50A 208v 3 Phase Rack PDU Load Ratings and Calculations Explained

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Issue

50A 208v 3 Phase Rack PDU Load Ratings and Calculations Explained

Product Line

3 Phase Rack Power Distribution models
- AP84XX
- AP86XX
- AP88XX
- AP89XX
- AP78XX
- AP79XX

Environment

All serial numbers
All firmware versions
50A 3 Phase Rack PDU, such as AP8868

Cause

This article will outline how phase current ratings and load calculations are determined as well as how load calculations are reached. AP8868 will be used as an example.

Resolution

In regards to the AP8868, each bank references two phases:

Bank 1 - Phase 1 and 2
Bank 2 - Phase 2 and 3
Bank 3 - Phase 1 and 3

AP8868 is one of the few units where the capacity is limited by the output circuit breakers and not the input cord/plug rating. The breaker on each bank has a maximum rating of 20A (rated to 16A per UL). Bank 1 draws current from phases 1 and 2 simultaneously. Bank 2 draws current from phases 2 and 3. This means that phase 2 is supplying current to both banks 1 and 2. However, like the phases, the banks are 120° out of phase. This means that part of the time that current is flowing one direction on bank 1, it is flowing the same direction on bank 2 and the two currents increase the draw from phase 2 to over 20A. However, at other times, when current is flowing one direction on bank 1, it is being returned on bank 2. This means that you can't just add the loads on the two banks to get the load on phase 2.

Here is another way to look at it. We are in essence supplying three 120V phases, but we are powering three 208V loads. If the loads are 16A, the total power consumed is (3 banks) x (208 Volts) x (16 Amps) = 9984 Watts. The current on the input must then be (20A x 208v x 3 banks)/(three phases x 120 Volts) = 34.6 Amps. This is how the total current per phase is determined but then please note the de-rating of values, per UL, which is 9984W.

Now that we know the load ratings, let's look at how to calculate the total power output.

For a single phase Rack PDU, Total Power = (Total Current) x (208V)
For a three phase Rack PDU, Total Power = (Total Three Phase Current) x (208V) x [sqrt(3)]

For a PDU like AP8868 with 3 banks, we'd use the single phase calculation for each of the three banks and then add them together to get the "Total Three Phase Current." The input of the Rack PDU is actually limited by the internal circuit breakers.

Why is AP8868 rated at 10.0kW, when AP7868 was rated at 12.5kW?

Previous generation rack PDUs, such as AP7868/AP7898 and AP7968/AP7898, were messaged as 12.5kW rated rack PDUs. This was because UL did not de-rate the internal circuit breakers. Since then, UL has mandated a de-rating of all components to 80% continuous load (such as input, output, and internal circuit breakers). This allows for short-term headroom for any power spikes that may occur. Best practice should have allowed for this headroom, even on the previous generation of products. The internal circuit breakers are the same type - 20A, UL489, branch rated, and hydraulic magnetic. The pictures below demonstrates the internal characteristics with the same medium delay curves in the circuit breakers for both models.

AP7868/AP7898 breaker time delay curve:

AP8868 breaker time delay curve: