PART I System Design Basics
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Load Analysis

Connected · Demand · Continuous · Diversity

The MEL gives you a list of loads. Load analysis turns that list into the numbers that size every transformer, every feeder, every breaker. Two loads matter most: the one if everything ran at once (connected), and the one that actually happens (demand).

Load Type Definitions — One Place

NEC and engineering practice define many overlapping load terms. Here they are, all in one table.

TermDefinitionSource
Continuous loadMaximum current expected to continue for 3 hours or moreNEC Article 100 (definitions)
Non-continuous loadLoads not classified as continuous (cyclic, intermittent, brief peaks)Implicit from NEC 100
Connected loadSum of all nameplate ratings, treating every load as if running at 100%Engineering practice
Demand loadMaximum kW (or kVA) the system actually carries at peak — connected × demand factorNEC 220, IEEE 141
Demand factor (Df)Max demand / total connected load. Always ≤ 1.0. NEC 220 publishes specific values per occupancy.NEC 220
Diversity factor (Dv)Σ individual peaks / system peak. Always ≥ 1.0. Applied across multiple feeders.IEEE 141
Coincidence factor (Cf)1 / Dv. Inverse of diversity. Common in residential utility load research.IEEE 141
Load factor (Lf)Average demand / peak demand over a period. Indicates how "flat" usage is.Utility tariffs
Coincident loadLoads that DO peak together (heater + lighting both peak in winter evening)Engineering judgment
Noncoincident loadLoads that CANNOT peak together (heating vs cooling, NEC 220.60)NEC 220.60
Inrush currentBrief peak (typically 6-12× FLA) when energizing motors or transformers. Lasts less than 1 second.Motor/transformer characteristics
Locked-rotor current (LRA)Current a motor draws if rotor cannot turn. Typically 6-8× FLA. Sustained until protection trips.NEC 430.7
Starting currentCurrent during motor acceleration. Decreases as motor reaches rated speed.Motor characteristics
Cyclic loadLoad that turns on/off in a regular pattern (elevators, welders, AC compressors)Engineering judgment
Intermittent loadBrief operations followed by rest periods. NEMA defines duty cycles by ratio.NEMA MG 1
Standby loadLoad that's normally OFF but ready to operate (backup pumps, redundant equipment)Engineering practice
Critical loadLoad that must remain energized at all times (IT, life safety, process)Engineering judgment
Sheddable loadLoad that can be dropped without significant impact (lighting, comfort HVAC)Demand response (§27)
Linear loadLoad that draws current proportional to voltage (resistive heaters, incandescent)Power quality (§15)
Nonlinear loadLoad that draws current in non-sinusoidal pulses (rectifiers, VFDs, LEDs, servers). Generates harmonics.Power quality (§15)

Connected vs Demand Load

Connected load is what you'd see if every load nameplate ran at 100% simultaneously. Demand load is what the system actually pulls at peak — after accounting for the fact that not everything runs, not everything runs at full output, and not everything peaks at the same moment.

CONNECTED LOAD "if everything ran at once" Lighting · 100 kW HVAC · 180 kW Process motors · 320 kW Receptacles · 60 kW TOTAL CONNECTED: 660 kW × DEMAND FACTOR (what really runs simultaneously)
Connected = sum of all nameplates. Demand = what the system actually carries at peak.

The Two Numbers Side by Side

Connected Load Demand Load
Definition Sum of every load's nameplate, as if all ran simultaneously at 100% Maximum kW (or kVA) the system actually sees at peak
Always larger by… 1.0× (reference) 0.4× to 1.0× (depends on diversity, demand factor)
Used for Equipment room space estimate · transformer thermal limit ceiling · fault current · MCC bus design Feeder ampacity · service entrance · transformer kVA · utility metering · generator sizing
NEC reference — Article 220 — demand factors per occupancy & load type
Atlas DC1 2.5 MW IT + 2.5 MW mech + 0.3 MW BOP ≈ 5.3 MW 2.5 MW IT (designed at 100%) + ~1.8 MW mech (at peak) + 0.2 MW BOP ≈ 4.5 MW
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Atlas DC1 is unusual — IT is 100% demand
For most facilities, demand << connected because not everything peaks together. But IT load in a fully-loaded data center IS demand load — every server runs continuously. This is why DC1 is sized to deliver 2.5 MW continuously (not 2.5 MW occasionally). The mech load, however, has demand diversity: chillers ramp with cooling need, not all motors at full speed all the time.

Demand Factor vs Diversity Factor

Both reduce a number; they reduce different numbers and they live in different parts of the calculation. Distinguishing them is the first thing the PE exam tests in load analysis.

