How Design Starts
The electrical engineer doesn't choose what loads exist. Other disciplines do. Your work begins when you receive a list of equipment that needs power — and ends when every single one is wired, protected, and labeled.
The Workflow — Where Each Document Lives
Every project follows the same five documents. They get bigger and more detailed as design progresses, but the order doesn't change. Memorize this flow — every section in Power Atlas slots into one of these stages.
An MEL row that's missed in Stage 1 becomes a missing breaker in Stage 5 — and a 3 a.m. RFI from the field
Meet Atlas DC1 — Your Reference Facility
Every section in this handbook returns to Atlas DC1. It's a representative 2.5 MW colocation data center built in 2N redundant topology — large enough to span every electrical concept, small enough to hold in your head all at once. Here is the full one-line. Spend a minute reading it. We'll dissect every piece across the next 31 sections.
IT load
2.5 MW
Critical load. PF ≈ 0.95 → ~2,632 kVA delivered by UPS at full load.
Topology
2N
Two independent paths (A and B). Either path alone carries the full IT load.
Tier rating
III Uptime
Concurrently maintainable — any single path can be removed for service without dropping load.
Service
12.47 kV
Utility primary. Two pad-mount transformers step down to 480V building distribution.
Genset capacity
5 MW
2 × 2500 kW. Each genset alone carries one full A-or-B path. Backup for utility loss.
Cooling
3000 tons
4 × 750-ton centrifugal chillers. One in standby (N+1 mech inside the 2N elec).
The MEL — What You Receive
The MEL is the input document. Other disciplines fill it out. Mechanical lists pumps and chillers. Process lists production equipment. Plumbing lists water heaters. IT lists racks and PDUs. Each row is a load you must power.
Anatomy of an MEL Row
Real MELs vary by employer, but the essential columns are universal. Here is a row from Atlas DC1's MEL — Chiller CH-1 — with each column annotated.
| MEL Column | Atlas CH-1 value | What you do with it | Source |
|---|---|---|---|
| Tag / Equipment ID | CH-1 | Becomes the panel-schedule row label, the SLD callout, the cable schedule reference | Mechanical assigns |
| Description | 750-ton centrifugal chiller | Confirms load type (motor) and informs duty cycle for power-quality + protection sections | Mech / process |
| Quantity | 4 (CH-1, CH-2, CH-3, CH-4) | Multiply for total capacity; consider redundancy / standby in load study | Mech |
| Voltage | 480V, 3φ, 60Hz | Determines which panel/MCC it connects to; selects correct calculation formula | Mech (per cutsheet) |
| HP / kW | 450 HP nominal | Starting point for FLA calculation; starting point for load study before efficiency & PF | Mech (per spec) |
| FLA / FLC | 480 A (NEC 430.250) | Direct input to wire size, breaker size, panel/MCC bus size | You fill in (or cutsheet) |
| MCA | 600 A (1.25 × FLC) | Minimum wire ampacity; sets conductor size | You calculate |
| MOCP | 1200 A (250% × FLC, NEC 430.52) | Maximum breaker size; set actual ≤ this | You calculate (or cutsheet) |
| Starting type | VFD (variable freq drive) | Affects starting current, harmonic mitigation, branch circuit conductor choice | Mech / electrical |
| Location | Mech room MR-1 | Determines feeder length → voltage drop calc; raceway routing | Architect / mech |
| Service factor | 1.15 | Affects motor overload setting; allows brief overloads | Cutsheet |
| Code letter | F | Locked-rotor kVA per HP — only relevant for DOL starting (less important for VFD) | Cutsheet |
| Notes | Standby duty (N+1) | One chiller is N+1 standby — load study uses 3 running, not 4 | Mech |
ANSI Standard Voltages
Voltages aren't arbitrary. ANSI C84.1 defines the discrete standard voltages used in North America — utilization (at the load), system (utility-supplied nominal), and the tolerances around each. You'll see these exact numbers on every cutsheet.
