20 Amp Circuit Breakers: a Blind Spot in Data Center Reliability

20 Amp Circuit Breakers: a Blind Spot in Data Center Reliability

The internet runs on a 20 amp circuit breaker.

By: Greg Lado

Banking, entertainment, social media, email, emergency response, weather reports, etc. All these applications run on servers commonly fed by power distribution units (PDU) filled with 20 amp single pole circuit breakers.

The problem is, those breakers aren’t being monitored by the majority of today’s data centers.

Data centers rely on tested backup systems for power reliability using UPS and generators, but since that equipment is upstream of the PDU branch breaker, unmonitored PDU breakers can become a reliability blind spot.

When you consider all the engineering and cost that goes into data center power reliability, redundancy, and backups, you realize just how important monitoring this humble PDU branch circuit breaker really is.


Power to the server!

In the world of dual-corded IT equipment, it’s important to monitor the loads that feed the servers.

Dual-corded simply means two power cords provide power to two redundant power supplies in a single server. These two cords are often fed from two PDU breakers that ‘share’ the load. If one trips, or the power supply in the server fails, the entire load will fall to the other breaker.

If you’re not constantly monitoring the load of the branch breaker, this could quickly become a problem.

Let’s make sure we all understand the load a 20 amp breaker can support.

A thermal magnetic circuit breaker is designed to trip on overcurrent at 80% of its rating. Therefore, a 20 amp circuit breaker will trip at 16 amps.

When two 20 amp circuit breakers feed dual corded IT loads, the max capacity has to accommodate the scenario where one breaker has to support both loads. This means the max capacity isn’t 16 amps, but instead must be divided in half to accommodate the total load.

In reality, the breakers in the PDU can only be loaded to 8 amps!

Data center maintenance staff know this and may periodically remove the panel covers in the PDUs and use a clamp-on ammeter to measure current on the branches in their data center.

That is a bad idea for many reasons:

  • Physical contact in live equipment is a life safety and equipment safety concern.
  • Periodic manual reading doesn’t give enough samples to be reliable.
  • Manual reading is a time consuming and expensive labor cost.


Why “smart plug strips” are not enough

Smart plug strips (aka rack PDU) seem to be a very logical place to monitor loads. However, if you skip the PDU branch breakers and just monitor the plug-strips, you can create monitoring gaps.

  1. Monitoring individual loads on a plug strip might give a false sense that loading is fine. However, it’s the cumulative effect of those loads as they near the 8-amp threshold ‘upstream’ in the PDU that cause issues. Best practice is to monitor both.
  2. Critical IT equipment like storage area networks (SAN) may not be fed from plug strips.
  3. Plug strip accuracy is poor. The BCPM can be up to 3X more accurate.


Monitor critical branch circuits with a branch circuit power monitor

The Schneider Electric BCPM (Branch Circuit Power Meter) is a device that has been specifically designed to monitor these critical branch circuits.

It is designed to be field installable without shutdown and provide visibility into these circuits. While its main purpose is to address power reliability, its value has been expanded due to the rise of ‘power as a service’ data center models.

Data centers are transitioning to service agreements that charge their customers actual energy (kWhr) consumed versus flat rate billing. This trend can leverage the fact that the BCPM is certified as a revenue accurate energy meter.

For the data center operator, the BCPM can kill two birds with one stone: monitoring for reliability, and energy billing.

Want to hear more about how a BCPM can help your facility’s reliability? Contact us today.


Greg Lado has worked for Schneider Electrics Power Monitoring Solutions group for 18 years in roles ranging from individual contributor to area sales manager, and always with the focus on solving customers energy and power needs with Schneiders proven solutions.