Factor Formula Typical range Where applied
Demand factor Df = max demand / connected load 0.4 – 1.0 One single load category (e.g., the lighting demand factor for a warehouse). NEC 220 publishes specific values.
Diversity factor Dv = Σ individual peaks / system peak 1.0 – 3.0+ Across multiple feeders/buildings — accounts for the fact that different consumers peak at different times.
Coincidence factor Cf = 1 / Dv 0.3 – 1.0 Inverse of diversity — sometimes published this way (especially in residential service/utility work).
Load factor Lf = average demand / peak demand (over a period) 0.3 – 1.0 Energy/revenue planning — not used directly for sizing, but tells you how "flat" or "peaky" your usage is. Atlas DC1: ≈ 0.95 (very flat).

NEC 220 Demand Factors — by Load Category

NEC Article 220 publishes the demand factors you must use for code-compliant sizing of feeders and services. Below are the most-used categories. Use these for the standard method (220.40).

Load type Threshold / Tier Demand factor NEC reference
General lighting (dwelling) First 3,000 VA 100 % Table 220.45
3,001 – 120,000 VA35 %
Above 120,000 VA25 %
General lighting (warehouse) First 12,500 VA / remainder 100 % / 50 % Table 220.45
General lighting (hospitals) First 50,000 / remainder 40 % / 20 % Table 220.45
Receptacles (non-dwelling) First 10 kVA 100 % 220.44
Remainder50 %
Cooking equipment (commercial) 1–6 units / 6+ units 100 % / 65 % / down to 50% Table 220.56
Range/oven (dwelling) 1 unit ≤ 12 kW 8 kW (per Table 220.55 — Column C) Table 220.55
Dryers (dwelling) 1–4 / 5+ 100 % / dropping per table Table 220.54
Motor feeder (mixed motor) Largest motor 125 % 430.24
All other motors100 %
HVAC (largest of) Heating OR cooling — pick the larger 100 % (largest noncoincident) 220.60
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220.60 — Noncoincident loads
When two loads will never run at the same time (heating & cooling are the classic example), only count the larger one. Same applies to electric heat vs gas heat backup. Don't double-count loads physically incapable of operating simultaneously.

Continuous vs Non-Continuous — The 3-Hour Rule

NEC 100 defines a continuous load as one whose maximum current is expected to continue for 3 hours or more. This trips a different multiplier than demand factor — the 125% rule for sizing wire and breakers.

0 1 hr 2 hr 3 hr time 0 Imax current CONTINUOUS — runs at Imax ≥ 3 hr 3-hr threshold NON-CONTINUOUS — peaks < 3 hr 3 hours determines the 125% multiplier
If max current persists ≥ 3 hours → continuous → 125% sizing on conductor + OCPD
For continuous loads — NEC 210.19(A) & 210.20(A)
Iwire ≥ 1.25 × Iload
IOCPD ≥ 1.25 × Iload
125% multiplier applied to both the conductor ampacity and the overcurrent device. NOT the same as demand factor — these are separate calculations that stack.
Mixed continuous + non-continuous
IOCPD ≥ 1.25 × Icont + 1.0 × Inon
Add the two pieces with their respective multipliers.
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Why 125%? — The thermal answer
Conductors and standard thermal-magnetic breakers are rated to carry 100% of nameplate continuously only at 80% of their maximum capability — for thermal margin. If you want to actually run 100% continuous load, you must derate the breaker/wire to 80%, which is the same as up-sizing by 1/0.80 = 125%. It's the same math, expressed two ways.

What Counts as Continuous?

Continuous loads (apply 125%)
  • Office & commercial lighting (open all day)
  • Retail floor lighting
  • Outdoor / street lighting
  • Server / IT loads (always on)
  • Refrigeration compressors (continuous duty)
  • Process equipment running production shifts
  • Battery chargers (sustained float)
  • Heaters running through cold weather
  • EV charging (often hours of continuous draw)
Non-continuous (no 125% multiplier)
  • Receptacles (cycle on/off through the day)
  • Welders (intermittent, duty-cycle sized)
  • Elevators (cyclic motor loads)
  • Cooking ranges (peaks < 3 hr)
  • Most HVAC (cycling on thermostat)
  • Snow melt, water heaters (typically cyclic)
  • Garage door openers, lift gates
  • Equipment with manufacturer-specified duty cycle

Motor Loads — Why They Have Their Own Rules

Motors get their own NEC article (430) because they violate two assumptions: (1) their starting current is 6–8× FLA for a few seconds, which would trip a normal-sized breaker; (2) they are typically continuous duty in industrial settings. The 125% multiplier appears, but for a different reason.

