the wire was specified as No. 6 AWG. This had to be a mistake. 

While checking the rest of the panel schedules, I found many more 20A, 120V circuits specified to be No. 8 and No. 6 AWG. Next, I looked at the plans, and the problem started to clear up. The circuits with large conductors were going up to 300 feet from the panel. The layout of the project included panelboards feeding loads located all the way across the building. Now that I understood the reason for the large conductors, it was time to finish an estimate based on the design.

Working with unusual engineering

This was an existing building. Cutting the floor was not allowed, so I had to measure the conduits overhead, with a building height that required the addition of 80 feet to the horizontal measurements. Good estimating practices required factoring the labor up 50% for work at that height. Even with combined circuit home runs, I had about 30 conduits crossing the building overhead. It was very expensive. At that time, I decided to redesign the project.

I moved two panels feeding the most distant loads to the other side of the building. Running two feeders across the building was far less expensive than running 30 branch conduits in the same path. No. 8 and No. 6 wires were no longer needed, which saved a lot of money. I considered moving the 480 to 120/208V transformers across the building, adjacent to the panels they fed, but space was not available.

This was a first for me, seeing a requirement for No. 6 AWG wires in a 20A, 120V branch circuit. As an estimator, always question when you see unusual engineering. Often, you can save your customer money.

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