Audit rack PDU failover capacity, 2N headroom, and the effect of losing either A or B. Model balanced or custom active/active PSUs, active/passive supplies, ATS-fed single-cord loads, and dual feeds that do not actually transfer.
Use the simple defaults first. Open Advanced settings only when you have metering, unequal feeds, documented limits, or reserve requirements.
Use approved peak watts when available; equipment-builder totals can replace this value.
Single-phase line voltage at the load.
By default this is the upstream branch-breaker rating. Change the basis in Advanced settings.
Failover protection is claimed only when the modeled topology transfers the full protected load.
Balanced active/active: normal demand is split 50/50; after either feed fails, the survivor is checked against the full design load.
Breaker ratings receive the continuous factor. PDU continuous ratings do not.
Expose a separate B rating and B-side limits.
Useful when the main entered value is a PDU continuous rating.
Used to convert watts to current.
Applied only to breaker ratings, not already-derated PDU continuous ratings.
Example estimates are not product ratings. Replace every estimate with metered, manufacturer-approved, or facilities-approved figures. Choose W or VA for each row; VA is converted using the selected power factor.
| Device name | Quantity | Each | Unit | Redundancy / A-B connection | Row watts | Action |
|---|
Run the calculator to compare normal and feed-loss states.
| Scenario | A demand | B demand | Limiting feed | Target utilization | Absolute utilization | Headroom | Decision |
|---|---|---|---|---|---|---|---|
| No calculation yet. | |||||||
With a balanced A/B rack, each feed normally carries half the transferable load. If one feed fails, its half moves to the survivor. When the surviving branch must stay within an 80% continuous-load target, the balanced normal target is therefore 80% ÷ 2 = 40% per feed. Forty percent is a consequence of that split and target—not a universal electrical limit.
| State | A feed | B feed | Meaning |
|---|---|---|---|
| Normal | 40% | 40% | Balanced sharing |
| A lost | 0% | 80% | B carries the protected load |
| B lost | 80% | 0% | A carries the protected load |
All examples use single-phase math. “Target utilization” compares demand with the selected continuous capacity; “absolute utilization” compares it with the entered hard rating.
Inputs: 4,200 W, PF 0.95, two 30 A branch circuits, 80% target, balanced active/active, no reserve.
target = 30 A × 0.80 = 24 A; design current = 4,200 W ÷ (208 V × 0.95) = 21.26 A.
Normal: 10.63 A per feed (44.3% of target). Either feed lost: survivor 21.26 A (88.6% of target; 70.9% absolute), leaving 2.74 A or 0.54 kW. Verdict: passes normal and either feed-loss case.
Inputs: 5,200 W, PF 0.95, 30 A at 208 V per feed, 80% target, balanced active/active.
design current = 5,200 ÷ (208 × 0.95) = 26.32 A; normal per feed = 26.32 ÷ 2 = 13.16 A.
Normal: 54.8% of the 24 A target. Either feed lost: 26.32 A, 109.7% of target and 87.7% absolute, with a 2.32 A (0.46 kW) target shortfall. Verdict: passes normal load but fails the selected failover target.
Inputs: 3,000 W, PF 0.95, 208 V, 80% target, balanced active/active. Targets are 24 A on A and 16 A on B.
design current = 3,000 ÷ (208 × 0.95) = 15.18 A; normal is 7.59 A per feed.
A survives: 15.18 A, 63.3% of A target. B survives: 15.18 A, 94.9% of B target, leaving 0.82 A or 0.16 kW. Verdict: passes, limited by B.
Inputs: 1,400 W, PF 0.90, 120 V, 20 A per branch, 80% target. A is preferred; B is standby.
design current = 1,400 ÷ (120 × 0.90) = 12.96 A; target = 20 × 0.80 = 16 A.
Normal: A 12.96 A, B 0 A. A lost: B 12.96 A (81.0% of target). B lost: A remains 12.96 A. Headroom is 3.04 A or 0.33 kW. Verdict: passes if transfer behavior is verified.
Inputs: normal readings A 9 A and B 8 A, measured surviving-feed peak 19.5 A, 30 A at 208 V, 80% target, PF 0.95 for the kW estimate.
normal total estimate = (9 + 8) × 208 × 0.95 = 3.36 kW. The measured 19.5 A replaces the calculated failure current.
Normal: A 37.5% and B 33.3% of target. Failure: 19.5 A, 81.3% of target and 65% absolute, leaving 4.5 A or 0.89 kW. Verdict: passes the entered limits; confirm the test covered the true workload peak.