Motor calc Single motor Multiple motor feeder NEC reference
Conductor ampacity (MCA) 1.25 × FLC of motor 1.25 × largest motor FLC + 1.00 × all other motor FLCs + other loads 430.22 / 430.24
Branch-circuit OCPD (MOCP) Per Table 430.52 (e.g., inverse-time CB ≤ 250% × FLC) Largest motor's MOCP + sum of other motor FLCs + other loads 430.52 / 430.62
Overload protection Separate device (in starter/MCC) at 115–125% of FLA, NOT in branch breaker Each motor has its own overload 430.32
FLA source NEC Table 430.247–250 (FLC), NOT nameplate Same — table values 430.6(A)
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Motor branch circuits — three protections in one
A motor branch has three devices: (1) short-circuit / ground-fault protection (the breaker — set high, up to 250% FLC), (2) overload protection (in starter, set at 115–125% FLA), (3) disconnecting means. The breaker is NOT the overload — those are separate devices. Confusing them is the most common motor-circuit error.

Worked Example 1 — Atlas DC1 Load Study (Side A)

Build the load study for one side of Atlas DC1. This sizes the 480V SWGR-A bus, the TX-A transformer, GEN-A, and the feeder from the utility to the building.

Example 01 · Atlas DC1 spine Build the load study for Side A · feeds half the IT + half the mech

Side A loads (per the MEL)

Load Connected (kW) Cont.? Demand factor Demand kW Multiplier Sized kW
UPS-A1+A2 input (1.25 MW IT, 96% UPS η, 0.95 PF)1,302 kWYes1.01,302 kW1.25 (continuous)1,628 kW
CH-1 + CH-2 chillers (450 HP × 2)674Yes1.0 (peak)6741.25 largest = 1.0625 ave716
CWP-1, CWP-2 (75 HP × 2)112Yes1.01121.0 (other motors)112
CRAH fans (50 HP × 4)149Yes0.95 (modulating)1421.0142
Lighting (mech + IT halls)22Yes1.0221.2528
Receptacles & misc15No0.5 (NEC 220.44 above 10kVA)91.09
TOTAL — Side A2,318 kW——2,292 kW—2,708 kW (sized) → 2,851 kVA at PF 0.95

Step-by-step

  1. Convert IT load to electrical (input) kW. 1.25 MW IT (mechanical/computational output equivalent) needs to account for UPS efficiency (~96%) and PSU efficiency (~94%) — net ~10% loss between utility and useful load.
    UPS input kW = 1250 / 0.96 / 0.94 ≈ 1,386 kW. But for sizing the upstream we use the UPS rating itself: 2 × 1250 kVA × 0.95 PF = 2,375 kVA × 0.95 = ~2,256 kW. Pick the larger — UPS rating governs.
  2. Apply 125% to continuous loads. Per NEC 210.20 / 215.3, the OCPD must be sized to 125% of continuous load.
    UPS feeders: 1,316 × 1.25 = 1,645 kW for OCPD sizing. Lighting: 22 × 1.25 = 28 kW.
  3. Apply NEC 430.24 to motor feeder portion. Largest motor (CH-1 at 337 kW) gets 125%, all others at 100%.
    Motor feeder: (337 × 1.25) + 337 + 56 + 56 + 142 = 421 + 337 + 56 + 56 + 142 = 1,012 kW for motor portion sizing.
  4. Sum to size the 480V SWGR — A bus + TX-A.
    Total demand at 480V = ~2,652 kW. Divide by PF (0.95 average) = 2,791 kVA. TX-A spec'd at 2,500 kVA — close to the line. Real designs would either oversize TX-A to 3,000 kVA or accept slight overload at full load (only happens during commissioning + 100% IT loading + max mech).
  5. Cross-check: 480V FLA at the SWGR.
    FLA = (2,791 × 1000) / (√3 × 480) = 3,357 A — fits within the 4,000 A bus. ✓
  6. Generator sizing. GEN-A must carry 100% of Side A demand on utility loss. 2,791 kVA × ~1.10 starting margin (motor inrush) → ~3,070 kVA.
    GEN-A is spec'd at 2,500 kW × ~0.85 PF = 2,941 kVA — borderline. Real design would step up to 3,000 kW (3,750 kVA) to provide motor-starting margin. Atlas DC1 currently uses load-shedding logic to drop non-critical loads on genset operation.
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kW vs kVA — keep them straight
All "Sized (kW)" values above are real power AFTER continuous + motor multipliers. Convert to kVA for transformer/genset sizing by dividing by PF. Conductors and transformers are sized for kVA (apparent), not kW (real). See Atlas DC1 Canonical Specs for the reconciled load study with TX-A sizing called out.
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Real-world sanity check
A real Atlas DC1 load study spans 50+ rows with sub-totals per panel, per UPS module, per ATS source, and per phase. The condensed version here shows the methodology. The two questions every load study answers: "will the equipment fit thermally?" and "will the genset start the load?"

Worked Example 2 — 50-Unit Apartment Building (NEC 220 Standard)

Apartment building service sizing is the textbook NEC 220 problem. Same principles, different demand factors, simpler geometry. Watch how the stacked diversity reduces the connected load to a fraction of itself.