| Class | System V (LL) | Phase / Wire | Where used | Typical Atlas DC1 use |
|---|---|---|---|---|
| Low (≤ 600 V) | 120 / 240 V | 1φ-3W | Residential, light commercial, control circuits | Office spaces |
| 208Y / 120 V | 3φ-4W | Small commercial, retail, downstream of step-down | Office, lighting at 277V (when 480Y) or 120V branches | |
| 480Y / 277 V | 3φ-4W | Industrial, commercial mech room, MV-fed buildings | Atlas main 480V bus — chillers, UPS input, large pumps | |
| Medium (1 – 35 kV) | 2400 / 4160 V | 3φ-3W or 4W | Large industrial motors, in-plant distribution | — |
| 13.8 kV | 3φ-3W | Common industrial primary, large MV motors | Used on bigger DCs (>10MW); not Atlas DC1 | |
| 12.47 kV / 7.2 kV | 3φ-4W | Utility distribution primary | Atlas DC1 utility service | |
| High (35 – 230 kV) | 69 kV | 3φ | Sub-transmission, large industrial direct service | Hyperscale (≥50MW) DCs |
| 115 / 138 / 230 kV | 3φ | Transmission, utility substation primary | Hyperscale campuses with on-site substations |
i
Utilization vs system voltage
Cutsheets specify utilization voltage (e.g., 460V, 230V, 200V — what the equipment actually expects to see at its terminals after voltage drop). The system voltage is the nominal source value (e.g., 480V, 240V, 208V). The standard pairs them: 480 system → 460 utilization. Don't confuse the two when reading nameplates.
SLD Symbol Legend
A single-line diagram is a schematic shorthand. Three-phase systems get drawn with one line, even though three conductors exist. Equipment is symbolic. Here are the symbols you'll see on every commercial / industrial drawing.
SLDs use single lines for 3-phase circuits. Counts (3φ, 3W vs 4W) are noted in text or with tick marks across the line.
Worked Example 1 — Atlas DC1 MEL Walk-Through
Below is a fragment of Atlas DC1's MEL exactly as you would receive it. Three rows: a chiller, a pump, and an IT-room PDU. Walk through what each row tells you.
Example 01 · Atlas DC1 spine
Three rows from the MEL — what to extract
| Tag | Description | V | HP/kW | FLA | Starter | Location | Notes |
|---|---|---|---|---|---|---|---|
| CH-1 | Centrifugal chiller | 480V 3φ | 450 HP | 480 A | VFD | MR-1 | N+1, water-cooled |
| CWP-1 | Cond water pump | 480V 3φ | 75 HP | 96 A | VFD | MR-1 | 1-per-chiller |
| PDU-A1 | IT power dist unit | 480→415Y/240V | 500 kVA | 602 A in | n/a | IT Hall A | Side A · UPS-fed |
What each row tells you
-
Row 1 — CH-1: A 450 HP motor on a 480V 3φ system, started by a VFD. Located in mechanical room MR-1. There is a fourth identical chiller (N+1 standby).Action: branch circuit on the 480V SWGR — A bus. FLA = 480 A from NEC table. Wire ≥ 600 A (MCA = 1.25 × FLA). Breaker ≤ 1200 A (MOCP = 250%, NEC 430.52). Connect ahead of VFD with output reactor — see §14 Motors.
-
Row 2 — CWP-1: Smaller motor, 75 HP. Same voltage, same MCC bus. Co-located with chiller (MR-1).Action: branch circuit on same MCC. FLA 96 A → MCA 120 A → wire #1 AWG (per NEC 310.16, 75°C). Breaker ≤ 240 A (per 430.52). Pump cycles with chiller via control logic — confirm with Mech.
-
Row 3 — PDU-A1: 500 kVA Power Distribution Unit. 480V input, 415Y/240V output. Located in IT Hall A.Action: feed from UPS-A1 output, NOT directly from 480V SWGR (it's a critical IT load). Input current 602 A — requires 800 A breaker on UPS-A1's distribution panel. PDU is itself a step-down transformer plus distribution; see §09 Transformers.
!
The "Notes" column is where the danger lives
Half the field RFIs come from electrical engineers ignoring MEL notes. "Standby" means N+1 — adjust load study. "Water-cooled" means there's a cooling tower load you might miss. "UPS-fed" means a different feeder source than the rest of the room. Read every notes column twice.
