This guide presents precise methods for sizing UPS bypass breakers and feeder conductors efficiently safely.
Rules cover continuous loads, NEC and IEC requirements, calculations, examples, and practical tips for electricians.
UPS bypass breaker and feeder sizing calculator (continuous load quick rules)
Technical framework for UPS bypass, feeders, and continuous load rules
When sizing UPS bypass breakers and distribution feeders, electricians must combine electrical physics with code rules and vendor requirements. The core tasks are: calculate actual current demand from the UPS/load, apply continuous-load multipliers, select an appropriate overcurrent protective device (OCPD), and choose conductor ampacity while applying correction and adjustment factors. Key regulatory drivers are the National Electrical Code (NEC, NFPA 70) continuous-load rule (loads expected to run for three hours or more) and equipment manufacturer instructions (UPS rating and bypass ratings). International projects will also reference IEC 62040-series for UPS performance and safety. Always coordinate final selections with system short-circuit study, manufacturer documentation, and local authority having jurisdiction (AHJ).Essential formulas and variable definitions
Single-phase apparent-current: I = P / (V × PF)
Three-phase apparent-current (kW or kVA): I = (S × 1000) / (√3 × V)
Where continuous sizing multiplier: I_continuous = I × 1.25
Where required breaker rating: Breaker ≥ next standard size ≥ I_continuous
Explain variables and typical values
- I — current (amperes). Typical units: A.
- P — real power (watts). For systems specified in kW, use P × 1000.
- S — apparent power (kVA). Use S × 1000 to convert to VA.
- V — voltage (line-to-line for three-phase). Typical site values: 208 V, 400 V, 480 V.
- PF — power factor (decimal). Typical UPS loads: 0.8 to 0.95; many modern UPS specify 0.9 or unity output for kVA = kW designs.
- √3 — square root of 3 (≈ 1.732).
- I_continuous — required continuous ampacity for conductor and design reference (125% multiplier per NEC for continuous loads).
Continuous-load rules and overcurrent device interaction
NEC baseline:- Continuous load definition: expected to run for three hours or more at sustained demand.
- Conductor ampacity for continuous load: must be at least 125% of the continuous load current (NEC requirement widely cited; verify local edition of NFPA 70).
- Overcurrent protection: OCPD rating must be selected consistent with conductor ampacity and manufacturer instructions. Historically, many designers size OCPDs at 125% for continuous loads; some modern equipment lists and 100%-rated breakers permit different configurations—check manufacturer guidance and NEC allowances.
- UPS bypass circuitry is commonly used for maintenance and tests; bypass breakers often must be sized equal to the maximum UPS input/output continuous current rating. Follow manufacturer bypass diagram and apply continuous multipliers.
- Where the UPS supplies a continuous critical load, size upstream feeders and bypass breakers for the larger of UPS full-load current or load continuous current multiplied by 1.25.
- Ambient temperature, conduit fill, and conductor bundling require ampacity derating (see NEC Table 310.x and adjustment rules). Always apply these corrections before final conductor sizing.
Common reference tables
| UPS Rating (kVA) | Voltage (V) 3ϕ | Calculated Current I (A) | Continuous Ampacity I×1.25 (A) | Typical Standard Breaker Size (A) | Notes |
|---|---|---|---|---|---|
| 30 | 480 | 36.1 | 45.1 | 50 | Use 50 A breaker; confirm conductor ampacity & derating. |
| 50 | 480 | 60.1 | 75.1 | 80 | 80 A breaker common; verify terminal ratings. |
| 75 | 480 | 90.2 | 112.7 | 125 | 125 A breaker typical; allow for transient inrush. |
| 100 | 480 | 120.3 | 150.4 | 175 | 175 A breaker recommended (150 A would be undersized). |
| 150 | 480 | 180.4 | 225.5 | 250 | Large feeders; coordinate with utility and transformer capacity. |
| 200 | 480 | 240.5 | 300.6 | 350 | 350 A is a conservative standard choice. |
| UPS Rating (kVA) | Voltage (V) 3ϕ | Calculated Current I (A) | Continuous Ampacity I×1.25 (A) | Typical Standard Breaker Size (A) | Notes |
|---|---|---|---|---|---|
| 30 | 208 | 83.3 | 104.2 | 125 | 208 V contexts commonly use 125 A breaker. |
| 50 | 208 | 138.9 | 173.6 | 175 | 175 A or 200 A based on coordination. |
| 75 | 208 | 208.3 | 260.4 | 300 | 300 A breaker typical; check bus ratings. |
| 100 | 208 | 277.8 | 347.2 | 350 | Large feeders and utility coordination required. |
| 150 | 208 | 416.7 | 520.8 | 600 | Consider paralleling feeders or larger single feeder designs. |
| 200 | 208 | 555.6 | 694.4 | 800 | Very large; typically requires substation-level distribution. |
Step-by-step methodology for quick estimates
- Obtain UPS rating: kVA or kW and nominal output voltage (line-to-line for three-phase).
