Instant 120/208V Wye Feeder Sizing Calculator — Accurate Electrical Service for Mixed Loads

This article explains Instant 120/208V Wye feeder sizing for mixed commercial and industrial electrical loads.

Practical calculations, code considerations, and a calculator methodology ensure accurate, compliant feeder selection every installation.

120/208 V Wye Feeder Sizing Calculator for Mixed Loads (Minimum Ampacity and Conductor Size)

Total three-phase load (kVA)

Total 120 V single-phase load (kVA)

System line-to-line voltage (V)

Continuous-load factor (%)

Advanced options

Additional design margin (%)

Ambient temperature (°C)

Number of current-carrying conductors

Conductor material Copper (Cu) Aluminum (Al)

Insulation temperature rating (°C) 60 °C 75 °C 90 °C

Upload a clear nameplate or one-line diagram image to suggest typical load and system values automatically.

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Enter the connected loads and system parameters to obtain the minimum feeder ampacity and recommended conductor size.

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Formulas used

1. Three-phase current from three-phase load

I_3φ (A) = S_3φ_kVA × 1000 / (√3 × V_LL)
where S_3φ_kVA is the total three-phase apparent power in kVA and V_LL is the line-to-line voltage in volts.

2. 120 V single-phase component (assumed balanced on three phases)

V_LN (V) = V_LL / √3 (for a wye system)
I_120_per_phase (A) = S_1φ_120_kVA × 1000 / (3 × V_LN)
where S_1φ_120_kVA is the total 120 V line-to-neutral apparent power in kVA.

3. Total base feeder phase current

I_base (A) = I_3φ + I_120_per_phase

4. Continuous-load adjustment

I_cont (A) = I_base × (Continuous_load_factor_% / 100)
Typical continuous-load factor is 125%.

5. Additional design margin (optional)

I_design (A) = I_cont × (1 + Additional_margin_% / 100)

6. Ampacity correction for temperature and number of conductors

F_temp = temperature correction factor (function of ambient temperature and insulation rating)
F_group = grouping correction factor (function of number of current-carrying conductors)
F_total = F_temp × F_group

7. Minimum base ampacity from tables

I_table_required (A) = I_design / F_total

The conductor size is selected as the smallest standard size whose tabulated ampacity at the chosen material and 75 °C rating is greater than or equal to I_table_required.

Quick reference: typical copper feeder ampacities (75 °C)

Conductor size (Cu)	Approx. ampacity (A)
6 AWG	65 A
4 AWG	85 A
3 AWG	100 A
2 AWG	115 A
1 AWG	130 A
1/0 AWG	150 A
2/0 AWG	175 A
3/0 AWG	200 A
4/0 AWG	230 A
250 kcmil	255 A
300 kcmil	285 A
350 kcmil	310 A
400 kcmil	335 A
500 kcmil	380 A
600 kcmil	420 A

Technical FAQ

Does this calculator assume perfectly balanced 120 V loads across the three phases?

Yes. The 120 V single-phase load is treated as evenly distributed on all three phases of the 120/208 V wye system. For strongly unbalanced loading or significant harmonic content, perform a more detailed phase-by-phase and neutral-current analysis.

How is the 125% continuous-load requirement applied?

The entered continuous-load factor multiplies the calculated base current. With the common 125% setting, the feeder is sized so that its ampacity is at least 1.25 times the calculated mixed-load current.

Does the ambient temperature and conductor count really affect the selected conductor size?

Yes. The calculator applies simplified correction factors for ambient temperature and for the number of current-carrying conductors. If these reduce the usable ampacity, a larger conductor size is selected to ensure the corrected ampacity still exceeds the design current.

Are the ampacities shown suitable for final code-compliant design?

No. The ampacity values and conductor selections are for preliminary engineering estimates. Always verify final feeder sizing against the applicable electrical code, manufacturer data, and project specifications before construction.

Scope and purpose of an instant 120/208V Wye feeder sizing calculator

This document defines methods and parameters required for an accurate, code-compliant feeder sizing calculator for 120/208V Wye systems serving mixed loads. The target audience includes electrical engineers, designers, contractors, and software developers building or validating an "instant" feeder sizing tool for commercial and light industrial installations.

System fundamentals: 120/208V Wye characteristics

A 120/208V Wye is a three-phase, four-wire system where line-to-line voltage is 208 V and line-to-neutral is 120 V. Balanced three-phase loads use 208 V (line-to-line); single-phase loads such as receptacles, lighting, and some small equipment use 120 V (line-to-neutral).

