PDU and branch circuit calculator
Every other calculator on this site answers 'how much load.' This one answers the question that comes right after: given a specific PDU input, how many of them do you order, and how does the load land once it is plugged in.
Enter the rack's IT load and pick the PDU input you are speccing or already have installed. The calculator returns how many circuits it takes to stay under the 80% continuous-load derate, and — for a three-phase input — the current on each leg if the branch outlets are balanced correctly.
Measured or expected draw, not nameplate.
8.42 kVA at the wall, 10.11 kVA with the 20% margin applied, split across the smallest number of IEC 60309 30A circuits that stays under the 80% continuous-load derate.
Sized to 2 circuits per rack, the minimum that keeps each one under 80%.
A three-phase PDU input feeding single-phase C13/C19 branch outlets only balances if the outlets are wired round-robin across L1–L2, L2–L3, and L3–L1. Loading one bank of outlets first is the most common cause of a rack PDU that reports a fault on one phase while the other two show plenty of headroom — check the PDU’s own phase-current display after cabling, not just the total.
Need the full picture — power, cooling, UPS, and circuits together? The full calculator does all of it at once.
PDU count is a different question from load
A rack has one number that matters most — its IT load in kW. But that number does not by itself tell you how many discrete circuits deliver it, because that depends on what the PDU is rated for and the 80% continuous-load ceiling on top of it.
A 15 kW rack fed by 208V single-phase 30A whips needs four of them. The same rack on a single 208V three-phase 60A input needs one, with headroom to spare. Same load, completely different bill of materials, and the difference is entirely about which PDU you chose — not about the rack.
This is why 'how many PDUs' deserves its own calculator rather than being a footnote on a load calculator: the answer depends on a decision — the PDU input — that a pure load figure does not capture.
Balancing a three-phase PDU is not automatic
Most rack-scale PDUs take a single three-phase input and break it out into single-phase C13/C19 branch outlets — typically in three banks, one per leg. Plugging in devices bank by bank instead of spreading them across all three banks concentrates the load on one leg while the other two sit idle.
The PDU still works when it is unbalanced. Nothing trips, nothing alarms, until that one leg's current climbs past what its breaker or the upstream circuit was sized for — usually discovered when someone racks the last few servers and a phase-current warning appears on the PDU display, not during design.
The fix is procedural, not electrical: wire new equipment round-robin across L1–L2, L2–L3, and L3–L1 as it goes in, and check the PDU's own per-phase current readout after cabling rather than assuming the math from a spec sheet held.
More PDUs for capacity is not the same as PDU redundancy
This calculator's 'PDUs required' figure is a capacity number — the fewest circuits that carry the load without exceeding the derate. It is not a redundancy design.
Redundant PDU feeds (commonly called A/B power) mean two independent PDUs, each on a separate upstream circuit and ideally a separate UPS, with every device in the rack dual-corded across both. Losing one PDU should not drop the rack. That is a different exercise from this calculator's circuit count, and it multiplies the PDU count rather than adding to it — a rack that needs 2 PDUs for capacity needs 4 for A/B redundancy (2 on each side), not 3.
If the rack is going into a facility with N+1 or 2N UPS redundancy already, as covered on the UPS sizing page, matching that with A/B PDU feeds is what actually carries the redundancy down to the equipment. A redundant UPS feeding a single PDU still has a single point of failure at the rack.
Reading a PDU that is already installed
If you are working against existing infrastructure rather than speccing new PDUs, the nameplate on the unit or its upstream breaker gives you volts, amps, and phase — the same three numbers this calculator asks for. Match those against the receptacle reference below to confirm the rating, then enter the rack's actual or planned load to see how much headroom is left.
The 0.8 in the usable-capacity figure is not a safety margin you can spend if you are in a hurry — it is the same NEC 210.20(A) continuous-load rule used throughout this site, and it applies to the PDU's input circuit exactly as it applies to any other breaker.
Common questions
- How many PDUs does a rack need?
- Divide the rack's load in kVA (kW ÷ power factor) by the usable capacity of one PDU input circuit — its rated kVA × 0.8 for the continuous-load derate — and round up. An 8 kW rack at 0.95 power factor on a 208V three-phase 30A input needs one PDU with room to spare; the same load on a 208V single-phase 20A input needs three.
- How do I calculate per-phase current on a three-phase PDU?
- Line current in amps equals kVA × 1000 ÷ (√3 × line voltage), assuming the load is balanced evenly across all three legs. A 5 kVA load on a balanced 208V three-phase PDU draws about 13.9 A per line — but that figure only holds if the branch outlets are actually wired round-robin across L1–L2, L2–L3, and L3–L1.
- What is the difference between PDU capacity and PDU redundancy?
- Capacity is how many circuits it takes to carry the load without exceeding the 80% derate. Redundancy (A/B feeds) is running two independent PDUs, each able to carry the whole load alone, with every device dual-corded across both. A rack sized for 2 PDUs on capacity needs 4 for full A/B redundancy, not 3.
- Can I mix single-phase and three-phase PDUs in the same rack?
- Electrically yes, as long as each PDU is fed from a circuit matching its own rating. It is unusual outside a transition period between standards, since it complicates spares and monitoring — most operators standardize on one PDU input type per row or hall.
- What growth margin should I add when sizing PDUs?
- 0% if the load is final and metered. 20% is a reasonable default for a rack that will keep growing over its first year. 50% or more for a new deployment where density is still being decided — oversizing a PDU input costs little; running out of circuits mid-deployment means a change order.