Inspecting Commercial Buildings and Their Power Supply

Electrical Power

In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10 or 20 kW in rating.
In contrast, in a three-phase system, the currents in each conductor reach their peak instantaneous values sequentially, not simultaneously; in each cycle of the power frequency, first one, then the second, then the third current reaches its maximum value. The waveforms of the three supply conductors are offset from one another in time (delayed in phase) by one-third of their period.

Defining the Terms

Amps vs. Volts:
Think of electricity as water flowing through a pipe. The amperage is analogous to the amount of water flowing through the pipe. Amperage is also called current. Larger diameter wires can handle more current, just as larger pipes can handle more flow.

Voltage is analogous to pressure, the force which moves the water through the pipe. A small pump (low voltage) would produce less pressure than a big pump (high voltage).

In most buildings the voltage will either be 208 volt (low voltage) or 600 volt (high voltage). The critical question is how much voltage and amperage the system is rated at, or in other words, how much equipment can I use in the building?

208 Volt vs. 600 Volt:
Most modern buildings are equipped with 600 volt services. Equipment such as air conditioning units (over 5 tons), larger exhaust fans, electric heaters, and some lighting will utilize 600 volts. However, standard outlets and most lighting operate at 208 volts.

In North America, individual residences and small commercial buildings with services up to about 100 kV·A (417 amperes at 240 volts) will usually have three-wire single-phase distribution, often with only one customer per distribution transformer. In exceptional cases larger single-phase three-wire services can be provided, usually only in remote areas where poly-phase distribution is not available. In rural areas farmers who wish to use three-phase motors may install a phase converter if only a single-phase supply is available. Larger consumers such as large buildings, shopping centers, factories, office blocks, and multiple-unit apartment blocks will have three-phase service. In densely populated areas of cities, network power distribution is used with many customers and many supply transformers connected to provide hundreds or thousands of kV·A, a load concentrated over a few hundred square meters.

Buildings equipped with 600 volt services will always have a transformer to reduce the 600 volts to 208 volts for the main building panels. These transformers are generally located near the main electrical service entrance.

When comparing the amount of power available for different voltages, a 200 amp, 600 volt service has nearly three times the power of a 200 amp, 208 volt service.

This is of less importance. All 208 volt and 600 volt services are three phase. This means there are three power wires coming into the building.

Single phase services may be found in older, smaller buildings and are found exclusively in houses.

In some older buildings you can find a single phase and a three phase service. This is usually identifiable, on the outside, by two separate services leading to the building.

Determining Amperage of Service

When you are inspecting the electrical room, the two items of information you are looking for; the are amperage and voltage. The presence of a transformer in the electrical room is usually indicative that it is 600 volts. They do make transformers that can used to step up a 208 volt service to 600 volts, for a specific piece of equipment.

What you should typically see is a small conduit (high voltage, low current) going into the transformer and a larger conduit (low voltage, high current) coming out and leading to a breaker panel or a splitter panel.

The ratings on the switches and splitter panel are not to be relied on; they only tell you the maximum amount of current or voltage the equipment can handle. Do not rely on the rating of the hydro meter(s), for the same reason.

The best way to verify the amperage is to open the door of the main power switch and read the rating of the main fuses. This is sometimes impossible to do without turning the power off, but is always dangerous, unless you know what you are doing. Even with the power off, half the box is live. You can end your real estate career, right there in somebody’s electrical room.

Reading the gauge (size) of the main power wires (in the meter cabinet or main splitter panel) can also help to determine the amperage of the service. The gauge number is typically printed on the wire sheathing. Common wire gauge sizes, for copper conductors and the allowable amperages are as follows:

Wire Gauge Allowable Amperage
3 100 amps
000 200 amps
350MCM 300 amps
500MCM 400 amps

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