- If kW provided, convert to kVA using kVA = kW / PF (use manufacturer-specified PF or conservative 0.9 if unknown).
- Calculate phase current: use I = (kVA × 1000) / (√3 × V) for three-phase, or I = (kVA × 1000) / V for single-phase.
- Apply continuous-load multiplier: I_continuous = I × 1.25 (NEC continuous load rule).
- Select next-higher standard breaker rating ≥ I_continuous (consider 100%-rated breakers if specified by manufacturer; otherwise use standard thermal-magnetic sizing rules).
- Select conductor ampacity ≥ I_continuous. Apply ambient temperature and adjustment factors per NEC 310.15 and other local tables, then choose conductor AWG or cross-section accordingly.
- Confirm short-circuit withstand and selective coordination of the chosen breaker and upstream protective devices; verify with manufacturer wiring diagrams for bypass and static transfer switches.
Worked examples with full development
Example 1 — 100 kVA UPS at 480 V three-phase, continuous data center load
Problem statement:- UPS rating: 100 kVA (manufacturer lists pf = 0.9 for output, but the UPS is rated in kVA; load is largely resistive so use kVA directly).
- Site voltage: 480 V three-phase (line-to-line).
- Load is expected to be continuous (24/7 critical load).
I = (S × 1000) / (√3 × V) = (100 × 1000) / (1.732 × 480)
I = 100000 / 831.36 = 120.25 A
I_continuous = I × 1.25 = 120.25 × 1.25 = 150.31 A
Next standard breaker rating greater than 150.31 A is 175 A (150 A would be undersized). Choose 175 A OCPD for the UPS input/bypass.
Conductor ampacity must be ≥ 150.31 A after applying any correction/adjustments. Using NEC tables (see NEC 310.16 for conductor ampacity and apply ambient corrections), choose a copper conductor with ampacity ≥ 150.31 A. For example, in many installations a 250 kcmil copper conductor (or other size per local ampacity table) may satisfy the requirement; confirm with final NEC column selection and terminal temperature rating (75 °C or 90 °C) and derating factors.
Step 5 — verify manufacturer and coordination:- Confirm the UPS manufacturer allows a 175 A bypass breaker at the specified terminal ratings.
- Coordinate upstream OCPD, bus rating, and transformer secondary rating; check for simultaneous loading and inverter short-circuit withstand capability.
- Calculated full-load current: 120.25 A
- Continuous ampacity design value: 150.31 A
- Selected breaker: 175 A (typical)
- Selected conductor: size that provides ampacity ≥ 150.31 A after derating (verify with NEC Table 310.16 and adjustments)
Example 2 — 50 kVA UPS at 208 V three-phase with 0.9 PF load
Problem statement:- UPS rating: 50 kVA.
- Output voltage: 208 V three-phase.
- Load power factor: 0.9 (loads are mixed with some non-linear equipment).
- Load is considered continuous (data center battery-backed equipment).
I = (S × 1000) / (√3 × V) = (50 × 1000) / (1.732 × 208)
Compute denominator: 1.732 × 208 = 360.256
I = 50000 / 360.256 = 138.88 A
I_continuous = 138.88 × 1.25 = 173.6 A
Nearest practical standard breaker rating ≥ 173.6 A is 175 A (or 200 A if coordination or other constraints push you to the next size). For this case select 175 A if manufacturer approvals allow.
Step 4 — conductor ampacity:Conductor ampacity must be ≥ 173.6 A once derating is applied. Select copper conductors per NEC ampacity tables; apply ambient temperature correction and conductor count adjustments before finalizing AWG or mm².