Key electrical relationships

Fundamental three-phase power and current relationships used by the calculator:

Variable explanations and typical values:

• P = real power in watts (W). Typical lighting and receptacles specified in watts or kW.
• S = apparent power in VA or kVA. Transformers and service equipment often specified in kVA.
• I = current in amperes (A).
• V_ll = line-to-line voltage; for 120/208V Wye, V_ll = 208 V.
• V_ln = line-to-neutral voltage; for 120/208V Wye, V_ln = 120 V.
• PF = power factor (unitless). Typical PF: lighting and resistive loads ~1.0; motors 0.8–0.9; mixed commercial ~0.9.
• sqrt(3) ≈ 1.732.

Regulatory and normative references

Design and calculation steps must follow applicable codes and standards. Key references:

• NFPA 70, National Electrical Code (NEC) — general feeder sizing, ampacity, conductor adjustment, continuous loads. See: https://www.nfpa.org/
• NEC 220 (Load Calculations), NEC 210 (branch circuits), NEC 215 (feeder requirements), NEC 310 (conductor ampacity), NEC 430 (motors).
• IEEE standards for harmonic considerations and power factor correction; e.g., IEEE Std 519 for harmonic control: https://standards.ieee.org/
• IEC 60364 for international wiring rules: https://www.iec.ch/
• Manufacturer datasheets for transformers, motors, and distribution equipment (e.g., Eaton, Schneider Electric, Siemens).

Calculator methodology: algorithmic workflow

An instant feeder sizing calculator must execute these ordered steps to produce safe, compliant results:

1. Input collection: list all loads (kW, kVA, single- or three-phase), load types (continuous vs non-continuous), motor nameplate data, diversity/demand areas (lighting, receptacles, HVAC), and service voltage (120/208V Wye).
2. Classification: separate 120 V single-phase loads and 208 V three-phase loads; tag continuous loads (operating >3 hours) for NEC 125% adjustment.
3. Compute currents per load using the relationships above; convert kW or kVA to amps appropriately.
4. Apply demand factors per NEC 220 for general lighting, receptacles, and appliances. For specific loads (motors, HVAC), apply relevant NEC motor and HVAC rules.
5. Sum currents on each phase considering load diversity and phase imbalance; compute both line currents and neutral currents for 4-wire wye feeders (harmonics affect neutral sizing).
6. Apply conductor ampacity selection and adjustment factors (temperature correction, conductor bundling/parallel runs per NEC 310). Increase continuous loads by 125% per NEC 210.24 and 215.3 where applicable.
7. Check overcurrent protection device (OCPD) sizing per NEC rules (e.g., 215.3, 240.4, 430.52). Select OCPD and coordinated disconnects; account for motor inrush and short-time withstand.
8. Perform voltage drop calculation and enforce maximum recommended drop (commonly 3% for feeders and 5% overall). Adjust conductor size if necessary.
9. If harmonic-producing loads exist, compute harmonic currents and assess neutral conductor sizing and derating for heat; consider K-factor or triplen harmonic treatment.
10. Output: recommended conductor AWG/size, insulation type, OCPD rating, transformer kVA (if calculating service transformer), voltage drop percent, and notes referencing code clauses used.

Formulas and computational details (HTML-only expressions)

Use these exact formulas in the calculator implementation. Replace the variable names with values when computing.

Three-phase current from real power (watts):

Three-phase current from apparent power (kVA):

Single-phase current (line-to-neutral):

Voltage drop for a three-phase feeder:

Where R_phase is the round-trip impedance per phase (ohms per conductor length multiplied by length).

Continuous load adjustment (per NEC):

Transformer secondary current (for 120/208V three-phase):

Neutral current for triplen harmonics (simplified approach):

I_neutral ≈ |I_h3 + I_h9 + ...| where triplen harmonic currents add algebraically; for high triplen content, neutral may exceed phase magnitude.

Variable examples (typical values)

• V_ll = 208 V (120/208V Wye).
• V_ln = 120 V.
• PF (lighting) = 0.95–1.00, (motors) = 0.8–0.9, (UPS and electronics) = 0.9–0.99.
• sqrt(3) ≈ 1.732.
• Voltage drop target: feeder ≤3% recommended.

Tables of common values

Detailed worked examples

Below are two full examples illustrating a step-by-step solver and final feeder/conductor sizing for instant calculation implementation.

