PDU Load Calculator — Single & Three Phase
Calculate usable rack PDU capacity in kW from voltage, breaker rating and phase, with the 80% continuous-load derate and N+1 redundancy applied.
Inputs
Line-to-line voltage for three phase (e.g. 208, 400, 415).
Modern server PSUs are typically 0.98-0.99. Use 1.0 to size in kVA.
2 for a typical A+B dual-corded rack.
Results
- Design capacity (what you may load)
- 8.65kW
- Usable capacity, all feeds
- 17.3kW
- Per feed — nameplate
- 10.8kW
- Per feed — continuous (80%)
- 8.65kW
- Current per feed at design load
- 24.0A
Derated to 80% and sized so the rack survives losing one feed.
How rack PDU capacity is calculated
A rack PDU's nameplate rating is apparent power, not the power you can actually plan around. Getting from a breaker rating to a usable kilowatt figure takes three steps, and skipping any of them is how racks end up tripping breakers at 2 a.m.
Step 1 — apparent power
For a single-phase feed, apparent power is simply voltage times current:
kVA = (V × A) ÷ 1000
For a three-phase feed you also multiply by the square root of three, because the three line-to-line voltages are 120° out of phase:
kVA = (√3 × V × A) ÷ 1000
A 208 V, 30 A three-phase PDU is therefore 1.732 × 208 × 30 = 10,807 VA, or 10.81 kVA. This is the number printed on the label, and it is not what you can load.
Step 2 — the 80% continuous-load derate
A circuit powering a load that runs for three hours or more is a continuous load, and electrical codes require it to be sized at 125% of the load — which is the same as saying you may only use 80% of the breaker rating. IT equipment runs continuously by definition, so the derate always applies:
Usable kW = kVA × 0.8 × power factor
Our 10.81 kVA PDU gives 8.65 kW of continuous capacity. Loading it to 10.8 kW would be within the nameplate and still wrong.
Step 3 — power factor
Power factor converts apparent power (kVA) into real power (kW). Modern server power supplies use active power factor correction and sit around 0.98-0.99, so the difference is small but not nothing. Legacy gear, large UPS bypass paths and some network hardware can be lower. If you are sizing in kVA rather than kW, leave it at 1.0.
Redundancy: why two feeds is not twice the power
This is the single most common mistake in rack power planning. A rack with A+B feeds has two PDUs, each rated 8.65 kW continuous — but you cannot design to 17.3 kW. The entire point of dual feeds is that the rack survives losing one, which means each feed must be able to carry the whole load on its own.
So a dual-fed rack with 30 A three-phase PDUs has a design budget of 8.65 kW, not 17.3 kW. The second feed buys availability, not capacity. Set the redundancy selector above to see both numbers side by side.
Worked example
| Step | Calculation | Result |
|---|---|---|
| Apparent power | √3 × 208 V × 30 A | 10.81 kVA |
| Continuous derate | 10.81 × 0.8 | 8.65 kW |
| Two feeds, no redundancy | 8.65 × 2 | 17.30 kW |
| Two feeds, N+1 redundant | 8.65 × 1 | 8.65 kW |
Common voltages and their usable capacity
| Feed | Nameplate | Continuous (80%) |
|---|---|---|
| 120 V / 20 A single phase | 2.40 kVA | 1.92 kW |
| 230 V / 16 A single phase | 3.68 kVA | 2.94 kW |
| 230 V / 32 A single phase | 7.36 kVA | 5.89 kW |
| 208 V / 30 A three phase | 10.81 kVA | 8.65 kW |
| 400 V / 32 A three phase | 22.17 kVA | 17.73 kW |
Things this calculator does not cover
- Phase balance. A three-phase PDU only delivers its full rating if load is spread evenly across the three phases. A badly balanced rack trips one phase long before the total looks full.
- Inrush current. Equipment draws far more than its steady-state current for the first moments after power-on, which matters when a whole rack restarts at once after an outage.
- Upstream capacity. Your PDU may be rated for 8.65 kW while the branch circuit, RPP or UPS feeding it is the real limit.
Treat the output as a planning figure. Confirm against the PDU datasheet, the upstream distribution design and local electrical code before committing to a rack layout.