Worked Example 2 — Tracing Power Flow on the Atlas DC1 SLD
Once you have the SLD, you should be able to start at any load and trace the full path back to the utility — naming every device, voltage, and protection along the way. This is the single most useful skill in commissioning, troubleshooting, and arc-flash work.
Example 02 · Atlas DC1 spine
Trace one server's power: from rack PDU back to the utility
The path (server → utility)
- Server power supply — 415V or 240V input
- Rack PDU — 415Y/240V outlet on the rack
- RPP / branch circuit — a 30A or 60A breaker in the row power panel
- PDU-A1 — 500 kVA distribution unit, 480V→415Y/240V step-down + sub-distribution
- UPS-A1 output bus — Critical UPS Bus A, 480V (before the PDU step-down)
- UPS-A1 — 1250 kVA double-conversion static UPS, with VRLA battery string
- 480V SWGR — A — main switchgear bus, 4000A rated, fed from TX-A or GEN-A via ATS-A
- ATS-A — automatic transfer switch, normal position = TX-A, transfers to GEN-A on utility loss
- TX-A — 2500 kVA pad-mount transformer, 12.47kV→480Y/277V, %Z = 5.75
- MV SWGR primary feeder — 12.47 kV, 3φ, 50kA fault current available
- Utility source — local distribution feeder, 12.47kV grounded-wye
What this trace gives you
i
Voltage transitions
12.47 kV → 480V → 415V → 240V at the load. Four voltage levels, three transformations. Each step = a separately derived system requiring its own grounding (see §13).
⚠
Arc flash points
Eight equipment locations on this trace need an arc flash label (NEC 110.16). Incident energy varies wildly — highest at PDU-A1 distribution panel (close to UPS source, fast trip), lowest at MV switchgear (longer trip times).
!
Why 2N matters in this trace
Side B is identical and independent. Each server has dual PSUs (A + B). If anything on the A path fails, the server pulls from B with no interruption. This is why the cross-tie breaker between SWGR-A and SWGR-B is normally open — closing it would couple the two sides and defeat 2N.
If You See THIS, Think THAT
| If you see… | Think / use… |
|---|---|
| "MEL" or "Equipment Schedule" handed to you | You're at Stage 1. Extract V, HP/kW, FLA, MOCP, location for every row before drawing a single line. |
| HP given but no FLA on the MEL | Use NEC Table 430.250 for FLC. Don't calculate from HP × 0.746 / V / PF for NEC sizing — that's nameplate, not table. |
| "480V" on a cutsheet | Means 480Y/277V 3φ-4W in commercial. Confirm wire count: 4-wire if 277V loads exist, 3-wire if motor-only. |
| "460V" on a motor nameplate | Utilization voltage — equipment expects 460V at terminals. System is 480V, with ~4% drop accepted. |
| Single-line shows a circle | Rotating machine — motor (M), generator (G), or sync condenser (SC). Letter inside identifies. |
| Single-line shows two intersecting circles | Two-winding transformer. Configuration (Δ-Y, Y-Y, etc.) labeled separately or shown with explicit windings. |
| Dashed line on the SLD | Either emergency/standby (often copper-colored), or a normally-open device. Read the label. |
| "2N" topology mentioned | Two completely independent paths. Cross-ties normally open. Each path sized for full load. |
| "N+1" topology mentioned | One redundant unit. Less expensive than 2N. Doesn't tolerate the failure of more than one unit. |
| "Tier III" in DC documentation | Concurrently maintainable — any single component can be taken offline for service without dropping load. 2N or 2(N+1) typical. |
| "PDU" in a data center | Power Distribution Unit — typically a 480→415Y/240V step-down transformer with downstream sub-panels. NOT a "plug strip." It's an entire piece of switchgear. |
| "RPP" in a data center | Remote Power Panel — a panelboard at the row level, fed from a PDU. Provides the actual rack-level branch circuits. |
| "Behind the UPS" or "critical bus" | Load is non-interruptible. Must be fed from UPS output, not utility-direct. Coordinated independent of mech loads. |