PART II Distribution
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Panel Schedules

Anatomy · phase balancing · bus sizing · main breaker

A panel schedule is one document that captures every branch circuit on a panel: which breaker, which wire, which load, which phase. It is the deliverable that contractors use to actually install the work.

Panelboard vs Switchboard vs Switchgear vs MCC

Before reading a panel schedule, know which kind of "panel" you're looking at. The word panel covers four very different pieces of equipment.

Equipment Typical use Voltage Bus rating Construction Atlas DC1
Panelboard Branch-circuit distribution from a feeder ≤ 600V ≤ 1200 A Wall- or floor-mounted; molded-case breakers; NEMA PB1 / UL 67 Office lighting, RPPs in IT halls
Switchboard Service entrance, feeder distribution ≤ 600V 800 – 5000 A typical Free-standing; molded-case or insulated-case CBs; NEMA PB2 / UL 891 —
Switchgear Main distribution at service or sub-station LV ≤ 600V or MV 1–38 kV 800 – 6000 A LV; up to 4000 A MV Free-standing; drawout breakers; protective relays; UL 1558 (LV) or IEEE C37 (MV) 480V SWGR-A & B (4000A) · 12.47kV MV SWGR
MCC (Motor Control Center) Motor starters & VFDs grouped together ≤ 600V (LV) or 5kV (MV) 800 – 5000 A bus Free-standing; modular "buckets" — combination starter, VFD, soft-starter; NEMA ICS18 / UL 845 Mech room MCC for chillers/pumps (often bolted to SWGR)
PDU (data-center context) 480→415Y/240V step-down + sub-distribution to racks 480V in / 415Y/240V out 225 – 1000 kVA typical Cabinet with isolation transformer + integral panelboard PDU-A1 (500 kVA), PDU-B1, etc.
RPP (Remote Power Panel) Row-level distribution from a PDU to racks 415Y/240V or 208Y/120V 225 – 400 A Slim panelboard at row end; sub-metered branches One per IT row, fed from PDU

Anatomy of a Panel Schedule

The panel schedule is a tabular document. Below is a real-format Eaton/Square-D-style schedule for a 42-circuit panelboard. Each row is one breaker; the table layout encodes the phase rotation.

PANEL: RPP-A1-1 · 415Y/240V 3φ-4W · 400A MCB · 42 circuits · NEMA 1 · Cu bus · IT Hall A · Fed from PDU-A1
Ckt# Description Wire Trip P A (W) B (W) C (W) P Trip Wire
1Rack A1-01 · servers#1030A15760——130A#10
2Rack A1-02 · servers#1030A1—5760—130A#10
3Rack A1-03 · servers#1030A1——5760130A#10
4Rack A1-04 · servers#1030A15760——130A#10
5Rack A1-05 · servers#1030A1—5760—130A#10
6Rack A1-06 · servers#1030A1——5760130A#10
7Rack A1-07 · GPU node#660A111520——160A#6
8Rack A1-08 · GPU node#660A1—11520—160A#6
9Rack A1-09 · GPU node#660A1——11520160A#6
10PDU controls#1220A1800——120A#12
11PDU monitoring#1220A1—800—120A#12
12Hot-aisle lighting#1220A1——800120A#12
… (circuits 13–42 follow same pattern)—————————
PHASE TOTALS (W) → 23,840 23,840 23,840  Σ = 71,520 W = 71.5 kW = 75.3 kVA @ 0.95 PF
PHASE CURRENT (A) → 99.3 99.3 99.3  Bus loaded to 25% of 400A — well within 80% target

What every column tells you

Column What it captures Why it matters
Ckt #Position in panel (odd numbers left, even right)Phase rotation: 1=A, 2=A, 3=B, 4=B, 5=C, 6=C — repeats. Enforces balance by geometry.
DescriptionWhat the breaker feedsField labeling, troubleshooting, future modifications
WireConductor size + typeBranch circuit conductor — sized per MCA
TripBreaker amp ratingOCPD — sized per MOCP, rounded to standard
P (poles)1, 2, or 3 pole1P = 277V or 120V, 2P = 240V or 480V, 3P = 3φ load
A / B / C (W or VA)Watts (or VA) attributed to that phaseUsed to calculate phase total + balance check
Phase totalSum of all loads on each phaseBalance check: phases should be within ~5–10% of each other
Phase currentVA/V calculation per phaseEnsures no phase exceeds bus rating

Phase Balancing — Why Circuit Numbers Are Geometry

The odd/even circuit numbering on a panel isn't decorative. The bus bars physically alternate A-A-B-B-C-C top-to-bottom. As long as you fill circuits sequentially with similar loads, the panel auto-balances itself.

