Size rack power and PDU breakers with the 80% continuous-load rule, watts-to-amps and VA conversion, growth headroom, and A/B redundancy checks that test the full load after either feed fails.
Start with four choices. Open Advanced settings only for custom assumptions, unequal feeds, or capacity planning.
Presets are planning examples, not wiring recommendations. Regional connectors, nominal voltages, codes, standard breaker sizes, and equipment ratings vary.
Use metered current when available; otherwise enter real watts or estimated equipment loads.
Choose A/B only when the rack has two separate sources; define the topology in Advanced settings.
One single-phase or phase-derived feed only. Use the voltage actually across the PDU input: line-to-neutral is one line to neutral; line-to-line is between two lines. Do not apply √3 here.
Enter the lowest applicable input or upstream rating. A separate B rating is available in Advanced settings.
Enter real power for everything that must remain powered.
Names identify rows; quantity must be a whole number; watts per device must be a non-negative real-power estimate. Quick-add wattages are rough estimates only—replace them with metered, datasheet, or approved planning values.
Current equipment total: 0 W
Use a representative peak reading, not only an idle snapshot.
Enter zero only when the B feed is normally unloaded.
Measured amps are converted to estimated watts with the selected voltage and power factor. The amperage checks themselves use your readings directly.
Added on top of the current load and shown separately.
PF relates real power to apparent power: W = V × A × PF.
Default 80%. Use another value only when your approved design supports it.
True redundancy tests the full design load on either surviving feed. Non-redundant dual feeds are checked only in their normal state.
Active/passive mode is normally concentrated on its active feed.
The remainder is assigned to B.
Defaults to the A rating; edit for asymmetric feeds.
Included in copied and downloaded results.
The calculator checks normal operation and, for true redundancy, each surviving feed.
Enter load and breaker details to check normal and failover utilization.
Standard size logic
The mathematical minimum and candidate standard sizes will appear here.
Candidate sizes are a comparison list, not a claim of universal availability or permission. Standard ratings, conductor sizes, connectors, installation choices, and approvals vary by equipment and jurisdiction.
| Scenario | A feed amps | A target headroom | B feed amps | B target headroom | Utilization | Status |
|---|---|---|---|---|---|---|
| No calculation yet. | ||||||
Inputs: 1,500 W current load, 120 V, PF 0.95, 20 A breaker, 80% target, 0% growth.
Substitution: load current = 1,500 W ÷ (120 V × 0.95) = 13.16 A; usable target = 20 A × 0.80 = 16 A.
Result: 82.2% of the 16 A planning target, with 2.84 A or about 324 W remaining. Apparent power is 1,579 VA.
Interpretation: Passes the selected continuous-load target. The 20 A rating itself is not the normal continuous planning target.
Inputs: 5,200 W design load, 208 V, PF 0.95, two 30 A feeds, balanced active/active, 80% target.
Substitution: normal per feed = 5,200 W ÷ 2 ÷ (208 V × 0.95) = 13.16 A; failover = 5,200 W ÷ (208 V × 0.95) = 26.32 A; target = 30 A × 0.80 = 24 A.
Result: Normal operation uses 54.8% of each target, but failover uses 109.6% and exceeds the target by 2.32 A or about 458 W.
Interpretation: Passes normal operation but fails the selected A- or B-feed-loss target. Reduce design load to about 4,742 W or use a fully approved higher-capacity path.
Inputs: 8 A on A, 12 A on B, 230 V, PF 0.95, two 32 A feeds, true redundant active/active, 80% target.
Substitution: current load = (8 A + 12 A) × 230 V × 0.95 = 4,370 W; failover current = 8 A + 12 A = 20 A; target = 32 A × 0.80 = 25.6 A.
Result: B is the limiting normal feed at 46.9% of target. Failover reaches 78.1%, leaving 5.6 A or about 1,224 W. Apparent power is 4.60 kVA.
Interpretation: Both normal and full-load surviving-feed checks pass, although the 40/60 imbalance should be understood before adding equipment.
