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Calculators

Fuse Sizing
Enter kVA and voltage
Enter kVA and voltage

1φ at 150% min. 3φ at 200% below 225 kVA, 100% at 225 kVA and above.

Load Amps
Enter kVA and service type

Shows 100%, 200% (2 hr), and 250% (flash) amps per phase.

Fusing

Voltage Tolerances
Range A (±5%) at meter. Range B for extreme conditions.
Nominal Range A Range B
120 V114.0–126.0104.4–127.2
208 V197.6–218.4181.0–220.5
240 V228.0–252.0208.8–254.4
277 V263.2–290.9241.0–293.6
480 V456.0–504.0417.6–508.8
Single Phase · 150% Min
25 kVA ÷ 7.97 kV = 3.14 A
× 1.5 = 4.7 A → 6K Fuse
kVA 2400 V 7970 V
56K3K
1010K3K
1510K6K
2520K6K
37.530K10K
5040K10K
7565K15K
10080K25K
167100K40K
Three Phase · 200% / 100%
200% min below 225 kVA · 100% min at 225 kVA and above
kVA 4160 V 13800 V
156K6K
3010K6K
4515K6K
7520K10K
112.530K10K
15040K15K
22540K15K
30050K25K
50065K25K
750100K40K
1000140K40K
1500N/A65K
2500N/A100K

Banks

3-phase bank schematics & vectors. Verify nameplate and company standards before energizing.

