PART III Sources & Service
§10 / 39

Transformers

kVA · %Z · Δ-Y configurations · inrush · NEC 450

Every voltage transition in your system has a transformer behind it. Sizing comes from the load study; %Z determines fault current downstream; the winding configuration determines grounding rules. Get all three from the cutsheet.

The Three Numbers Every Transformer Cutsheet Has

Sizing is from the load study (kVA). %Z determines downstream fault current. The winding configuration determines grounding rules. Get all three from the cutsheet — every other characteristic follows.

ParameterWhat it doesAtlas DC1 TX-A
kVA ratingMaximum continuous output. Sized at ~110-125% of demand load to allow thermal cycling.2,500 kVA
Voltage ratingsPrimary / secondary nominal voltages. Determines turns ratio and tap settings.12,470 / 480Y/277V
%Z (impedance)Per-unit impedance. Lower %Z → higher fault current downstream. Standard values 4.5–7%.5.75%
Winding configurationΔ-Y, Y-Y, Δ-Δ, Y-Δ. Determines neutral availability and grounding strategy.Δ-Y (delta primary, wye secondary, neutral grounded)
Cooling classHow heat is removed. ONAN (oil-natural air-natural), ONAF (oil-natural air-forced), KNAN (less-flammable fluid), Dry-type.KNAN (less-flammable fluid for indoor use)
Insulation classTemperature rise rating. 65°C standard for new equipment.65°C rise
Tap settings±2.5% no-load taps for fine voltage adjustment. 4 taps each side of nominal typical.±5% in 2.5% steps

Standard kVA Sizes

ClassStandard sizes (kVA)
Single-phase1, 1.5, 3, 5, 7.5, 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500
Three-phase15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500, 3000, 5000, 7500, 10,000

Winding Configurations — Δ vs Y

ConfigurationWhere usedProsCons
Δ-Y (delta-wye)Most common. Distribution transformers, all utility step-downs to commercial/industrialWye secondary provides neutral for 1φ loads. Delta primary blocks zero-sequence currents from secondary fault → quieter primary.30° phase shift between primary and secondary (leading or lagging by 30°, depending on labeling)
Y-Y (wye-wye)Some utility distribution, autotransformers, industrial step-downs where primary and secondary both need neutralsNo phase shift. Both sides have neutrals available.Requires careful 3rd-harmonic management; ground faults transfer between primary and secondary
Δ-Δ (delta-delta)Industrial 480V-480V step transformers, isolation transformersNo phase shift. Open-delta operation possible (one bank can fail and system continues).No neutral. Cannot serve 1φ phase-to-neutral loads.
Y-Δ (wye-delta)Step-up transformers (generator to grid), some industrial applicationsGenerator side has neutral. Delta secondary blocks 3rd harmonics from grid.30° phase shift (opposite direction from Δ-Y).

Why %Z Matters — Fault Current Downstream

%Z (per-unit impedance) determines how much current the transformer can deliver into a downstream fault. Lower %Z = higher fault current. This sets the AIC requirement for downstream switchgear.

Fault current at transformer secondary (infinite primary bus assumption)
Ifault = IFLA / (%Z / 100)
Quick approximation assuming infinite primary bus (utility impedance = 0). Real fault current is somewhat lower because the utility has real impedance — see §12 for the rigorous MVA method that includes utility contribution. The error is small (~5%) for stiff utilities, larger (20%+) for weaker connections.

Inrush Current — Why Upstream Protection Sees the Pain

When energizing a transformer, the inrush can be 8–12× rated current for the first half-cycle, decaying to normal in 6-10 cycles. Upstream OCPD must allow this without tripping.

AspectDetail
Magnitude8-12× FLA peak first half-cycle; decays in 6-10 cycles to normal
CauseDC offset in flux when energized — depends on point-on-wave of switching
MitigationUpstream OCPD picked to coordinate above inrush curve. NEC 450.3 specifies primary protection ≤ 250% of rated primary current for transformers ≥ 1000V.
Sympathy inrushEnergizing a new transformer can trigger inrush in already-energized adjacent transformers — must consider in protection coordination

Worked Example 1 — Atlas DC1 TX-A Sizing & Fault Current

Example 01 · Atlas DC1 spine2,500 kVA, 12.47kV-480Y/277V, %Z = 5.75 — what does this mean for the system?
  1. Why 2,500 kVA? Side A demand was ~2,791 kVA (from §03). 2,500 kVA appears slightly undersized — but real installation oversizes the genset side and accepts brief overload at full IT loading. Many real DCs would spec 3,000 kVA.
  2. Secondary FLA:
    FLA = 2,500,000 / (√3 × 480) = 3,007 A
  3. Fault current at 480V bus (using %Z):
    Ifault = 3,007 / 0.0575 = 52,300 A symmetric
    → 480V SWGR-A bus must be rated for ≥ 52 kA AIC. Standard ratings: 65 kA. ✓
  4. Inrush: 10× × 3007 = ~30 kA peak first half cycle. Primary breaker (12.47 kV side) must let this through.
  5. Primary protection (NEC 450.3 for transformers ≥ 1000V): 250% × primary FLA = 2.5 × (2,500,000 / (√3 × 12,470)) = 2.5 × 116 = 290 A. Primary CB sized at 300 A or fuse at 250 A. ✓

Worked Example 2 — Office Building Step-Down (480→208/120V)

Example 02 · Alternate scale75 kVA, 480-208Y/120V, %Z = 5%, dry-type indoor — sized for office lighting + receptacles
  1. Office demand load: 50 kW lighting + 10 kVA receptacles + 5 kVA misc = ~65 kVA total demand. → Use 75 kVA standard size.
  2. Primary FLA: 75,000 / (√3 × 480) = 90 A
  3. Secondary FLA: 75,000 / (√3 × 208) = 208 A
  4. Primary protection (NEC 450.3(B), < 1000V): 125% of primary FLA → 113 A → use 125 A breaker.
  5. Secondary protection (NEC 450.3 not required if primary is sized at <125%): Many designs add secondary protection anyway — 225 A panelboard MCB matches the 208 A secondary FLA.
  6. Fault current at 208V bus: 208 / 0.05 = 4,160 A. 208V panelboards routinely rated for 10 kA AIC — ample margin.

Drill — Quick Self-Check

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

Drill 1 · FLA from kVA

1,500 kVA, 480V 3φ secondary. FLA?

Drill 2 · %Z fault current

1,500 kVA, %Z = 5.5%. Approximate fault current at secondary (infinite primary)?

Drill 3 · Δ-Y vs Δ-Δ

A transformer secondary serves both 3φ motors AND 1φ-N lighting. Configuration?

Drill 4 · NEC 450.3 primary

1,500 kVA at 4160V (primary). NEC 450.3 max primary OCPD?

Drill 5 · Atlas TX-A

Atlas TX-A: 2,500 kVA, 480V secondary, %Z = 5.75. Fault at secondary (infinite primary)?

Worked Example 3 — Setting Transformer Taps for Voltage Optimization

Real utility primary voltage often runs slightly off nominal. Distribution transformers have ±2.5% no-load taps to compensate. Setting the tap correctly delivers nominal voltage at the secondary loads.

Example 03 · Tap optimizationAtlas DC1 TX-A — utility primary measured at 12,830V instead of nominal 12,470V

The problem

  1. Utility primary measured = 12,830V (2.9% high above 12,470V nominal). Common when transformer is close to substation.
  2. TX-A secondary with all taps in nominal position would deliver: 12,830 / 12,470 × 480 = 494V at light load.
    Per ANSI C84.1, utilization range is 456V to 504V (95-105% of 480V). 494V is within range but tight, especially if drops accumulate downstream.

Tap selection

TX-A has 5 no-load primary taps in 2.5% steps: +5%, +2.5%, Nominal, −2.5%, −5%.

  1. Choose the +2.5% tap. This raises the primary turns by 2.5%, requiring 2.5% more primary voltage to deliver the same secondary voltage.
  2. New secondary at no-load:
    Vsec = (12,830 / 12,470) × (1 / 1.025) × 480 = 1.029 × 0.9756 × 480 = 482V
    Centered in the 456-504V range. Plenty of margin for downstream voltage drops.
!
"No-load" taps must be set with transformer DE-ENERGIZED
No-load (NLT) taps require the transformer to be completely de-energized — they cannot be switched under load. Load Tap Changers (LTC) are different equipment that can switch under load (used in utility substation transformers, not pad-mounts). For Atlas DC1 pad-mount TX-A, tap changes happen during scheduled maintenance with full LOTO (§29).
i
Why measure first, set tap second
Don't set taps from the design assumption. Wait for actual primary voltage measurements over several days/weeks (some utilities have seasonal variation). Then set the tap that centers your secondary in the ANSI band.

If You See THIS, Think THAT

If you see…Think / use…
"%Z" or "5.75%" on cutsheetPer-unit impedance. Drives fault current. Lower %Z = more fault current downstream.
"Δ-Y" windingStandard utility/commercial config. 30° phase shift. Wye secondary has neutral.
"Y-Y" windingBoth sides have neutrals. Watch for 3rd harmonic issues. Less common.
"Δ-Δ" windingNo neutral. Industrial 480-480V isolation. Cannot serve 1φ-N loads from secondary.
"K-factor 4" or "K-13" transformerDesigned for harmonic loads (servers, VFDs, LEDs). Larger neutral, special core. Used in DCs.
"Pad-mount" transformerOutdoor utility-grade. 12.47kV/480V typically. Used at service entrance for commercial/industrial.
"Dry-type" transformerIndoor, no oil. NEMA 1 enclosure. Lower %Z = louder. Cooling: AA (ambient air), AFA (forced air).
"ONAN/KNAN" transformerLiquid-cooled. ONAN = mineral oil, KNAN = less-flammable fluid (FM-200, Envirotemp). KNAN is required for indoor liquid-cooled.
"Inrush current"8-12× rated for ~half cycle. Upstream OCPD must coordinate above inrush curve. NEC 450.3 sizing.
NEC 450.3Transformer overcurrent protection. ≥ 1000V: ≤ 250% primary; < 1000V: ≤ 125% primary (with exceptions).
Tap settings shown±2.5% no-load taps. Adjust if utility primary voltage is consistently high or low.