[AusNOG] Fwd: Derating of PDU

[Mitch Kelly]

Based on previous experience in power distribution design, here’s my
general approach to derating power circuits:

*Disclaimer: Always refer to AS/NZS 3000, AS/NZS 3008, and consult a
licensed electrician for all practical installations and compliance.*

For ambient temperatures around 40°C, a thermal derating factor of 0.87 is
typically used (based on AS/NZS 3000/3008).

Real-world usable power also depends on:
The power factor (PF) of the connected equipment.

Whether the PDU is installed in a hot aisle or cold aisle.
If the load is continuous, you must also apply the 80% continuous load rule
as per AS/NZS 3000.

Example — 30A Circuit @ 230V
Cold Aisle Scenario (23°C, PF = 0.9):
No temperature derating needed at 23°C.

Apply 80% derating for continuous load:
30A × 0.8 = 24A usable

Real usable power:
24A × 230V × 0.9 = 4968 W

Usable continuous load = ~4968W
Peak load capacity (non-continuous):
30A × 230V × 0.9 = 6210 W
(for short-term bursts only)

Hot Aisle Scenario (40°C, PF = 0.8):
Apply both continuous and thermal derating:
30A × 0.8 (continuous) × 0.87 (thermal) = 20.88A

Real usable power:
20.88A × 230V × 0.8 = 3848 W

Usable continuous load = ~3848W
Cold Aisle (23°C) 24A 0.9 4968 W
Hot Aisle (40°C) 20.88A 0.8 3848 W


On Tue, Apr 8, 2025 at 2:48 PM Alex Samad <alex at samad.com.au> wrote:

> Hi
>
> I'm having the pleasure of looking at PDU's and trying to standardise over
> US / UK / Japan / Aus
>
> Something I didn't know was the standard in the US for derating electrical
> devices. whilst I normally plan for max normal draw of around 80% of
> rating. seems like PDU for USA have a derating value.
>
> So I am seeing 30amp @ 230v in and line rating of 24amp and max 30amp.
> The notes seems to suggest you can go over 24 amps but only for a very
> short period of time - seems like minutes.
>
> This seems a bit of a pain for me - whilst I wouldn't normally run over
> the 24 amps there have been times i have had to run over - but under 30 for
> days if not a couple of weeks.
>
> Currently I'm trying to order into japan (omg), lot of their stuff is
> imported which means it comes with the USA specs.  so 24/30 amp setup..
> Interestingly also noted that when you read the fine prints its actually 2
> x 20amp breakers - 1 breaker per bank .. PDU has 2 banks
>
> I can't find anything about derating for Aus - do we not do it in Aus or
> mandate it ?
>
>
> Alex
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