Example 02 · Alternate scale 50 dwelling units · 1,200 sq ft each · electric range & dryer in each · 208Y/120V 3φ service

Per-unit connected load (NEC 220.42 + 220.55)

ItemValueNotes
General lighting + receptacles1,200 ft² × 3 VA/ft² = 3,600 VANEC 220.41
2 small-appliance circuits2 × 1,500 = 3,000 VANEC 220.52(A)
Laundry circuit1,500 VANEC 220.52(B)
Range (12 kW nameplate)8 kVA (Table 220.55, Col C)NEC 220.55
Dryer (5 kW nameplate)5 kVANEC 220.54
Per-unit connected:21,100 VA—

Step-by-step (multi-family demand)

  1. Total connected lighting + small appliance + laundry across 50 units
    (3,600 + 3,000 + 1,500) × 50 = 405,000 VA
  2. Apply NEC 220.45 demand factors: 100% of first 3,000 + 35% of next 117,000 + 25% of remainder.
    3,000 + (117,000 × 0.35) + (285,000 × 0.25) = 3,000 + 40,950 + 71,250 = 115,200 VA
  3. Range demand for 50 ranges (Table 220.55, Col C, 50+ units):
    Per Table 220.55 — for 50 units of 8 kW each: total demand = 90 kW + (0.75 × 50) = 90 + 37.5 = 127.5 kW = 127,500 VA
  4. Dryer demand for 50 dryers (Table 220.54):
    First 4 at 100% = 4 × 5 = 20 kW. Next 8 at 85% = 8 × 5 × 0.85 = 34 kW. Next 8 at 75% = 30 kW. Continue per table → roughly 110,000 VA total dryer demand.
  5. Sum total demand load:
    115,200 + 127,500 + 110,000 = 352,700 VA = 352.7 kVA
  6. Convert to amps at 208V 3φ:
    I = (352,700) / (√3 × 208) = 352,700 / 360 = 980 A
    Service entrance sized at 1200 A, 208Y/120V 3φ with appropriate margin.
  7. Compare to connected: 50 units × 21,100 VA = 1,055,000 VA = 1,055 kVA connected. Demand is 33%. Two-thirds of the apparent load disappears via NEC 220 demand factors — the diversity of human behavior across 50 households.
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Why the demand factor stack works
Across 50 units, not every range fires at 6 PM. Not every dryer runs Saturday morning. Not every TV + microwave + lights peak together. NEC's tables encode decades of utility metering data into fixed multipliers. Trust them — but only for the load types they cover. An assisted living facility, hospital, or industrial plant uses different factors.

Drill — Quick Self-Check

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

Drill 1 · Continuous load

An office lighting circuit draws 12 A continuously. What's the minimum breaker size?

Drill 2 · Demand factor — receps

A commercial building has 30 kVA of receptacles. What's the demand load?

Drill 3 · NEC 430.24 motor feeder

Motors on one feeder: 50 HP (FLC 65A), 25 HP (FLC 34A), 10 HP (FLC 14A). What's the minimum feeder ampacity?

Drill 4 · Noncoincident loads

A panel has 50 kW of heating + 30 kW of cooling. NEC 220.60 demand?

Drill 5 · Atlas DC1

Atlas DC1 Side A demand was 2,708 kW after sizing multipliers. At PF 0.95 and 480V 3φ, what's the FLA?

If You See THIS, Think THAT

If you see…Think / use…
"Connected load" in a problemSum of all nameplates. NO demand factor. Use for fault analysis, equipment-room ceiling, transformer thermal limit.
"Demand load"What the system actually carries at peak. Use NEC 220 factors. Use for feeder, service, and transformer sizing.
Load operates ≥ 3 hours at maxContinuous → apply 125% to wire AND breaker (NEC 210.19, 210.20).
"Sum of connected load" + "demand factor"That demand factor is per NEC 220 Tables. Multiply category-by-category, not in bulk.
Multiple motors on a feederNEC 430.24: 125% of largest motor FLC + 100% of all other motor FLCs + other loads. Largest motor only gets the bonus.
Heating AND cooling on the same panelNEC 220.60: only count the larger one (noncoincident). They can't run together.
"Diversity factor" mentionedGreater than 1 — applied to peaks across multiple feeders. Don't confuse with demand factor.
Receptacles in commercialNEC 220.44: 100% of first 10 kVA, 50% of remainder.
50+ dryers in apartment buildingTable 220.54 — demand factor drops below 50%; very significant savings.
"Service factor" of 1.15 on motorAllows brief overload up to 115% — affects overload protection setting (430.32), NOT branch circuit sizing.
"Largest motor" called out in feeder problemNEC 430.24 applies. Tag it; the 125% bonus belongs to it.
"Load factor" mentionedEnergy-efficiency / utility metric. NOT a sizing factor. Don't apply to feeder calc.