PDU amperage vs. Circuit - Please help clarify

[Anonymous user]

Hi everyone, I've been looking around for answers but no one is able to help me so far.

We have a rack at a datacenter with a 208V/60 amp circuit coming into a 208V/60 AMP PDU and I was wondering if we lower the circuit to let say 208V/30 amp or 120/30 amp do I need to change the PDU to a 208V/30 or 120/30 as to match the amperage on the circuit?

I guess my question is if the PDU has to always match the amperage and volts of the circuit?

Thanks in advance

[Anonymous user]

upsguy wrote:
Ideally you would want to change the supply breaker. You could probably get away with it if the PDU has overload protection built in. What model rack mount PDU do you plan on using?

I already have a 60 amp PDU but planning on lowering the 60amp breaker to 30amp.

Just a thought though, if you already have the 60A breaker and the PDU but aren't using the full amount of current you aren't wasting any power by having the larger distribution source.

I agree, but the difference in price for a 60 and 30 amp breakers are relatively high.

Thanks.

[upsguy_apc]

If cost is an issue then why would you consider buying anything if you already have as I see it.

PDU w/60A CB -------60A cordset-------- Rackmount 60A PDU.

You only loose capacity by replacing the 60A pdu with a 30A pdu. If you don't need the 60A capacity only use 30A of it. It doesn't hurt anything to keep it at a low load level.

[Anonymous user]

upsguy wrote:
If cost is an issue then why would you consider buying anything if you already have as I see it.

PDU w/60A CB -------60A cordset-------- Rackmount 60A PDU.

You only loose capacity by replacing the 60A pdu with a 30A pdu. If you don't need the 60A capacity only use 30A of it. It doesn't hurt anything to keep it at a low load level.

If I would only get charged for what I consume then I agree with you, but this is not the case. We get billed for having 60 amp brakers on our rack no matter what the consumption is.

[BillP]

I suspect you have your rack hosted at a co-location centre and they're charging you for the size of the service to your rack regardless of the actual consumption. This is an all too common tactic of many hosting sites, but, it's money for them; they charge you to install the service, then they charge a monthly rental.

If you can confirm your load is under 24 Amps (the code limit for a 30 Amp service), then you could downgrade the service to 30 Amps. The electrical service will be protected from overload by the 30 Amp breaker. While you're at it, set the warning thresholds on the PDU to correspond to the 30 Amp Service; 20 Amps for near overload, 24 Amps for overload (for example). This is not a preferred solution as it introduces the possibility of overloading the service and nuisance tripping of the breaker.

The better option would be to use a 30 Amp PDU on a 30 Amp Service.

[Anonymous user]

mfitzsimmons wrote:
I suspect you have your rack hosted at a co-location centre and they're charging you for the size of the service to your rack regardless of the actual consumption. This is an all too common tactic of many hosting sites, but, it's money for them; they charge you to install the service, then they charge a monthly rental.

Thanks for your response, you mention that this is a common tactic but what is better? to get charged for the size of the service or for consumption?

[BillP]

geneticx wrote:
Thanks for your response, you mention that this is a common tactic but what is better? to get charged for the size of the service or for consumption?

It would be fair for the hosting site to charge to install the service, then charge a factor of the actual consumption. The actual load is an accurate indication of the power used, it is also an accurate indication of the load your system places on the cooling system.

What is more common is for the hosting site to charge to install the service, then also charge to rent the service monthly to cover the cost of power consumption and cooling load. In some jurisdictions it is not legal to re-sell electrical power, so this a method to get around this restriction. The problem arises when you don't use the capacity of the service. For example, a 60 Amp service at 208 Volts is rented as a 12.5 kW service and charged at something like $2,500.00 per month. The rational being that 12.5 kW is the maximum the service can supply and it takes the same amount of power to cool that load. Of course you don't use the maximum the service can provide, and, therefore do not require the maximum cooling either. This gets compounded if you have redundant electrical services. (on top of this you have your square footage charge, connection charges, management charges and so on)

This practice leads to the same issues of power consumption we see in condominium offices and residences. Since you're paying the maximum for power, why not use it. There is no incentive to upgrade to more power efficient equipment or to try to control your power consumption.

[Anonymous user · Accepted solution]

Hi everyone, I've been looking around for answers but no one is able to help me so far.

We have a rack at a datacenter with a 208V/60 amp circuit coming into a 208V/60 AMP PDU and I was wondering if we lower the circuit to let say 208V/30 amp or 120/30 amp do I need to change the PDU to a 208V/30 or 120/30 as to match the amperage on the circuit?

I guess my question is if the PDU has to always match the amperage and volts of the circuit?

Thanks in advance