Panel bus geometry: phases alternate every 2 circuits A B C 1 2 3 4 5 6 7 8 … cycle repeats every 6 circuits … Phase A bus Phase B bus Phase C bus
Three-phase panels rotate A-A-B-B-C-C-A-A-B-B-C-C ... by circuit pair, top to bottom

Balance rules

  • Spread similar loads across phases. If you have 30 identical 30A breakers, fill them sequentially. The geometry handles the balance.
  • 2-pole breakers connect to consecutive bus stabs (positions 1+3 = A+B, 3+5 = B+C). They span two phases.
  • 3-pole breakers connect to all three (positions 1+3+5). Ideal for 3φ loads — inherently balanced.
  • Target imbalance: ≤ 10% between heaviest and lightest phase. Below 5% is excellent.
Imbalance percentage
imb % = (Imax − Iavg) / Iavg × 100
NEMA limit for motor operation = 1% (motors derate quickly). Panel balance target = 10%.

Bus & Main Breaker Sizing

The panel's bus must carry the worst-case phase current. The main breaker (MCB) protects the bus. Or, with a Main Lug Only (MLO) panel, the upstream OCPD protects.

Decision Rule NEC reference
Bus rating ≥1.0 × (heaviest phase current after demand factor)NEC 408.30
Bus rating ≥1.25 × continuous load on the heaviest phaseNEC 215.3 (applies to feeder OCPD)
Main breaker (MCB) ≤Bus ratingNEC 408.36
Main breaker (MCB) ≥Same logic as feeder OCPD: 1.25 × cont + 1.0 × non-contNEC 215.3
MLO panelUpstream feeder breaker provides the bus protection. Bus must equal or exceed feeder breaker rating.NEC 408.36(A) Exception
Number of branch breakers ≤42 per panelboard (lighting & appliance)NEC 408.54 (deleted in 2008+ but many AHJs still enforce; otherwise UL 67 governs)
i
Standard panelboard bus sizes
100 · 125 · 150 · 200 · 225 · 250 · 400 · 600 · 800 · 1000 · 1200 A. For data center RPPs (415Y/240V), 400A and 600A are standard. For office panelboards (208Y/120V), 100–225A is typical. PDU sub-panels: 400–800A.

Worked Example 1 — Atlas DC1 RPP-A1-1 Panel Schedule

Continue the panel schedule shown earlier. We need to verify bus and main breaker are correctly sized.

Example 01 · Atlas DC1 spine RPP-A1-1 · 415Y/240V 3φ-4W · serves 8 racks + supporting loads

Panel summary (from schedule)

Voltage
415Y/240V 3φ-4W (downstream of PDU-A1)
Connected loads
71.5 kW total (continuous — server load)
Phase A total
23.84 kW = 23,840 / 240 = 99.3 A
Phase B total
99.3 A
Phase C total
99.3 A (perfectly balanced — sequential rack loading)

Bus + main breaker sizing

  1. Apply 125% to continuous (server) load.
    99.3 × 1.25 = 124 A per phase required
  2. Bus selection. Standard sizes ≥ 124 A: 125, 150, 200, 225, 400.
    Choose 400 A bus to allow future capacity (you don't want to swap the panel when racks expand). Loaded to 31% currently — leaves ~270 A spare.
  3. Main breaker. Sized to protect bus AND ≥ 1.25 × continuous load. Standard breakers ≤ 400: 400, 350, 300, 250, 225, 200, 175, 150, 125.
    400 A MCB matches the bus. Could also use 200 A MCB and call it derated for current load, but a 400A MCB allows the panel to grow.
  4. Feeder from PDU-A1 to RPP-A1-1. Sized to 125% × 99.3 = 124 A minimum.
    Feeder breaker at PDU-A1 = 400 A (matches downstream MCB). Wire = 500 kcmil Cu THWN-2 (320 A × 1.25 derating headroom = 400 A capable). Or 1/0 Al MC cable.
!
Why oversize the panel for a data center?
Server racks grow over time. A "24 kW rack" today becomes a "44 kW rack" in 18 months when GPU nodes get upgraded. Sizing the bus at 4× current load is industry standard — replacing an RPP requires shutting down the row, and DC operators avoid that at all costs. Oversize at install; never resize after.

