Panel Schedules on the Drawings, Column by Column

Why this matters

When a tenant asks for "one more circuit in the break room," the panel schedule on the electrical drawings is where the answer starts, and the panel directory taped inside the door is where it usually goes wrong. The two look alike and are different documents. The design schedule is the engineer's statement of what the panel was meant to carry, phase by phase, at the time the set was drawn. The directory is whatever someone last wrote by hand. A shop that adds load from the directory alone can stack it on the heaviest phase, use a "spare" that was put to work years ago, or read a bus rating as spare capacity the feeder never had.

The artifact

A design schedule for a small 208Y/120 V lighting and receptacle panel, in the single-column format some firms use. Others print it two-sided, odd circuits left and even right, to mirror the panel; the columns are the same.

PANEL LP-2       LOCATION: RM 112 STORAGE, SURFACE MTD
208Y/120V 3PH 4W BUS 225A  MAIN 125A 3P MCB
AIC 10,000A SYM  FED FROM: T2 SECONDARY (SEE E-601)
CKT DESCRIPTION            A      B      C   P-TRIP
1   LTG OPEN OFFICE      1200                 1-20
2   RECEP OFFICE N        900                 1-20
3   LTG CORRIDOR                800           1-20
4   RECEP OFFICE S             1080           1-20
5   RECEP BREAK CTR                   1500    1-20
6   REFRIGERATOR                       600    1-20
7   RTU-3 (7/9/11)       2400                 3-30
8   MICROWAVE            1500                 1-20
9   RTU-3                      2400
10  COPIER                     1200           1-20
11  RTU-3                             2400
12  SPARE                                     1-20
13-30 SPACE
CONNECTED VA            6000   5480   4500
TOTAL 15,980 VA, 44.4 A BAL; PHASE A 50.0 A

The header, field by field

Panel name and location. LP-2 is the tag every other sheet uses; the one-line, the riser and the circuit tags on the plans all point back to it. The location is the room, which matters in a building where the one-line draws panels next to each other that are floors apart (see related: One-Line Diagrams and How They Differ From a Riser).

Voltage, phase and wires. 208Y/120 V, three-phase, four-wire: three phase conductors plus a neutral, giving 120 V phase to neutral and 208 V phase to phase. A 208 V load needs two or three poles; a 120 V load needs one pole and the neutral.

Bus rating and main. The bus is what the panelboard itself can carry, 225 A here. The main breaker, 125 A, is what actually protects the panel and sets its usable capacity. The gap is not spare capacity. The feeder and transformer behind this panel were sized for 125 A, and a reader who sees "225A BUS" and plans to 225 A has skipped the two things that would fail first. A panel listed MLO (main lugs only) has no main of its own, and its protection is the feeder breaker upstream.

Interrupting rating (AIC). 10,000 A symmetrical is the fault current the panel's breakers are rated to interrupt. It has to be at least the available fault current at this bus, which the one-line or fault study gives. Replacement breakers must carry the same or higher rating, or match the series combination on the panel label if one is listed.

Fed from. The source, here the secondary of transformer T2. This is the line that tells you what to open to make the whole panel dead, including the line side of the main.

The circuit rows, column by column

Circuit number. Odd numbers down one side of the panel, even down the other. On a three-phase panel the phases rotate every two circuit numbers: 1 and 2 on A, 3 and 4 on B, 5 and 6 on C, then 7 and 8 back on A. That rotation is why RTU-3, a 3-pole load, occupies circuits 7, 9 and 11 and lands one leg on each phase.

Description. What the engineer intended the circuit to serve. It is the most perishable field on the sheet, because tenants change and nobody updates the drawings.

Load per phase, in volt-amperes. The connected load the engineer assigned to each circuit, placed in the column of the phase it lands on. Receptacle circuits here follow the NEC's general allowance of 180 VA per receptacle yoke (NEC 220.14, as adopted by your jurisdiction), which is why office north reads 900 VA for 5 duplexes and office south 1,080 VA for 6. Appliances carry their nameplate rating. These are connected figures. They are not what a clamp meter will read on a Tuesday afternoon, and they are not the demand load the engineer's calculation uses after applying continuous-load and demand factors. Some schedules print that calculation in a block beneath; this one does not.

Poles and trip. "1-20" is a single-pole 20 A breaker; "3-30" a three-pole 30 A. A 3-pole breaker appears once with its trip, and the rows below it show only its loads on the other phases.

