In short. A/B distribution gives a load two independent power paths, A and B, from separate sources through separate equipment. If one path fails or is maintained, the other carries the load. The architecture works only if the paths are truly independent, and each can carry the full load alone.
The idea
Dual-corded equipment, such as many servers and network devices, accepts power from two inputs. Each input connects to a different path. The two paths run from source to rack, passing through separate switchgear, UPS, distribution boards and cables. A fault or a planned shutdown on one path leaves the other running.
Elements of each path
| Level | Path A | Path B |
|---|---|---|
| Source | Utility or generator feed A | Utility or generator feed B |
| Main switchgear | Board A | Board B |
| UPS | UPS A | UPS B |
| Distribution | PDU A | PDU B |
| Cabling | Route A | Route B |
| Rack feed | Rack PDU A | Rack PDU B |
The capacity rule
Each path must carry the full load when the other is out. If each path normally carries 50 per cent, it must be rated for 100 per cent. Many sites keep each path below about 40 to 50 per cent loading in normal operation so the surviving path is not overloaded on failover. If paths load beyond that, one failure becomes two.
Independence in practice
- Separate rooms or at least separate fire-rated sections for A and B equipment where feasible.
- Separate cable routes, not one tray with both.
- Separate control supplies and monitoring.
- Separate generators or separate sections of a generator bus. See bus sectionalisation.
Single-corded loads
Single-input equipment can only use one path. Use a static transfer switch or rack transfer switch so it can take either path, and understand that the switch itself is a single point of failure. See static transfer switches and dual-cord and single-cord loads.
Where A/B designs fail
- A shared maintenance bypass that bridges both paths.
- A common neutral or earth arrangement that couples faults.
- Both paths loaded to 60 per cent so a failover overloads the survivor.
- Wrong cord connections on site, with both cords on one path.
- Cooling or controls shared by both paths.
Worked illustration
A room has 200 kW of dual-corded IT. Paths A and B each feed 100 kW. Each path is rated 200 kW, so each can carry everything. After a year, new equipment adds 60 kW to each path, giving 160 kW per path. If path A fails, B carries 320 kW against a 200 kW rating and trips. The architecture was sound; the loading discipline was not. Monitor per-path load and cap each at its limit.
Commissioning checks
- Verify each rack’s A and B cords are on different paths.
- Switch off one path at a time and confirm all equipment stays up.
- Check each path’s load stays within its limit after failover.
The A/B principle
Two independent distribution paths, A and B, each able to carry the full critical load. Dual-corded equipment draws from both. If one path fails or is under maintenance, the other carries the load without interruption.
| Element | Requirement |
|---|---|
| Sources | Separate UPS, separate generators or buses |
| Paths | Separate cable routes and switchboards |
| Rooms | Separate rooms where fire is a concern |
| Loading | Each path under 50 percent in normal operation if it must carry 100 percent on failure |
| Control | No shared single controller or DC supply |
Loading rule
Maximum normal loading per path = 50 percent of path capacity, for a true 2N A/B design
If path A is rated 500 kW and carries 300 kW while path B carries 200 kW, a failure of B puts 500 kW on A, which is the limit. Add the margin for growth and inrush and the true limit is lower. Monitor both paths and alarm when the combined load approaches the capacity of one.
Single-corded equipment
Single-cord loads need a rack-level static transfer switch or similar to benefit from A/B. See dual-cord and single-cord loads.
Common mistakes
- A and B cables in the same tray.
- Shared ventilation or cooling failure that takes both.
- Gradual load growth beyond 50 percent per path.
See critical-power bus architecture.
What to send us
Send the load, the number of cords per device and the space for equipment. Send your application details. See also critical-power bus architecture.
Questions engineers ask
Is A/B the same as 2N?
A/B describes two paths to the load. 2N describes two complete systems each able to carry the full load. A properly built A/B distribution with full-capacity paths is a form of 2N.
Why keep each path at about half load?
So that when one path fails the other can carry everything without overload.
What about single-corded equipment?
Use a transfer switch to select a path, accepting the switch as a single point of failure.
How do I check my paths are independent?
Trace each path from source to load and list every shared element, including controls, cooling, earthing and cable routes.
This guide gives general principles. Specific designs should be reviewed against your load and standards.