Step 5 — coordination and manufacturer checks:- Confirm the UPS bypass switch and terminal lugs are rated for the selected breaker rating and conductor size.
- If in an IT enclosure with multiple feeders, ensure parallel feeder rules and load sharing are addressed per manufacturer instructions.
- Calculated current: 138.9 A
- Continuous ampacity requirement: 173.6 A
- Recommended breaker: 175 A (common)
- Conductor: size to meet ≥173.6 A after derating (verify per NEC table selection)
Practical considerations and advanced adjustments
Ambient temperature and bundling
- NEC requires ampacity correction for ambient temperatures above 30°C (86°F) and adjustment for more than three current-carrying conductors in a raceway or conduit. Apply these corrections to the base ampacity before selecting conductor size.
- If the corrected ampacity falls below the continuous requirement, upsizing conductor cross-section is necessary even if the uncorrected ampacity would have been adequate.
100%-rated breakers and listed equipment
- Some breaker manufacturers provide 100%-rated breakers allowed to carry 100% of their rating continuously (e.g., certain bolted breakers). If the selected breaker is 100%-rated and listed for continuous operation, the breaker selection can sometimes align differently, but conductor ampacity must still comply with the 125% rule unless specific code sections and equipment listings allow otherwise.
- Always document manufacturer guidance in the installation files to present to the AHJ.
Coordination with bypass switches and utility transformers
- Bypass breaker sizing must match not only the steady-state current but also ensure the equipment can handle transfer transients. Check static bypass or maintenance bypass switch ratings in the UPS manual.
- Transformer secondary and primary protection must be coordinated so that a UPS bus transfer or bypass does not cause nuisance tripping upstream.
Regulatory and normative references
- NFPA 70, National Electrical Code (NEC) — see requirements on conductor ampacity, overcurrent protection, and continuous loads. Official site: https://www.nfpa.org/NEC
- IEC 62040 series — international standards for UPS performance, safety, and testing. See IEC site for standards and purchasing: https://www.iec.ch/
- Manufacturer technical guides and installation manuals (APC by Schneider Electric, Eaton, Vertiv, etc.) provide specific wiring and bypass instructions. Example vendor technical library: https://www.eaton.com/ (search UPS manuals) and https://www.se.com/
- NEMA and IEEE publications for power distribution practice and equipment coordination (e.g., IEEE Std 141, IEEE Std 242) provide guidance on coordination and reliability: https://standards.ieee.org/
Checklist for electricians before final installation
- Confirm UPS nameplate: kVA, rated input/output voltages, power factor, maximum continuous current, and bypass ratings.
- Calculate I using the correct formula for single-phase or three-phase, and apply 1.25 for continuous loads.
- Select an OCPD rating that is ≥ I_continuous and compatible with the UPS manufacturer’s instructions.
- Choose conductor sizes based on corrected ampacity per NEC tables and consider derating for temperature and groupings.
- Verify terminal lug temperature limits (60 °C, 75 °C, 90 °C) as they determine which ampacity column to use in NEC tables.
- Coordinate short-circuit and selective protection studies; ensure breaker interrupting rating is adequate for available fault current.
- Document decisions and obtain AHJ approval where required.
Additional references and useful links
- NFPA — National Fire Protection Association: NEC resources and code explanations: https://www.nfpa.org/NEC
- IEC — International Electrotechnical Commission: standardization and IEC 62040 overview: https://www.iec.ch/
- Schneider Electric / APC UPS technical library (installation manuals and wiring diagrams): https://www.se.com/ or https://www.apc.com/
- Eaton UPS technical documentation and application notes: https://www.eaton.com/
- IEEE Standards association for power system design best practices: https://standards.ieee.org/
Key takeaways for quick adoption
- Always compute UPS currents from kVA or kW using the correct three-phase or single-phase formulas and include PF where appropriate.
- Apply the continuous-load multiplier of 1.25 to determine conductor ampacity needs and guide breaker selection.
- Select the next standard breaker rating equal to or greater than the continuous ampacity; confirm equipment listings and AHJ expectations.
- Apply NEC temperature and adjustment factors before final conductor selection; verify lug temperature ratings.
- Cross-check with UPS manufacturer bypass and transfer switch guidance; coordinate with system protection and transformer constraints.