These are reference examples for branch-circuit ratings with an 80% continuous target, balanced A/B sharing, and PF 0.95 for the final watts column. They are not a list of universally standard breaker sizes. Product, code, conductor, plug, and regional requirements can differ.
| Voltage / rating | Nameplate VA | 80% capacity VA | Balanced normal per feed | Protected failover VA | Protected watts at PF 0.95 |
|---|---|---|---|---|---|
| 120 V / 15 A | 1,800 | 1,440 | 720 VA (6 A) | 1,440 | 1,368 W |
| 120 V / 20 A | 2,400 | 1,920 | 960 VA (8 A) | 1,920 | 1,824 W |
| 120 V / 30 A | 3,600 | 2,880 | 1,440 VA (12 A) | 2,880 | 2,736 W |
| 208 V / 20 A | 4,160 | 3,328 | 1,664 VA (8 A) | 3,328 | 3,162 W |
| 208 V / 30 A | 6,240 | 4,992 | 2,496 VA (12 A) | 4,992 | 4,742 W |
| 208 V / 50 A | 10,400 | 8,320 | 4,160 VA (20 A) | 8,320 | 7,904 W |
| 208 V / 60 A | 12,480 | 9,984 | 4,992 VA (24 A) | 9,984 | 9,485 W |
| Voltage / rating | Nameplate VA | 80% capacity VA | Balanced normal per feed | Protected failover VA | Protected watts at PF 0.95 |
|---|---|---|---|---|---|
| 230 V / 16 A | 3,680 | 2,944 | 1,472 VA (6.4 A) | 2,944 | 2,797 W |
| 230 V / 32 A | 7,360 | 5,888 | 2,944 VA (12.8 A) | 5,888 | 5,594 W |
| 230 V / 63 A | 14,490 | 11,592 | 5,796 VA (25.2 A) | 11,592 | 11,012 W |
| 240 V / 16 A | 3,840 | 3,072 | 1,536 VA (6.4 A) | 3,072 | 2,918 W |
| 240 V / 32 A | 7,680 | 6,144 | 3,072 VA (12.8 A) | 6,144 | 5,837 W |
| 240 V / 63 A | 15,120 | 12,096 | 6,048 VA (25.2 A) | 12,096 | 11,491 W |
Reviewed: 17 July 2026. Maintained by the Starlight Tools product and engineering team. Corrections can be reported through the contact page.
current amps = watts ÷ (volts × power factor)
design load = current load + future growth reserve + workload peak reserve
calculated surviving demand = design load × (1 + failover overhead)
breaker continuous limit = breaker rating × selected continuous target
PDU continuous limit = entered PDU nameplate continuous rating (no second derating)
usable feed limit = minimum applicable breaker, PDU, and internal bank limit
Defaults are 208 V, PF 0.95, an 80% continuous target, equal 30 A branch circuits, balanced active/active sharing, and no added reserve. All recommendations use deterministic browser-side arithmetic; inputs are not uploaded.
The calculator does not validate conductor ampacity, code classification, breaker type, phase balance, plug/cord/receptacle limits, individual outlets, temperature derating, harmonics, inrush, ATS transfer time, PSU ride-through, source independence, maintenance procedures, or protection coordination. A pass is a capacity result, not certification or proof of end-to-end redundancy.
With a balanced A/B load, two feeds at 40% become one surviving feed at 80% after transfer. The 40% value follows from a 50/50 split and an 80% continuous target; it is not a universal electrical limit.
Yes, if the goal is end-to-end redundancy. Two rack PDUs connected to the same breaker, UPS, switchboard, or upstream path retain that common failure point.
Active/active supplies share normal load, often approximately 50/50, then the survivor takes the transferable load. Active/passive systems place normal demand on the preferred feed and transfer it to the standby feed after failure.
Choose the matching rating basis. A branch-breaker rating is multiplied by the selected continuous target. An already-derated continuous PDU nameplate rating is not derated again; the calculator then uses the weakest entered breaker, PDU, or internal limit.
kVA describes apparent power and current demand; kW describes real power. For this single-phase model, kW = volts × amps × power factor ÷ 1,000, while kVA = volts × amps ÷ 1,000.
Use approved peak or metered failover data when available. Nameplate values are conservative planning inputs but can overstate typical demand; average measurements can miss startup and workload peaks.
Yes, but protected capacity is limited by the weaker surviving path. Enable unequal ratings in Advanced settings and check both A-survives and B-survives scenarios.
Yes. A PDU input rating does not override a smaller internal bank, outlet, plug, cord, or device inlet limit. Enter an internal aggregate limit when known and review every downstream component separately.
An ATS can transfer a single-cord load between independent inputs when its ratings and transfer behavior are suitable. The calculator models capacity, not transfer time, phase synchronization, device ride-through, or ATS reliability.
The result checks deterministic capacity from the values entered. It cannot verify independent utility, generator, UPS, switchgear, cabling, ATS, PDU, outlet, PSU, monitoring, or maintenance paths, nor prove that equipment actually transfers as modeled.
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