Delta–Delta · 0° displacement · no system neutral required
Polarity check: Before closing the last secondary tie, measure across the open loop. Near 0 V = correct. ~double voltage (e.g. ~480 V on a 240 V delta) = reversed / bucked coil — do not close.
Bank schematic
PRIMARY (Δ) A B C Xfm A–B H1 H2 X1 X2 Xfm B–C H1 H2 X1 X2 Xfm C–A H1 H2 X1 X2 SECONDARY (Δ) a b c
Primary & secondary closed delta
Vector diagram
Primary Δ A B C Secondary Δ a b c In-phase · 0° displacement
  1. Hang three same-ratio transformers; float tanks (or ground per standard).
  2. Primary Δ: H1→A & H2→B on unit A–B; H1→B & H2→C on B–C; H1→C & H2→A on C–A.
  3. Secondary Δ (series loop): Connect X3 (or X2) of unit 1 to X1 of unit 2; X3 of unit 2 to X1 of unit 3; X3 of unit 3 to X1 of unit 1. (Do not land X1–X1 or X2–X2 in parallel — that shorts the bank if polarities are series-additive.)
  4. Final tie voltage check: Read across the last open secondary jumper before closing. ~0 V = OK to close. ~2× rated secondary (e.g. ~480 V on 240 V delta) = reversed polarity — swap leads and recheck.
Wye–Wye · H1→phase · H2→Npri · X1→phase · X2→Nsec
Warning: Always ensure the primary neutral is solidly tied to the system multi-grounded neutral (MGN). Floating or ungrounded Wye–Wye primary neutrals cause neutral shift, unstable line-to-neutral voltages, and severe ferroresonance under light load or single-phase switching.
Bank schematic
PRIMARY (Y) A B C Xfm A H1 H2 Xfm B H1 H2 Xfm C H1 H2 Npri SECONDARY (Y) X1 X1 X1 a b c Nsec (all X2)
Primary N solidly on system MGN · secondary N grounded
Vector diagram
Primary Y A B C N Secondary Y a b c n L–N phasors · 120° apart
  1. Three transformers, same ratio; H1 to each primary phase.
  2. Primary neutral (critical): Bond all H2 bushings solidly to the system multi-grounded neutral (MGN). Never leave primary N floating or open.
  3. Secondary: X1 to each phase; bond all X2 to secondary neutral and ground.
  4. Open primary N → neutral shift, wild L–N voltages, ferroresonance risk on 1φ switching.
  5. Common for 208Y/120 V and 480Y/277 V service when both sides are grounded wye.
Wye–Delta (Y–Δ) · primary Y · secondary Δ · typically 30° displacement
Primary neutral trap: Grounding the primary neutral on a closed Wye–Delta bank turns it into a grounding / backfeed bank. If an upstream primary phase opens, the bank can try to supply the system’s missing phase through the secondary and rapidly overheat or blow fuses.
Most utilities float (leave ungrounded) the primary H2 neutral bus on closed Y–Δ during normal operation. If Npri is landed for switching or lightning protection, open it after secondary is closed — or follow your utility SOP exactly.
Bank schematic
PRIMARY (Y) A B C Xfm A H1 H2 Xfm B H1 H2 Xfm C H1 H2 H2 bus · FLOAT SECONDARY (Δ) X1 X2 X1 X2 X1 X2 a b c
H1→phase · H2s tied together but typically floating · secondary closed Δ
Vector diagram · ~30° shift
Primary Y A B C n* Secondary Δ a b c *Primary N usually floated · 30° shift (per roll/ANSI)
  1. Three same-ratio transformers. Primary wye: H1→A/B/C; bond H2s together on a primary neutral bus.
  2. Normal operation: Leave primary H2 bus floating (ungrounded) unless your SOP says otherwise — prevents backfeed / grounding-transformer action on primary phase loss.
  3. If Npri is temporarily grounded for energizing/switching or lightning, open that bond after secondary is closed (per utility SOP).
  4. Secondary Δ: Series-connect X3→X1 around the loop (same polarity rules as Δ–Δ). Voltage-check last tie before closing.
  5. Confirm ANSI clock (e.g. Dy1 / Dy11) and company print before paralleling or phasing.
Delta–Wye (Δ–Y) · primary Δ · secondary Y · typically 30° displacement
Most common step-down for 208Y/120 V and 480Y/277 V. Primary needs no neutral; secondary neutral available for line-to-neutral loads.
Bank schematic
PRIMARY (Δ) A B C Xfm A–B H1 H2 Xfm B–C H1 H2 Xfm C–A H1 H2 SECONDARY (Y) X1 X2 X1 X2 X1 X2 a b c Nsec (all X2)
Primary closed delta · secondary wye with grounded neutral
Vector diagram · ~30° shift
Primary Δ A B C Secondary Y a b c n L–N secondary · ~30° from primary · ground Nsec
  1. Primary Δ: H1→A & H2→B on A–B; H1→B & H2→C on B–C; H1→C & H2→A on C–A.
  2. Secondary Y: X1 to each secondary phase; bond all X2 to Nsec and ground.
  3. No primary neutral required for pure closed delta primary.
  4. Phase secondary vs primary carefully — 30° displacement is normal; match ANSI clock / company print.
  5. Standard for 208Y/120 V and 480Y/277 V bank secondaries.
Open-Wye / Open-Delta · 2 transformers only
Neutral required: Both H2 bushings must land on the multi-grounded primary neutral. With only two units, return current flows through N — omit or float N and the bank fails.
Capacity: ~57.7% of an equivalent 3-transformer closed bank (same unit rating), or ~86.6% of the combined nameplate kVA of the two units.
Bank schematic
PRIMARY (open-Y) Neutral REQUIRED A C B (open) Xfm 1 A–N H1 H2 Xfm 2 C–N H1 H2 Npri — MUST CONNECT SECONDARY (open-Δ) X1 X2 X1 X2 a b c
H2 of both units on primary neutral · third phase open
  1. Install two transformers only (third primary phase open).