Example 1 — Small office floor, mixed 120 V and 208 V loads

Scenario: A 120/208V Wye feeder supplies a small office floor with the following connected loads:

• Lighting: 12,000 W (all 120 V line-to-neutral), considered continuous.
• Receptacles and small equipment: 8,000 W (120 V), non-continuous.
• Small three-phase HVAC unit: 15 kW (three-phase, 208 V), non-continuous, PF = 0.9.
• Server rack with UPS: 5 kVA (three-phase), continuous; PF = 0.95.
• Motor loads: none additional.

Step 1: Separate single- and three-phase loads.

• Single-phase total P_1φ = 12,000 + 8,000 = 20,000 W.
• Three-phase real power P_3φ = HVAC 15,000 W; UPS apparent S_ups = 5 kVA.

Step 2: Apply demand factors (NEC 220) — for a typical office, some demand factors apply for receptacles and lighting; for conservatism in an instant calculator we can use full connected or apply typical diversity. For this example apply moderate diversity:

• Lighting: use 100% of connected (continues) => 12,000 W (NEC requires continuous increase).
• Receptacles: apply typical 75% demand factor => 8,000 * 0.75 = 6,000 W.

Step 3: Continuous load adjustment (NEC 210/215): continuous loads must be increased to 125% for conductor and OCPD sizing.

• Lighting is continuous: I_lighting at 120 V = 12,000 / 120 = 100 A. Adjusted per NEC for conductor/OTC: I_lighting_calc = 100 * 1.25 = 125 A.

Step 4: Compute single-phase current for receptacles (non-continuous):

I_receptacles = 6,000 / 120 = 50 A.

Step 5: Compute three-phase currents.

Calculate numerically: denominator = 1.732 * 208 * 0.9 ≈ 324.7; I_hvac ≈ 15,000 / 324.7 ≈ 46.2 A.

For UPS: I_ups = (S *1000) / (sqrt(3) * V_ll) = (5 * 1000) / (1.732 * 208) ≈ 5000 / 360.1 ≈ 13.88 A.

Step 6: Phase allocation and summation.

Assume single-phase loads are balanced across phases where feasible. For conservative sizing, sum worst-case phase currents. For this example assume even distribution of single-phase loads across all three phases; therefore each phase receives 1/3 of single-phase total.

Total single-phase load current (post-demand): I_1φ_total_operating = (12,000 + 6,000) / 120 = 18,000 / 120 = 150 A operating. But lighting is continuous and has been adjusted earlier for conductor selection. For per-phase distribution: distribute 150 A / 3 = 50 A per phase operating.

Now add three-phase currents per phase: per-phase current = 50 A (single-phase share) + I_hvac per phase contribution (46.2 A is already three-phase current) + UPS three-phase (13.88 A).

Per-phase total I_phase_operating = 50 + 46.2 + 13.88 ≈ 110.08 A.

Step 7: Continuous adjustment for conductor sizing.

• Lighting required conductor current was increased to 125 A for continuous portion; adjusting distributed share: lighting portion per phase increased from 40 A to 50 A * 1.25? To be precise, we should increase only the continuous portion. Earlier distribution assumed full values. For conservative approach, increase the total phase current by 25% of continuous portion. Lighting distributed per phase = 12,000/3/120 = 33.33 A per phase; adjusted = 33.33*1.25 = 41.67 A. Receptacles remain 16.67 A per phase.
• Recompute per-phase: lighting adj 41.67 + receptacles 16.67 + HVAC 46.2 + UPS 13.88 ≈ 118.42 A.

Step 8: Select conductor ampacity.

Choose next standard conductor with ampacity >= 118.42 A. From table: 2 AWG (115 A) is insufficient; 1 AWG (130 A) is suitable. Therefore recommend 1 AWG Cu THHN per phase, subject to temperature correction and bundling.

Step 9: Voltage drop check.

Assume feeder length = 100 ft one-way. Conductor resistance for 1 AWG Cu approx 0.000321 ohm/ft (round numbers). Round-trip phase loop length = 200 ft.

Phase R_total ≈ 0.000321 * 200 = 0.0642 ohms.

VD = sqrt(3) * I_phase * R_total = 1.732 * 118.42 * 0.0642 ≈ 13.12 V. Percent VD = (13.12 / 208) * 100 ≈ 6.3%.

6.3% is above recommended 3% for feeders. Increase conductor size to reduce VD or accept longer run mitigation. Try 750 kcmil? Practically choose 4/0 reduces R significantly. For brevity, pick 4/0 Cu (230 A ampacity) with R ≈ 0.00005 ohm/ft => R_total ≈ 0.00005*200 = 0.01 ohm. VD ≈ 1.732 * 118.42 * 0.01 ≈ 2.05 V => 0.98% VD — acceptable. However 4/0 may be overkill; iterative sizing required (2/0 or 3/0 typically). Calculator must iterate to find minimal conductor meeting both ampacity and VD constraints. Final recommendation: 3/0 Cu (200 A) may produce acceptable VD (~1.5–2.5%).

