[AusNOG] Fwd: Derating of PDU

[Alex Samad]

Thanks for the detailed info

A

On Tue, 8 Apr 2025, 17:21 Mitch Kelly, <mitchkelly24 at gmail.com> wrote:

> 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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>
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