Regardless of size and type, disconnect switches are typically shown as the same, nondescript symbol. If you don’t find out and know everything you can about the disconnect switches you are estimating, you could be losing your company thousands of dollars and even a bid here and there.

Start with the basics

The most commonly seen disconnects come in staged amperage sizes of 30, 60, 100, 200 and 400 amps. There also are 600-, 800- and 1,200-amp sizes, but these are very large and are usually found on the power one-line diagram. Each step up in size carries a large increase in both material and labor costs. The cost difference between 30- and 60-amp sizes is not so extreme, but the difference between 200 and 400, or 600 and 1,200 is quite dramatic. It’s not just the price of the disconnect unit either. Larger amperage units are going to be fed with larger conduits, flex, fittings and wire. Additionally, the enclosures of each size get bigger, so the mounting and support (and possibly seismic bracing) costs also will increase.

There are several different manufacturing quality ratings to consider: standard duty, heavy duty and industrial duty. Voltage is another consideration, as there are 250V, 480V and 600V. There also are DC-rated disconnects. The enclosures also vary: NEMA 1, 3R, 4, 4x, 7, 12 and 13. Then there are the explosion-proof models, which you absolutely do not want to inform your boss you missed—especially if your company won the bid.

A few other equally important qualities must be known, such as fused or nonfused, single-throw or double-throw, and others you’ll find only in a manufacturer’s catalog. You also need to determine how each disconnect is fed, though always check the specs. Sometimes seal-tight is required.

What is the amperage?

This one isn’t too hard to figure out. However, it isn’t typically found with the symbol. You probably will need to look at the equipment connect schedule, a panel schedule or the one-line diagram. If none of those are available or they do not show the disconnect size, you may need to size the switch based on the wire feeding it. If that information isn’t shown (often it is not), then off to the mechanical schedules you go. Perhaps that engineer did his work?

Now, since they come in staged sizes, the disconnect amperage must be higher than the amperage of your circuit. For example, if you have a 20-amp circuit, you need a 30-amp disconnect. If you have a 40, you need a 60, etc. Be careful. If you have a 200-amp feeder, you may actually need a 400-amp switch because you shouldn’t max out or exceed the switch’s capacity.

Price it right

Another key reason for knowing exactly everything about these “squares with the little handle” is pricing their material costs. Labor isn’t a huge factor until you get into the larger sizes.

Except for the smaller, standard units, I don’t recommend pricing them yourself or using a national pricing service. Get quotes directly from your vendors, and make sure they know everything about the units. Along with your counts, send them a copy of the disconnect or safety switch specifications and any sheet notes, schedules or other information you have. Make sure they know the enclosure type, voltage ratings and any special features that are required. Also, make sure the quote secures the pricing for at least 30 days, if not 60 or 90.

Review the schedule and details

Electrical engineers often do not clearly design the entire installation, and they leave out critical requirements, such as mounting and connection details and control wiring.

Review the mechanical equipment schedules to confirm the size and voltage characteristics of each disconnect switch. Sometimes these schedules do not list the amperage size. More often they list the horsepower of the heating, ventilating and air conditioning unit. However, they usually list the voltage and phasing.

These schedules also may have very subtle, but often powerful, numbered notes placed next to each unit on the schedule. Do not ignore these notes. They often list additional control requirements and other important qualities about the disconnect symbol.

Read the mechanical specifications

I often read in the mechanical specifications two possibly job-winning words: “factory installed.” They can be overlooked easily, so read carefully. And make sure you fully understand what they mean and to which disconnects they are referring. Sometimes the electrical engineer will show disconnects that are factory installed on the electrical drawings. If you include the material and labor costs for these switches, your bid is going to be high. On the other hand, if you don’t include them, your competition might, and you will be low.

You should always confirm who is responsible for furnishing and installing disconnects. If you can’t, make sure your proposal clearly states what your company included.

Disconnects are pretty simple, as long as you know what the symbol really stands for.