Step 10: OCPD selection.

Select OCPD per NEC: continuous loads 125% accounted; OCPD must protect conductor; choose standard OCPD size not exceeding conductor termination ratings and per NEC rules. For 1 AWG conductor, typical OCPD is 150 A; but since conductor current demand ~118 A, use 125 A or next standard rating, depending on installation. Full coordination needed. The calculator outputs recommendations and notes for final engineering review.

Example 2 — Light manufacturing shop with motors and harmonic loads

Scenario: A 120/208V Wye feeder serves a small manufacturing shop with the following connected loads:

• Lighting and general: 10 kW (continuous), 120 V.
• Three-phase motor #1: 10 HP, 208 V, nameplate FLC = 32 A (PF ~0.9), starts across-the-line.
• Three-phase motor #2: 20 HP, 208 V, nameplate FLC = 58 A (PF ~0.9), VFD-controlled (inverter), contributes harmonics.
• Welders and inverter loads: 15 kVA (single- and three-phase mix), significant harmonics, treat as nonlinear.

Step 1: Compute currents.

• Lighting current: I_lighting = 10,000 / 120 = 83.33 A operating; continuous => conductor design increase 1.25 factor applies.
• Motor #1 FLC given = 32 A.
• Motor #2 FLC given = 58 A; VFD-driven motors have additional considerations: the input current to the drive and harmonic spectrum must be considered; use FLC for fundamental contribution and account for harmonic currents separately.
• Welders/inverters S = 15 kVA. Assume three-phase distribution: I_inverters = 15,000 / (1.732*208) ≈ 41.6 A.

Step 2: Motor starting and TVA.

NEC requires feeder conductors to be sized to carry 125% of continuous loads; motor inrush and starting must be considered for OCPD selection and short-time capacity but do not necessarily require conductor up-sizing unless locked-rotor/starting current persists.

Step 3: Harmonic and neutral considerations.

• VFDs and welders introduce triplen harmonics (3rd, 9th, etc.) that add in the neutral conductor. Calculator must compute harmonic currents using recommended harmonic spectrum or K-factor equivalent for non-linear loads.
• Estimate harmonic current magnitude: for a VFD-fed motor, typical 3rd harmonic content might be 15–40% of fundamental line current depending on device and filters. For conservative design assume 30% 3rd harmonic.
• Compute neutral current contribution: I_h3_phase ≈ 0.30 * I_phase_fundamental for each phase; because triplen harmonics are in phase among phases, neutral current ~ sum of triplen currents ≈ 3 * I_h3_phase = 0.9 * I_phase_fundamental — which can be nearly equal to phase current.

Step 4: Numerical harmonic estimate and conductor sizing.

Assume balanced fundamental currents per phase: lighting 83.33 A distributed => 27.78 A/phase. Motor currents: distribute motor phasing; three-phase motors add fully to each phase: motor #1 = 32 A; motor #2 = 58 A; inverter load 41.6 A.

I_phase_fundamental = 27.78 + 32 + 58 + 41.6 ≈ 159.38 A.

Estimated 3rd harmonic per phase = 0.30 * 58 (for VFD motor portion only) + 0.30 * 41.6 (for inverter) ≈ 17.4 + 12.5 ≈ 29.9 A per phase. But if other loads have lower harmonic content, adjust accordingly. For conservative neutral sizing sum triplen from all sources: I_neutral_triplen ≈ 3 * 29.9 ≈ 89.7 A. This is in addition to fundamental neutral unbalance currents (which are small if balanced).

Neutral conductor must be sized to carry up to I_neutral_total ≈ sqrt(I_unbalanced^2 + I_triplen_sum^2). If balanced fundamentals assumed, neutral ≈ 89.7 A. Thus neutral size must be at least equivalent of 4 AWG (85 A) is slightly under, choose 3 AWG (100 A) or 2 AWG to be safe. For harmonic-rich environments NEC permits upsizing neutral or adding dedicated neutral conductor sized for harmonic currents.

Step 5: Phase conductor sizing.

I_phase with continuous lighting increased by 25% for continuous portion; lighting per-phase originally 27.78 A, adjusted = 27.78 * 1.25 = 34.73 A. Recompute I_phase_design = 34.73 + 32 + 58 + 41.6 ≈ 166.33 A.

