🆓 Free Tool · IEEE C57.12.00 · NEC Compliant
Transformer Voltage Drop Calculator
Calculate transformer voltage regulation AND secondary wire voltage drop — two tools in one, single-phase
& three-phase.
3Φ & 1Φ Support
NEC 210.19 Check
Wire Gauge Upgrade Guide
V_terminal = V_rated × (1 − VD%/100)
Example: 1000 kVA, Z%=5.75%, R%=1.5%, PF=0.85 at
full load → VD% = 3.1%, Terminal voltage = 465V from rated 480V.
Two Types of Transformer Voltage Drop — Know the Difference
When people search for a transformer voltage drop calculator, they often mean two different
things. This tool covers both:
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① Transformer Voltage Regulation
The internal voltage drop inside the transformer winding. Caused by winding resistance (R%) and
leakage reactance (X%). Depends on load current and power factor. Calculated with Z% from the
nameplate.
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② Secondary Wire Voltage Drop
The drop in the conductors between transformer and load. Depends on wire gauge (AWG), wire length,
conductor material (copper vs. aluminum), and load current. Calculated per NEC 210.19(A).
Total voltage drop at the load = Transformer regulation + Wire drop. I’ve seen many
engineers only calculate wire drop and ignore transformer regulation — a mistake on large transformers.
A 2000 kVA transformer can have 3–4% internal regulation, and if wire drop adds another 2%, the load
sees 5–6% total drop, causing motor starting issues and LED lighting flicker at the end of long runs.
Transformer Voltage Regulation Formula (IEEE C57.12.00)
Step 1: Find X% = √(Z%² − R%²) → leakage reactance
Step 2: Find sinφ = √(1 − cos²φ) → from power factor
Step 3: Load Factor = Actual Current / Full Load Current
Step 4: VD% = LF × (R% × cosφ + X% × sinφ)
Step 5: V_terminal = V_rated × (1 − VD%/100)
| Transformer | Z% | R% | Power Factor | VD% (Full Load) | 480V → Terminal V | Rating |
|---|---|---|---|---|---|---|
| 75 kVA | 3.0% | 1.2% | 0.85 | 1.89% | 470.9 V | Excellent |
| 150 kVA | 3.5% | 1.4% | 0.85 | 2.21% | 469.4 V | Excellent |
| 300 kVA | 4.0% | 1.4% | 0.85 | 2.56% | 467.7 V | Good |
| 500 kVA | 4.5% | 1.5% | 0.85 | 2.88% | 466.2 V | Good |
| 1000 kVA | 5.75% | 1.5% | 0.85 | 3.10% | 465.1 V | Acceptable |
| 1000 kVA | 5.75% | 1.5% | 0.70 | 3.58% | 462.8 V | Acceptable |
| 1500 kVA | 6.0% | 1.6% | 0.85 | 3.24% | 464.4 V | Acceptable |
| 2000 kVA | 5.75% | 1.5% | 0.80 | 3.34% | 463.9 V | Acceptable |
Values at rated secondary 480V, full load, infinite primary source.
R% estimated as ~26% of Z% (typical for distribution transformers).
Wire Voltage Drop — AWG Reference Table
Use this reference table alongside our voltage drop calculator to select the right
wire gauge for your transformer secondary feeder. Values assume copper conductor, 75°C
rating per NEC Table 9.
| Wire Gauge | R (Ω/1000 ft, Cu) | VD% at 100A, 100ft, 480V 3Φ | VD% at 100A, 100ft, 208V 3Φ | NEC 3% Limit at 100A |
|---|---|---|---|---|
| 6 AWG | 0.510 | 1.85% | 4.26% | ✅ 480V OK |
| 4 AWG | 0.321 | 1.16% | 2.68% | ✅ Both OK |
| 2 AWG | 0.202 | 0.73% | 1.69% | ✅ Both OK |
| 1/0 AWG | 0.128 | 0.46% | 1.07% | ✅ Both OK |
| 3/0 AWG | 0.0808 | 0.29% | 0.67% | ✅ Both OK |
| 250 kcmil | 0.0671 | 0.24% | 0.56% | ✅ Both OK |
| 500 kcmil | 0.0354 | 0.13% | 0.29% | ✅ Both OK |
Formula: VD_3ph = √3 × I × R × L / (1000 × V_LL). Copper, 75°C. For
aluminum, multiply R by 1.64.
How Power Factor Affects Transformer Voltage Drop
Power factor is the single biggest variable in transformer voltage regulation — often underestimated in
design. The reactive component (X% × sinφ) can dominate the total drop at low power factor.
| Power Factor | Load Type | VD% (1000kVA, Z=5.75%, R=1.5%, Full Load) | Terminal V (from 480V) | Effect |
|---|---|---|---|---|
| 1.00 | Pure resistive (heaters) | 1.50% | 472.8 V | Minimal drop |
| 0.95 | Office loads, LED lighting | 2.15% | 469.7 V | Low drop |
| 0.85 | General industrial | 3.10% | 465.1 V | Moderate |
| 0.80 | Motors at partial load | 3.34% | 463.9 V | Moderate |
| 0.70 | Welding, induction heating | 3.58% | 462.8 V | High drop |
| Leading (cap.) | Capacitor banks, lightly loaded cables | Negative | > 480 V | Voltage RISE |
Frequently Asked Questions
FLA, X% = √(Z%²−R%²). Terminal voltage = V_rated × (1−VD%/100). Example: 1000 kVA, Z=5.75%, R=1.5%,
PF=0.85 at full load → VD% = 3.1%, terminal = 465V.
drop?+
voltage drop = resistive drop in the conductors. Total drop at load = transformer regulation + wire
drop. Both must be calculated to find the actual voltage available at the equipment terminals.
3–5%: Acceptable (general purpose). >5%: Poor — consider lower Z% transformer, voltage tap changer,
or power factor correction. NEC recommends ≤3% branch circuit wire drop and ≤5% total.
transformer?+
At PF=0.8 (lagging), VD% = LF × (R%×0.8 + X%×0.6) — much larger since X% is typically 3–5× R%. At
leading PF (capacitive loads), the terminal voltage can actually rise above rated.
copper gives 1.85% drop (OK). For 208V 3Φ same setup — need 4 AWG for 2.68% (just under NEC 3% limit).
Longer runs or higher currents need larger wire. Aluminum conductors need 2 AWG sizes larger than copper
equivalent.