Monthly Archives: September 2010

Combining Branch Circuits in Electrical Estimates

When I was a junior estimator, branch takeoff was easy. The engineer always designed the branch. All we had to do is measure what was shown on the drawings. Many things have changed since then, and the biggest change has been in the practice of combining branch circuits.

For those who may not be familiar with this practice, it is mostly done in commercial projects with a three phase service. The N.E.C. allows 3 hot and 1 neutral conductor to be run in a single conduit without de-rating the capacity. In a three phase system, this could be three phase conductors and one neutral. By combining these circuits into a single conduit, two conductors and two conduits can be eliminated. This saves a lot of money.

Changes have slowly been taking place in the implementation of this practice. The designs and specifications of many projects now restrict, or do not allow combining of branch circuits. The reason I hear most often is harmonic feedback, which can cause all sorts of damage to electrical systems. The affect on combined circuits is the addition of excess current on the neutral conductor, which can result in overheating. Since the neutral is not protected by a fuse or circuit breaker, there is a risk of heat damage and fire.

Harmonic feedback is increasing due to the growth in use of switch mode power supplies. These power supplies, common in number of electronic devices such as dimmers, computers, microwave ovens and lighting ballasts, create distortion and harmonic feedback on the neutral conductor.

 This creates several problems for the estimator. The first is in regards to competitiveness. If the circuits are not combined, the cost of the project will be higher since you may have three conduits instead of one, and three neutrals instead of one.

 Another problem is the change I have seen in the engineer’s designs. Many projects are being designed with single circuit branch runs, including the homerun. If combing circuits is allowed, the estimator has to take additional time to re-design the project before the takeoff can be finished. Just what we need with the short bid times we are getting these days.

The third problem is the addition of more complexity to the plans and specifications. The engineers are handling the feedback problem in a number of ways. Some forbid combining circuits. Others call for up-sized neutrals. Another approach is allowing combined circuits in a single conduit with a neutral for each circuit. The bid documents have to be studied closely to see which branch wiring method is allowed.

As estimators, we have to be sure that the bid documents have been adhered to, and at the same time come up with the low bid. As observed in a previous article (Electrical Estimating Per The Electrical Specs? Are You Kidding?), I always wonder if the competition is following the bid requirements. If they are not, they will have a lower bid. One strategy to deal with this is to qualify your bid as being per the requirements, but offer a deduction if you are allowed to combine circuits. Another strategy would be to ignore the requirements in your base bid, and offer an adder to follow the bid requirements.

In summary, this has three impacts on us. First, another level of complexity has been added to our estimates. Second, this is another opportunity for less vigilant estimators to make a mistake. Third, this is another requirement for some estimators to ignore, thereby taking work away from firms that follow the plans and specification. Sometime in the future I’ll start a discussion about ethics and bidding practices. It should be interesting.