Reading One-Line Diagrams and Panel Schedules

When a building loses power, or you need to know what feeds a machine, the question is simple: where does this power come from? A one-line diagram answers it for the whole building. A panel schedule answers it for each circuit on a panel. These two documents are the power side of a drawing set. Facility crews, HVAC techs, electricians and inspectors all use them, because they show the path to shut off and the capacity that is left.

What You Will Be Able to Do

• Explain what a one-line diagram shows, and what it leaves out.
• Recognize common one-line symbols and ratings.
• Trace power from the utility to a load.
• Explain what a delta-wye transformer symbol tells you.
• Read a panel schedule and check whether the panel is balanced.

What a One-Line Shows

A one-line diagram, also called a single-line diagram, shows how power is distributed. A single line stands for all the conductors in a circuit, even though a three-phase feeder has three phases and often a neutral and a ground. That simplification keeps a whole building on one sheet.

A one-line shows:

• Sources, such as the utility, generators and solar.
• Switchgear, panels and motor control centers (MCCs).
• Transformers, breakers and disconnects, with ratings.
• Feeders, with their conductor and conduit sizes.
• How each piece connects to the next.

A one-line does not show control logic, physical location or exact wire routing. It also does not show every branch circuit. You will need other drawings for those.

Common Symbols

Symbols vary a little between engineers. Always check the legend on the drawing set.

Ratings You Will See

The frame size is the physical breaker body, and it sets the maximum. The trip rating is where it actually opens. The interrupting rating must be at or above the fault current available at that point.

Reading a One-Line

Most one-lines read from the top down, from the source toward the loads.

Here is a small example, drawn in text form:

 Utility 13.8 kV
      |
 [1000 kVA transformer, delta-wye, 480Y/277V]
      |
 Main switchboard MSB, 1600 A
      |--- 400 A breaker --- MCC-1
      |--- 225 A breaker --- [75 kVA, 480 to 208Y/120V] --- Panel LP-1
      |--- 100 A breaker --- Panel HP-1, 277 V lighting

Worked Example: Tracing Panel LP-1

You need to shut off Panel LP-1 for service.

1. Find LP-1 at the bottom of the drawing.
2. Follow the line up to the 75 kVA transformer that feeds it.
3. Follow up to the 225-amp breaker in MSB.
4. That breaker isolates both the transformer and LP-1.
5. Check the drawing for any other source, such as a generator or a tie to another panel.

Tracing on paper first tells you which device to lock out and what else will go down with it. Everything downstream of the breaker loses power, so tell those areas first. Then have a qualified person confirm in the field that the drawing is right.

Reading a Transformer Symbol

A transformer symbol often shows how its windings are connected, with a triangle for delta and a Y for wye. A delta primary with a wye secondary is the most common arrangement in buildings. It tells you several things:

• The secondary has a neutral. The center of the wye gives a point that can be grounded and brought out.
• That neutral is the reference. Line-to-neutral loads such as 277-volt lighting and 120-volt receptacles use it, and ground-fault protection depends on it.
• There is a 30-degree phase shift between the primary and secondary voltages.

That phase shift matters when two sources are connected together. Transformers can run in parallel only if their voltages and phase shifts match. If you connect a delta-wye to a delta-delta, the voltages are out of step, and a huge current flows between them.

A ground symbol on the wye shows that the system is solidly grounded. If you see a resistor between the neutral and ground, the system is resistance grounded, which limits ground-fault current.

Separate Sources

A one-line can warn you of something that a quick look at a panel will not. A machine can get power from more than one place:

• A generator, through a transfer switch.
• Solar or batteries.
• A second feeder or a tie breaker.
• A control circuit fed from a different panel.

If a diagram shows more than one source, opening one breaker may not make the equipment dead. Control transformers, for example, are sometimes fed from a separate source, and they may not appear on the one-line at all. A qualified person must test for voltage before anyone relies on the drawing.

