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Determining the
Size and Type of an Electrical Service
This is an
excerpt from one of my advanced electrical courses for home and commercial
building inspectors. I have modified it slightly for this tutorial.
George wells, BSEE, MBA, CMI, Licensed
Master Electrician
Knowing the number or size of
conductors entering a building does not provide enough information
to determine the Voltage, number of phases, or the capacity of the
electrical service. Single-phase services will usually have
two to four conductors. Two-phase services will usually have
three to five conductors, and three phase services will usually have
three to five conductors.
The capacity ratings of
smaller electrical services, such as in houses, are usually listed in Amperes.
The capacity ratings of larger electrical services are usually listed in kVA and
mVA.
Whenever possible, look
at the transformer connections to determine the number of phases, Voltages, and
capacity of the electrical service.
There are at least five
distinct arrangements possible with a service drop having four conductors. The
service can be Single-phase, Two-phase, or Three-phase. The five possibilities
of a four wire service entrance are Single-phase, Two-phase, Three-phase
center-tapped Delta, Three phase corner-grounded Delta, and Three-phase Wye.
Unless otherwise noted,
references to transformer connections are for the secondary side; in other
words, the side to which the service conductors are connected.
1. Single-phase
I've encountered four wire arrangements many times. It is not an arrangement
that an inspector is likely to see in a residential electrical system but it was
a popular arrangement in the late 1970’s and the early 1980’s on small and
medium commercial installations.
Major computer companies
in the late 1970’s and early 1980’s where demanding floating grounds for the
mini-computers (not micro-computers, aka PC’s) that were being installed in
small commercial facilities. As an electrician in the 1970’s and 1980’s I could
not reconcile the demands of the computer makers with the requirements of the
NEC, the NESC and IEEE Red Book and Green Book guidelines for power distribution
and grounding. At that time FIPS Pub 94, and today the IEEE Emerald Book, would
have four conductors going back to the transformer in a perfect world.
A generally accepted
compromise was to carry a fourth conductor back to the transformer. By the end
of the 1980’s the computer companies had stopped asking for floating grounds.
Inspectors and even
electricians sometimes mistakenly referred to three-wire single-phase systems as
Two-phase. A Single-phase system has a center tap on the secondary side of the
transformer to develop a neutral. Therefore, it is a split-phase system, not
two-phase. The actual secondary Voltage of transformer supplying a modern
residential electrical system is 240V. Adding a center tap provides the two
120V sides and a neutral conductor. The neutral conductor is solidly grounded.
2. Two-phase
A single-phase system has
phase legs that are 180 electrical degrees apart. A two-phase system has phase
legs that are 90 electrical degrees apart. It is possible for a two-phase
system to have only three conductors but that is not the usual practice because,
unlike single-phase and three phase systems, the phases are not symmetrical so
there is no true neutral.
Electricians and inspectors on the West coast may have
never seen a two-phase four-wire system but the farther Northeast you go, the
more likely you are to encounter them. There was only one major installation of
two-phase generation in the Western United States. In California, the Pacific
Power and Light Company installed a two-phase system in
San Gabriel Canyon, near Los
Angeles, to serve the town of Azusa. I don’t know when that system was taken out
of service.
Most of the two-phase
systems were taken out of service by 1980. There are no major two-phase systems
currently in use in the US by public utility companies but there still thousands
of privately owned systems throughout the US. Privately owned systems include
those that buy primary power from a public utility and redistribute it.
Two single-phase
transformers connected in a “T” connection, also known as a “Scott” connection
can be used to convert the two-phase to three-phase once inside the building.
A traditional Two-phase
system has four wires. Two-phase five-wire systems are, strictly speaking, not
two-phase systems. They are four-phase systems but they are known as Two-phase
five-wire systems.
Two-phase systems have
been almost universally retrofitted to either single-phase or three phase. The
presence of the fourth conductor on a commercial building could indicate that a
two-phase system has been retrofitted to a single-phase system. As I mentioned
earlier, Single-phase systems are sometimes mistakenly referred to as two-phase.
3. Three
phase
Three-phase systems have
three symmetrical phases. That is, the phases are an equal 120 electrical
degrees apart. A three-phase system requires neither a ground nor a neutral to
be able to function properly. At any given time, in a properly balanced
Three-phase system, two of the three phase conductors serve as the return path
for the current.
-
Wye
This has become the most popular secondary
connection in small to medium size interior electrical distribution systems
because its center tapped neutral enables it to provide a wide range of useful
Voltages and handle a wide variety of loads.
You can explore the relationship of the
currents in the phases to the current in the neutral of a Wye system by entering
various values in my 3p4w Neutral Current Calculator.
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Ungrounded Delta
The Ungrounded Delta is NOT one of the
five possible arrangements of a typical four-wire service. I have listed it
here to demonstrate further why it is not possible to determine the number of
phases based on the number of conductors. An ungrounded system would typically
have the three ungrounded conductors and a messenger cable but the
messenger cable would not enter the building. It is possible, however, that an
overly zealous electrician or facility manager would connect it to an electrode
as a supplementary grounding electrode. There would be nothing wrong with doing
that.
Delta systems may or may not have neutral
conductor but they do not require a neutral because the phases are symmetrical
(120 electrical degrees apart). The Ungrounded Delta is still very popular for
both the primaries and secondaries of substations; particularly, privately owned
substations.
Most ungrounded Deltas are connected in a
Delta Wye configuration. It is called an Ungrounded Delta because it is not
grounded on the primary. Even though it is called an “Ungrounded Delta”, there
is a ground on the secondary at the center of the Wye.
In instances where there is an Ungrounded
Delta secondary, the neutral is developed in a separately derived system
downstream of the substation. This is still probably the most popular choice
for manufacturing facilities.
c. Delta with a corner ground
Corner grounded Delta’s are no longer common
in new installations but there still thousands of existing installations. Their
purpose was to stabilize Voltages and reduce the cost of the system.
-
Delta with a center
tap ground
Known by a variety of names such as “Hi-leg”,
Wild-leg”, “Stinger”, B-phase” and “Red-leg”. The high leg is usually colored
orange, red, or brown. Local customs vary. In some locations even yellow is
used to identify the high leg.
Center tapped Delta systems are still in wide
use but are becoming less popular. The Voltages present in a center-tapped
Delta often confuse less experienced electricians. It is easy for an
inexperienced electrician to mistake the center-tapped Delta for a Wye
connection because of the Voltage relationships.
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Open Delta
The Ungrounded Delta is NOT one of the
five possible arrangements of a typical four-wire arrangement. I have listed it
here to demonstrate further why it is not possible to determine the number of
phases based on the number of conductors.
Open Deltas are used to
keep costs down or to operate a three phase system at reduced capacity when one
of the transformers in a three single-phase transformer arrangement is lost.
The general rule is that the system will run at 57.7% of capacity with two
Single-phase transformers. However, additional cooling with fans can often
increase the capacity.
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