Failure to properly design, size and implement dual power infrastructure at the cabinet may lead to breaker trip during restart. Improperly loaded circuits may support a running load in a failover situation, but the restarting of connected servers during single source operation could then trip the upstream circuit breaker (thereby causing momentary starting loads as drives spin up).

Most thermal magnetic breaker manufacturers recommend limiting current through the breaker to 80% of the breaker rating. Limiting the total load for an A-B whip pair to 80% is an essential element of dual power distribution design.

This margin of safety allows for surges of power to the load often experienced on start up of certain devices or other unforeseen momentary loads. The 80% rating is a time versus temperature relationship, so the breaker is able to handle the start up surge for a limited time, after which the loads returns to normal.

The total load on both power whips should be distributed evenly between the circuits and the total of those two loads should not exceed 80% of the breaker rating.

Recall the example from Dual Powered Data Center – Preventing Breaker Trip on Failover:

Setting up A-B Power to Prevent Breaker Trip on Failover

A server cabinet contains eight servers, each with dual power supplies consuming 2 amps per server at full running load. If a 120 volt, 20 amp A-B power whip pair is delivered to the cabinet the load will be distributed as follows:

Power Circuit A – 8 amps (with both A-B circuits active)

Power Circuit B – 8 amps (with both A-B circuits active)

Total power draw for the A and B circuits is 16 amps. If power circuit B fails or must be de-energized for maintenance, the A power circuit in all eight servers will be required to deliver twice the power to the server – for a total input load of 16 amps on a 20 amp power circuit.

Remember…8 servers, each server needs 2 amps and only one power supply is now energized per server. This is within the 80% breaker rating design criteria, so this example is within the design specification.

In a failover situation with one power circuit inoperative, the running load for one circuit in the example above would be 16 amps. If one of those devices were a large RAID array, the starting current could easily exceed 200% of the running load and exceed the breaker rating, thereby causing a breaker trip and resulting downtime.

The 80% rating should always be followed for safety and additional margin.


  1. You can also deploy intelligent rack PDU’s (power strips / iPDU’s) that include outlet switching and sequencing. With these, the user can establish the order of outlets turning on, and the time delay between each outlet coming on. This allows you to control what order equipment in the rack comes on, and allows time for a device to fully start up before the next device starts (if this is desired.)

    The key benefit is that you don’t see the in-rush current form all devices starting at the same time. You still should not exceed 80% of rated capacity on the circuit (or 40% on each circuit if you want fully redundant fail-over power.)

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