Starlight Tools
Check the weakest feed
A redundant design is limited by the smaller breaker, cord, receptacle, or PDU rating on either side.
Weakest link
Redundant Power Feed Capacity Calculator
Check A/B feed capacity for dual-corded racks and critical equipment. Convert watts to amps, model the normal A/B split, and verify whether either surviving feed can carry the protected load after the other side fails.
Calculator
Inputs
Total load
Device count
Measured normal feed current
Measured mode treats the entered A and B currents as the normal operating state, then estimates protected load from voltage and power factor.
For dual-corded equipment in A/B mode, the failover check assumes either surviving feed may need to carry 100% of the protected load.
For 100%-rated breakers or other approved designs, change the continuous-load target to match your documented requirement.
Results
Protected load--
Normal worst feed--
Failover worst feed--
Overall status--
Target capacity--
Recommended feed--
Enter load and feed details to check normal and failover utilization.
| Scenario | A feed amps | A target headroom | B feed amps | B target headroom | Usable target | Status |
|---|---|---|---|---|---|---|
| No calculation yet. | ||||||
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Assumptions and formulas
This calculator models single-phase A/B feed loading. It is useful for rack PDUs, branch circuits, transfer planning, and dual-corded IT equipment where protected load can be estimated in watts or measured in normal amps.
Continuous-load target
target amps = feed rating amps x continuous target percent
target watts = voltage x target amps x power factor
Watts to amps
amps = watts / (voltage x power factor)
A/B redundant failover
surviving feed amps = total protected watts / (voltage x power factor)
Recommended feed rating
minimum feed amps = worst-case amps / continuous target percent
The calculator defaults to an 80% continuous-load planning target because standard overcurrent protection is often planned with a 125% continuous-load allowance, which is equivalent to using 80% of the rating. Use a different target only when your equipment labels, installation, and local requirements support it.
Limits and safety notes
This tool does not evaluate conductor ampacity, transfer switches, receptacle ratings, PDU internal branch breakers, temperature derating, harmonic current, inrush behavior, three-phase phase balance, grounding, or local code details. Treat the results as planning math for conversations with facilities, electrical contractors, and equipment vendors.
In A/B redundant systems, normal operation can look comfortable because the load is split between feeds. The important 2N check is whether either A or B can carry the full protected load during maintenance or failure without exceeding the chosen continuous-load target.
Frequently asked questions
Does a 30 A feed mean I can run 30 A continuously?
Not usually for a standard 80%-rated design. With an 80% target, 30 A becomes 24 A of continuous planning capacity. Only use a higher target when the breaker, PDU, installation, and applicable rules support it.
Why does power factor matter?
Feeds, breakers, and PDUs are limited by current, while IT equipment is often estimated in watts. Power factor converts real power to apparent current for planning. If you have measured amps, measured mode is more direct.
Why is failover load higher than normal A/B load?
Dual-corded equipment often shares load across A and B during normal operation. If one side fails, the remaining feed may carry the full protected load, so each feed must be sized for that condition.
What if my A and B feeds are different sizes?
The redundant capacity is limited by the smaller approved feed target. A larger A side does not protect a rack if the B side cannot carry the full failover load.
Can I use this for three-phase PDUs?
Use this page for single feed breaker headroom or a single phase-derived feed. For per-phase balance, use the 3-Phase PDU / Phase Load Balance Calculator linked above.
Are my inputs uploaded?
No. The calculator runs in your browser. Copy and CSV export actions use only the values already on the page.
Practical planning notes
Normal load is not enough
A/B racks can pass normal utilization and still fail the surviving-feed check after one side drops.
Failover check
Metered amps win
Nameplate watts are useful early, but live PDU metering captures real utilization and power factor behavior.
Best input
Leave room for moves
Reserve capacity makes future rack adds and maintenance transfers less likely to create a breaker event.
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