Approximate real-power capacity below assumes PF 0.95. Nameplate VA is V × A; 80% usable watts are V × breaker A × 0.80 × 0.95. “100% approved” is a comparison only and applies only when the entire configuration is listed, installed, and permitted for that use.
| Feed | Nameplate VA | 80% usable amps | Approx. watts at 80%, PF 0.95 | Approx. watts at 100%, PF 0.95 |
|---|---|---|---|---|
| 20 A at 120 V | 2,400 VA | 16 A | 1,824 W | 2,280 W |
| 30 A at 120 V | 3,600 VA | 24 A | 2,736 W | 3,420 W |
| 20 A at 208 V | 4,160 VA | 16 A | 3,162 W | 3,952 W |
| 30 A at 208 V | 6,240 VA | 24 A | 4,742 W | 5,928 W |
| 16 A at 230 V | 3,680 VA | 12.8 A | 2,797 W | 3,496 W |
| 32 A at 230 V | 7,360 VA | 25.6 A | 5,594 W | 6,992 W |
| 30 A at 240 V | 7,200 VA | 24 A | 5,472 W | 6,840 W |
The weakest link governs: use the lowest applicable rating among the upstream circuit, breaker, plug, cord, receptacle, PDU input, internal branch or outlet-bank protection, and connected equipment.
This calculator models one single-phase input or one phase-derived feed at the voltage entered. It does not use
the √3 multiplier for balanced three-phase input. A 120 V line-to-neutral feed measures from one line
to neutral; a 208 V line-to-line feed measures between two lines. Use the voltage actually presented to the PDU.
Real and apparent power
W = V × A × PF VA = V × A A = W ÷ (V × PF)
Watts are real power consumed; VA or kVA are apparent power carried by the circuit and commonly used on PDU and UPS ratings. When trustworthy peak current is metered, it avoids estimating current from nameplate watts and an assumed PF.
Growth reserve
reserve W = current W × growth %
design W = current W + reserve W
Continuous-load target
target A = breaker A × continuous target %
target W = V × target A × PF
A/B redundant failover
surviving feed A = design W ÷ (V × PF)
Recommended breaker rating
mathematical minimum A = worst-case A ÷ continuous target %
The displayed standard size is the first value in the visible candidate list at or above that mathematical minimum. It is not an installation recommendation.
True redundant active/active: both power supplies or feeds normally share the load. Choose balanced, manual, or concentrated normal distribution. The tool then tests the full design load on A and on B separately for 2N planning.
True redundant active/passive: one feed normally carries the protected load while the other is standby. The normal active feed and each full-load failover direction are checked.
Non-redundant dual feed: two inputs serve different loads or are not known to transfer the full protected load. The tool checks normal loading only and deliberately does not claim that either feed can survive a loss.
This tool does not evaluate conductor ampacity, receptacle ratings, PDU internal branch breakers, temperature derating, harmonic current, inrush behavior, three-phase phase balance, or local code details. Treat the results as planning math for conversations with facilities, electrical contractors, and equipment vendors.
A design can pass the arithmetic and still be unsuitable. Confirm the transfer behavior of the actual PSUs, breaker trip characteristics, connector and cord ratings, bank limits, required redundancy level, and the complete approved installation.
Updated 14 July 2026. Starlight Tools calculates deterministic planning values in the browser. Defaults are PF 0.95, 80% continuous target, balanced active/active A/B loading, and no growth reserve. Equipment preset watts are explicitly estimates. No reviewer credential is claimed.
These sources support the general method, not a site-specific design. Available standard sizes and permissible choices vary by jurisdiction, product listing, and manufacturer.
At an 80% target and PF 0.95, a 20 A feed is about 1,824 W at 120 V or 3,162 W at 208 V. A 30 A feed is about 2,736 W at 120 V or 4,742 W at 208 V. Actual capacity is the lowest approved component limit.
With balanced A/B loading, 40% on A plus 40% on B becomes about 80% on the surviving feed after a failure. That preserves an 80% continuous target while allowing the full protected load to transfer.
Prefer a reliable measured current or power reading from a representative peak. Nameplate watts are useful for early conservative planning but can overstate typical demand; still account separately for startup peaks and growth.
For true redundant A/B mode, the calculator places the full design load on each surviving feed in turn. Active/passive equipment is also tested at full load on its active feed. Non-redundant dual feeds receive no failover pass claim.
Yes. A PDU can have internal branch or outlet-bank breakers in addition to the upstream building branch breaker. They protect different sections, and the lowest applicable current or power limit governs.
Not usually as a continuous planning load on a standard 80%-rated circuit: 30 A becomes 24 A. Use 100% only when the complete equipment and installation are specifically approved for it.
A bank breaker can limit a subset of outlets before the PDU input or upstream limit is reached. Check both the total PDU reading and each protected bank, and redistribute equipment only within manufacturer instructions.
A redundant design is limited by the smaller breaker, cord, receptacle, or PDU rating on either side.
A/B racks can pass normal utilization and still fail the surviving-feed check after one side drops.
Nameplate watts are useful early, but live PDU metering captures real utilization and power factor behavior.
Reserve capacity makes future rack adds and maintenance transfers less likely to create a breaker event.
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