Choose conductor ampacity >= 166.33 A. From table 3/0 is 200 A, 2/0 is 175 A (sufficient), so 2/0 CU (175 A) is marginally sufficient; but also consider derating for conductor bundling/ambient temperature. If derating reduces ampacity below 166 A, step up to 3/0. For conservative selection choose 3/0 Cu (200 A).

Step 6: Voltage drop and final check.

Compute voltage drop; if feeder length moderate, 2/0 or 3/0 will typically satisfy VD. Provide final recommendation and include note to install harmonic filters or oversize neutral to handle triplen currents, and to coordinate motor starting OCPDs per NEC 430.

Practical rules, derating, and special considerations for instant calculators

• Continuous loads: apply 125% to continuous loads for conductor and OCPD sizing.
• Ambient temperature and conductor bundling: apply NEC adjustment factors to ampacity before selecting conductor size. Common multipliers: 0.91, 0.88, etc., depending on temperature and number of conductors in raceway (see NEC 310.15).
• Motor full-load current: use nameplate FLC. For multiple motors, NEC 430 allows demand factors in some cases; the calculator should reference NEC 430.6 or local code rules.
• Harmonics: for installations with VFDs, UPS, or nonlinear loads, implement K-factor neutral sizing or calculate triplen harmonic additive currents and oversize neutral accordingly. Consider harmonic filters or phase-shifted VFDs to reduce triplen accumulation.
• Voltage drop: recommended maximum: feeder ≤3%, total from service to furthest point ≤5%. The calculator must offer iterative resizing to meet VD constraints.
• Transformer sizing: include inrush and continuous loads; size primary and secondary appropriately and ensure OCPD selection follows NEC 450 and 240 rules.
• Emergency and standby systems: follow NFPA 110 and NEC Article 700–701 for transfer switches, loads, and feeder sizing for alternate sources.

Implementation tips for an "instant" calculator

1. Use a structured input form: allow CSV or JSON upload of load lists with fields: load ID, kW/kVA or HP, phase type (1φ/3φ), voltage, PF, continuous flag, motor starting type, harmonic factor.
2. Automate demand factor tables by occupancy and load category with configurable defaults and override options.
3. Perform automated iterative sizing: compute base ampacity, then apply correction factors (temperature, bundling) and voltage drop check. Iterate conductor sizes until all checks pass.
4. Provide explanation text and normative references for each sizing decision (e.g., "Continuous load, 125% applied per NEC 210.24").
5. Include safety margins and warnings if unusual conditions exist (high harmonic content, extreme ambient temperatures, long feeders).
6. Offer downloadable summary reports with calculated values, equations used, and standard code citations for review by licensed engineers.

References and external authority links

• NFPA 70, National Electrical Code (NEC) — authoritative code. https://www.nfpa.org/
• NEC Handbook and specific articles: 220 (Branch-Circuit, Feeder, and Service Load Calculations), 215 (Feeder Requirements), 310 (Conductors for General Wiring), 430 (Motors, Motor Circuits, and Controllers). Refer to latest edition.
• IEEE Std 519 — Recommended Practices and Requirements for Harmonic Control in Electric Power Systems. https://standards.ieee.org/standard/519-2014.html
• IEC 60364 — Electrical installations of buildings (international standard). https://www.iec.ch/
• Manufacturer application notes for VFDs, UPS and transformers (e.g., Schneider Electric, Siemens, Eaton) — consult for inrush and harmonic characteristics.

Summary of deliverables for a compliant instant calculator

• Clear input schema for mixed loads and motor data.
• Accurate implementation of three-phase and single-phase equations (shown above in HTML form).
• NEC-based handling of continuous loads, demand factors, and conductor ampacity adjustments.
• Harmonic assessment routine and flexible neutral sizing rules.
• Iterative voltage drop and conductor selection logic that returns the smallest conductor meeting ampacity, OCPD, and VD constraints.
• Documented output with code citations and suggested equipment sizes.

Final engineering notes

• The calculator should be a decision-support tool, not a replacement for licensed electrical engineer review. Local amendments to the NEC, utility requirements, and equipment manufacturer instructions must be considered.
• Where conductor terminations or equipment ratings limit OCPD sizes, the calculator must flag potential violations and require manual intervention.
• For installations with high harmonic content, include options for harmonic mitigation (filters, oversized neutrals, phase-shifting transformers) with cost/benefit notes.

Use the formulas, workflows, and worked examples above to implement or validate an instant 120/208V Wye feeder sizing calculator that produces safe, code-compliant, and verifiable engineering outputs.