Spare versus space. These two words mean different things. A spare is a breaker installed and not connected, circuit 12 here. A space is a position with no breaker, ready to take one, circuits 13 to 30. Both can disappear in the field without the drawings knowing.

Totals. Each phase column sums to its total. Recount them yourself whenever you use a schedule; transcription errors in long ones are common. Phase A is 1,200 plus 900 plus 2,400 plus 1,500, which is 6,000 VA. Phase B is 800 plus 1,080 plus 2,400 plus 1,200, which is 5,480 VA. Phase C is 1,500 plus 600 plus 2,400 plus 0, which is 4,500 VA. The three phases sum to 15,980 VA, matching the total line. Balanced, that is 15,980 divided by (1.732 x 208), or 15,980 divided by 360.3, which is 44.4 A. The heaviest phase is what loads the main, though: 6,000 divided by 120 is 50.0 A on phase A.

Worked example: one more break-room circuit

The tenant wants a dedicated 1,500 VA circuit for a second countertop appliance. The shop holds the electrical license, and its tech reads the schedule above, then opens the panel door.

The field directory disagrees. Circuit 12, the design spare, is written in as "RECEP CONF RM." Someone used it after the drawings were issued. Clamping it would give today's draw, which is the wrong basis to add to a column of connected loads. The like-for-like figure is the connected load the engineer would have assigned: the conference room has 4 duplex receptacles on it, so 4 times 180 is 720 VA, on phase C.

Restate the phases with the correction. Phase A stays 6,000 VA. Phase B stays 5,480 VA. Phase C becomes 4,500 plus 720, which is 5,220 VA. The corrected connected total is 15,980 plus 720, or 16,700 VA. This corrected panel, not the drawing, is the baseline for the new circuit.

Pick the phase. The first spaces are 13 and 14, both on phase A, the heaviest phase. Putting 1,500 VA there makes A 7,500 VA. Space 15 is phase B, making B 6,980 VA. Space 17 is phase C, making C 6,720 VA. Phase C ends lightest of the three options, so the circuit goes in 17, and the panel reads A 6,000, B 5,480, C 6,720 VA.

Check it against the main. The new total is 16,700 plus 1,500, which is 18,200 VA; the three phases sum to the same, 6,000 plus 5,480 plus 6,720. Balanced that is 18,200 divided by 360.3, or 50.5 A. The heaviest phase, C, is 6,720 divided by 120, which is 56.0 A against a 125 A main, about 45 percent of its rating on connected load. There is room. Continuous loads such as the office lighting get counted at 125 percent in the engineer's calculation while demand factors pull other loads down, so on a panel this lightly loaded the connected figure is a fair screen, and on one within a few circuits of its main it is not.

What would change the answer. A panel already near its main on connected load sends this back to the engineer rather than to arithmetic, because whether the demand calculation still closes is their call. A tenant asking for a 208 V, 2-pole load would put 1,500 VA split across two phases, changing which pair of spaces balances best. And a panel labeled MLO puts the check on the upstream feeder breaker instead of a main.

Before anyone touches it. Adding a breaker means working inside the panel. That is a qualified person's work, and a licensed electrician's where your state or locality licenses electrical work, with the upstream source on the "fed from" line locked and tagged and absence of voltage proved live-dead-live on a known source before and after, per NFPA 70E Article 120 (120.5 in the 2021 edition, renumbered in 2024), with work practices at 29 CFR 1910.333(b)(2). Opening only LP-2's main leaves its line side live.

Close the loop. The directory gets the new circuit and the corrected description for 12, and the shop's redlined set gets the same two changes, so the next person reads a schedule that matches the door (see related: How to Redline the Set as You Build).

Design schedule against the panel directory

Neither is the truth on its own. On an inherited building, reconciling the two is a procedure of its own (see related: Panel Schedule Audit on an Inherited Building).

References

• NFPA 70 (NEC), as adopted by your jurisdiction: Article 220 load calculations (220.14 receptacle outlet loads) and Article 408 panelboards.
• NFPA 70E, current edition, Article 120; 29 CFR 1910.333(b)(2) for lockout and tagging during electrical work.
• See related: Panel Schedule Audit on an Inherited Building
• See related: How to Trace a Circuit From Panel Schedule to Plan; How to Read a Schedule on a Drawing Set
• See related: One-Line Diagrams and How They Differ From a Riser