Drawings Can Be Out of Date

Buildings change. Panels get added, and breakers get swapped, and not every change reaches the drawings. An as-built drawing shows what was installed when the job ended, and it can still be out of date.

If what you find does not match the drawing, stop and report it. Mark up a copy, and get the record updated.

Reading a Panel Schedule

A panel schedule lists what every circuit in a panel does. It is on the drawings, and a directory is often inside the panel door. A typical heading tells you the panel name, voltage, main or lug size, bus rating, feed and mounting.

Here is part of a three-phase schedule. Panels place odd numbers on the left and even numbers on the right. In a three-phase panel, phases repeat every three rows: circuits 1 and 2 on phase A, 3 and 4 on phase B, 5 and 6 on phase C.

Each line tells you the size of the breaker, what it serves and how much load was planned. Multi-pole breakers take up more than one circuit number. A two-pole breaker at circuits 1 and 3 lands on both A and B.

A single-phase 120/240-volt panel uses two phases, in an A, B, A, B pattern down the panel.

Checking the Balance

Loads should be spread evenly across phases. On a 120/240-volt panel the neutral carries the difference between the two legs. Uneven phases put more heat in the neutral and in part of the transformer, which wears it out sooner. A common target is to keep phases within about 10 percent of each other.

Worked Example: Balance

A panel has 8,200 VA on phase A, 6,400 VA on phase B and 7,000 VA on phase C.

1. Total: 21,600 VA.
2. Average: 21,600 / 3 = 7,200 VA.
3. Highest difference: 8,200 - 7,200 = 1,000 VA.
4. Percent: 1,000 / 7,200 = about 14 percent.

That is over the target. Moving a 1,000-VA circuit from A to B would give 7,200, 7,400 and 7,000 VA, which is close. Any change needs to be made by a qualified person, and the schedule has to be updated.

Finding Spare Capacity

The total connected load tells you how much room remains. A 225-amp, 208-volt, three-phase panel can carry about 225 x 208 x 1.732 = 81,000 VA at full rating. Spare spaces and spare breakers are listed in the schedule. Adding a load also needs a look at the feeder and the demand.

Colors on the Job

Drawings call out conductors, but in the field you also see color. The code sets a few rules: white or gray for a grounded conductor, green or bare for an equipment grounding conductor, and orange for the high leg on a high-leg delta. Other colors are common practice.

Gray shows that a neutral belongs to a 277/480-volt system, which helps keep it apart from a 120-volt system's white neutral in the same building.

On a high-leg delta, the orange conductor reads about 208 volts to neutral. Connecting a 120-volt load to it destroys the equipment.

Always prove what a wire is with a meter, or have a qualified person do it. Colors in older buildings can be anything.

Common Mistakes

Check Your Understanding

1. A one-line shows a breaker marked 800AF / 600AT. What does that tell you?

The breaker body is an 800-amp frame, and it is set to trip at 600 amps.

2. A diagram shows a delta primary and a wye secondary with a ground symbol. Can it supply 277-volt lighting from a 480-volt secondary?

Yes. A grounded wye gives a usable neutral, so line-to-neutral loads can use it.

3. A pump shows two incoming lines through an ATS. You open the normal-side breaker. Is the pump safe?

No. The second source can still feed it. Both sources must be isolated, locked and tested.

4. Phases on a panel show 5,000, 5,100 and 7,300 VA. Is it balanced?

No. The average is 5,800 VA, and 7,300 is about 26 percent above it. Circuits should be moved from the heavy phase.

Key Takeaways

• A one-line shows power distribution, using one line for all the conductors in a circuit.
• Read it from the source down, and check the legend.
• A delta-wye transformer gives a neutral and a 30-degree shift.
• Look for second sources before you rely on a single disconnect.
• A panel schedule shows what each breaker does, and how the load is spread.
• Keep phases within about 10 percent, and keep drawings up to date.