Worked Example 2 — Apartment Unit Panel

A single 200A residential panel for a typical 2-bedroom unit. The panel schedule for residential is simpler — fewer phases (just split-phase A-B), but more circuit types.

Example 02 · Alternate scale 200 A · 120/240V 1φ-3W · 30-circuit · MCB
PANEL: UNIT-101 · 120/240V 1φ-3W · 200A MCB · 30 circuits · Square D Homeline
CktDescWireTripPA (W)B (W)PTrip
1, 3Range — 50A 240V#65028000———
5, 7Dryer — 30A 240V#10302—5000——
9, 11Water heater — 30A 240V#103024500———
13, 15A/C condenser — 40A 240V#8402—3500——
17Kitchen receps #1 — 20A#122011500—120
19Kitchen receps #2 — 20A#12201—1500120
21Bath receps GFCI#12201800—120
23Bedroom receps AFCI#14151—600120
25Living receps AFCI#14151600—120
27Lighting#14151—800115
29Laundry recep#122011500—120
PHASE TOTALS (W) → 16,900 11,400  Σ = 28,300 W
CONNECTED PHASE I (A) → 141 95  After NEC 220 demand: ~ 118 A actual peak
IMBALANCE → 39%  Re-balance: move dryer + WH to opposite phases

Issues caught by the schedule

  1. Heavy imbalance. Phase A = 16.9 kW, Phase B = 11.4 kW. 39% imbalance — well above 10% target.
    Fix: swap range and dryer phase positions. Range is single-phase 240V (uses both A and B equally — already balanced). Move water heater (4500W) from A to B; result: A = 12.4 kW, B = 15.9 kW. Imbalance now ~ 14%. Better. (Perfect balance impossible with discrete loads of different sizes.)
  2. 200A MCB sizing check. Heaviest phase = 141A connected. With NEC 220 demand factors applied, peak ~ 118A. Within 200A bus. ✓

Drill — Quick Self-Check

Work each problem mentally; reveal to check. Goal: reflex, not deliberation.

Drill 1 · Bus sizing

Panel demand on the heaviest phase = 156 A continuous. Minimum bus rating?

Drill 2 · Phase rotation

On a 3-phase panelboard, what phase does circuit 7 connect to?

Drill 3 · Imbalance check

Phase A = 80 A, B = 65 A, C = 75 A. Avg = 73.3. % imbalance?

Drill 4 · Panel vs switchgear

A 3,000 A free-standing distribution panel with drawout breakers — is it a panelboard, switchboard, or switchgear?

Drill 5 · Atlas RPP

Atlas DC1 RPP-A1-1 had 99.3 A on each phase (continuous). Required main breaker minimum?

If You See THIS, Think THAT

If you see…Think / use…
How a breaker's poles map to the panel bus"One pole, one hot. Two poles, no neutral. Three poles, three phases." 1P touches one phase (A, B, or C). 2P spans two adjacent positions (A+B, B+C, or C+A). 3P spans all three. 3P loads are inherently balanced; 1P loads must be distributed across phases for balance (see Phase Rotation section above).
"Panel schedule" requestedTabular doc with one row per breaker. Captures wire, breaker, phase, load. Used by contractor to install.
"42-circuit panelboard"21 left + 21 right. Lighting & appliance branch panels traditionally limited to 42; modern UL 67 allows more.
"MCB" vs "MLO"Main Circuit Breaker (panel has its own main); vs Main Lug Only (no main, fed protected from upstream).
2-pole breaker on 3φ panelSpans 2 phases (e.g., A-B, B-C, A-C). Used for 240V or 480V single-phase loads (split-phase or 3φ panel).
3-pole breakerSpans all 3 phases. Used for 3φ motor / pump / panel-feed loads.
"PDU" in DCPower Distribution Unit — 480→415Y/240V step-down + integrated panelboard. Not a power strip.
"RPP" in DCRemote Power Panel — branch panel at the row/aisle, fed from PDU.
"MCC" — Motor Control CenterFree-standing modular cabinet with starter/VFD/disc combo "buckets" per motor.
Phases imbalanced > 10%Re-arrange branch positions to redistribute load. The panel layout itself is the lever.
Bus loaded > 80%Either upsize to next standard bus, OR re-shed loads to another panel. Don't run panels at 100%.
"Series-rated" breakersDownstream CB has lower interrupting rating than upstream — only valid if combination is UL-listed for series rating. Many AHJs prohibit; verify.