  2. Primary neutral: Both H2 bushings bond to multi-grounded primary N — mandatory return path.
  3. H1 of xfm 1 → A; H1 of xfm 2 → C (or company phase pair).
  4. Secondary open-Δ: Series secondaries for three phases; leave one corner open.
  5. Optional: center-tap one secondary for 120/240 V lighting + 3φ power.
  6. Derate: Plan load at ≤57.7% of closed 3-unit bank capacity (or ≤86.6% of two units’ total kVA).
120/240 V 4-Wire Δ · high-leg / wild-leg on B · A–N & C–N = 120 V · B–N ≈ 208 V · φ–φ = 240 V
Warning: NEC 110.15 requires orange identification of the high leg. Never connect 120 V single-phase loads between high-leg (B) and neutral — B–N is ~208 V (√3/2 × 240 V) and will destroy 120 V equipment.
Bank schematic · high-leg on B
PRIMARY (Δ or open) A B C Power A–B Power B–C Lighting C–A + CT SECONDARY 4-wire Δ N (CT) Lighting Xfm A C B HIGH-LEG wild-leg · orange
Lighting on A–C with CT · high-leg B opposite CT
A to Neutral
120 V
B to Neutral
208 V
C to Neutral
120 V
Any phase–phase
240 V
  1. Close secondary delta; center-tap lighting transformer on the A–C leg (X2 mid-point = neutral).
  2. Identify high-leg on B (phase opposite the lighting CT) — orange marking per NEC 110.15 / 230.56.
  3. A–N and C–N = 120 V lighting only. B–N ≈ 208 V — 3φ or 208 V loads only, never 120 V L–N.
  4. Phasing check: A–N ≈ C–N ≈ 120 V; B–N ≈ 208 V; all φ–φ ≈ 240 V.
AC / BD Secondary Paralleling
What it is: Most pole transformers have two secondary coils inside (tagged A, B, C, D). You can wire them in series (higher voltage) or parallel (lower voltage, full kVA). AC–BD is the parallel hookup.
Memory aid: Alley Cat · Bad Dog — put A with C, put B with D.
Why it matters: For a wye secondary bank (e.g. 120/208 V), coils must be paralleled so each unit puts out 120 V L–N. Series leaves you at ~240 V L–N (wrong for 208Y/120). Voltage can look “usable” on a meter in some setups, but kVA is cut in half if you don’t parallel — you only use one coil’s capacity.
What each setup does
SERIES Factory default · higher V X1 A–B C–D X2 120 + 120 = 240 V Coils end-to-end Typical 120/240 single-phase PARALLEL · AC–BD Lower V · full nameplate kVA X1 A–B C–D X2 Both coils = 120 V Needed for 120/208 Y banks
Series stacks voltage · Parallel shares load across both coils at lower voltage
Lead tags · Alley Cat / Bad Dog
Inside the secondary compartment Coil 1 ~120 V winding A B Coil 2 ~120 V winding C D A + C → one bushing B + D → other bushing
Alley Cat (A–C) together · Bad Dog (B–D) together = parallel
Series vs AC–BD parallel (leads)
SERIES (not paralleled) X1 A coil B jumper C coil D X2 B jumpered to C · A→X1 · D→X2 · ~240 V total PARALLEL · AC–BD X1 A coil 1 B C coil 2 D X2 / N A & C landed together · B & D landed together Both coils see ~120 V · full kVA · use for 208Y/120 Industry habit: X2 (or common) often serves as neutral on Y banks
Always match the nameplate diagram on the tank — tags may vary by manufacturer
Quick compare
Connection
Result
Series (A–B–C–D chain)
~240 V L–L / higher V
Parallel (AC–BD)
~120 V per coil · full kVA
Use parallel when…
Wye secondary · 120/208
Skip parallel when…
120/240 single-phase series
How to do it (field steps)
  1. De-energize & ground the transformer. Open secondary compartment cover only when it is safe and allowed by your SOP.
  2. Find the tags on the secondary coil leads: A, B, C, D (stamped or tagged). Confirm against the nameplate connection diagram.
  3. See what you have now. Series usually has a jumper from B to C, with A and D on opposite bushings. Parallel has no B–C series jumper — A is with C, B is with D.
  4. To parallel (AC–BD):
    • Land A and C together on one secondary bushing (Alley Cat).
    • Land B and D together on the other secondary bushing (Bad Dog).
    • Remove any series B–C jumper that would leave coils in series.
  5. For a 120/208 Y bank: After paralleling each unit, land each unit’s phase lead (often X1) to its secondary phase and bond the common/neutral (often X2) of all three units to the secondary neutral and ground — per company print.
  6. Check every unit in the bank the same way. One unit still in series will fight the bank (wrong voltage / circulating issues).
  7. Verify before livening: nameplate diagram matches your leads; connections tight; cover secured. After energizing (per SOP), confirm L–N ≈ 120 V and L–L ≈ 208 V on a Y secondary.

Standards

OH Standards
Install with fiberglass cutout standoff, squirrel guard on transformer, and cutout animal guard.
Secondary wire: 1/0 triplex crib · #2 triplex services · 1/0 CU tap · #6 stranded CU neutral/tank ground to down ground.
Crib splitting: Prefer two transformers when 9+ customers on a crib.
# Customers Recommended kVA
1–425
5–837.5
9–1150
12+75

Guideline only — does not account for full load.

Vertical · Mid-Span
Lowest sag. NESC 232 / 234.
Surface Sec Pri
Pedestrian12 ft14 ft
Res. Drive12 ft16 ft
Commercial16 ft16 ft
Public Roads18 ft18 ft
Highways22 ft22 ft
Over Roof10 ft14.5 ft
At Pole · NESC 235
Primary above secondary. Vertical separation.
Pair At Pole Mid-Span
Pri → Sec40 in30 in
To Neutral12 in12 in
Sec → Drip12 inN/A
To Guy/Arm6 in—

Always confirm local standards and latest NESC edition.