Calculating Amperage on a PDU

[BRaley]

I have a 480V to 208/120V PDU that has a 225 KVA transformer with 3- 225 Amp Panels. I need to know how to calculate maximum amperage for the whole PDU at 208/120. One way to figure it is to take the rating 625 @ 208/120V and multipy it by 3 to get 1875 Amps single phase. The other way to figure it is to take the 625 Amp rating, multiply it by 1.73 and get 1081. I need to know which value to use to calculate how many breakers I can put in the PDU. Thanks, Brian

 

[brian john]

It is load dependent this PDU can have 1000's of circuit breakers if the loads are minimal.

But a 225 KVA can feed 625 amps if you figure it is continuous loads-500 amps

 

[Smart $]

Your available power (in simple form :grin is 225kVA. The number of breakers is not dependent on the available power. You can use up your available power with any number of loads. The number of breakers is solely determined by how many circuits will be distributing power to the forementioned loads.

 

[BRaley]

To be more thorough, we know the customer is using only 120V, 30A breakers we calculate these at 24 Amps (80%), so when I do the math, I can have 23 of these breakers in each of the 3 panels.
23*24= 552 amps
552*120V= 66240 watts
66240*3= 198720 watts (for whole PDU) or 198.720 KW

Please let me know if this is accurate. Thank you.

 

[Smart $]

To be more thorough, we know the customer is using only 120V, 30A breakers we calculate these at 24 Amps (80%), so when I do the math, I can have 23 of these breakers in each of the 3 panels.
23*24= 552 amps
552*120V= 66240 watts
66240*3= 198720 watts (for whole PDU) or 198.720 KW

Please let me know if this is accurate. Thank you.

Your calculation is only accurate if each and every 30A branch circuit powers a continuous load of 24A. Otherwise, it is inaccurate

 

[BRaley]

Thank you very much. Brian Raley

 

[Smart $]

Thank you very much. Brian Raley

Somehow, I get the impression that you are not grasping the entire concept...

Just because the customer uses 30A breakers does not mean the branch circuit is loaded to the max of the permitted continuous or non-continuous loads, or a combination thereof.

Each branch circuited is permitted to be loaded to 100% of the non-continuous load plus 125% of the continuous load (unless using 100% duty-rated equipment)... but the actual loading on the panel, its feeder, and the transformer is determined by actual connected loads and/or the minimum lighting and receptacle loads permitted, whichever is greater.

The only true way to determine panel loading is to do a load calculation per Article 220.

 

[BRaley]

I do understand. The challenge is the way they sell the power to the customers. They sell each circuit at the 80% maximum (30Amp is 24Amp, etc.). So when we load the PDU's we have to assume they are using all of it, because the customer is paying for all of it. It's weird. It's more sales than electrical. Anyway, thank you for all your input.