Simple Panel Calculation Question
Simple Panel Calculation Question
(OP)
Hello,
I have a 480/277V PDU with an onboard transformer that steps down to 208/120V, and then branches into 2 panels (or 3, I'm not sure) supplying 208V L-L/ 120V L-N (3 phase of course).
We need to add two new circuits for two AC units that run on 208V and need 50A breakers each. We're calculating the potential KVA of the panel based on circuit breakers amps and their voltages, which is conservative, but nevertheless my question is for the 2-pole breakers in the panel (20A for example) do we calculate the potential power as follows:
208V x 20A = 4.16KVA ?
And for the 3-pole breakers in the panel (30A for example):
120V x 30A x 3 = 10.8KVA ?
Also, is there a good estimate for the percentage of current breakers typically use? (I've heard 80% is good for calculation purposes)
Thanks for the help.
I have a 480/277V PDU with an onboard transformer that steps down to 208/120V, and then branches into 2 panels (or 3, I'm not sure) supplying 208V L-L/ 120V L-N (3 phase of course).
We need to add two new circuits for two AC units that run on 208V and need 50A breakers each. We're calculating the potential KVA of the panel based on circuit breakers amps and their voltages, which is conservative, but nevertheless my question is for the 2-pole breakers in the panel (20A for example) do we calculate the potential power as follows:
208V x 20A = 4.16KVA ?
And for the 3-pole breakers in the panel (30A for example):
120V x 30A x 3 = 10.8KVA ?
Also, is there a good estimate for the percentage of current breakers typically use? (I've heard 80% is good for calculation purposes)
Thanks for the help.






RE: Simple Panel Calculation Question
RE: Simple Panel Calculation Question
Keith Cress
kcress - http://www.flaminsystems.com
RE: Simple Panel Calculation Question
To answer your basic questions, your load calculations are correct for 2-pole and 3-pole breaker loads on a 208Y/120V panel board. (3x120Vx30A is same as 1.732x208Vx30A)
If the breakers are properly sized, meaning 125% of the continuous load it may see, they should not see load more than 80% of its rating.
RE: Simple Panel Calculation Question
RE: Simple Panel Calculation Question
There is also the LRA of an individual moter, the Locked Rotor Amperage which is the Amps when the motor is started, which is used to size the short time protective device.
As stated by the other posters, the Amp rating of a breaker is not an indication of the load on the circuit except to let you know that the running load on the circuit may not be greater than 80% of the rating for a standard breaker.
So for your units that "need 50A breakers each", (3-phase?), you really need to know what the nameplate ratings of the units are. Then as Wareagle says, get a load reading of the panel and you can determine what the total load will be.
RE: Simple Panel Calculation Question
rbulsara
Motors and A/C unit breakers are sized differently from other continuous loads. The A/C unit breaker is allowed to be 175% of the FLA to allow for starting.
RE: Simple Panel Calculation Question
Yes. You are right to the extent if the motor is the only load and it is a single motor, and it could be even more than 175%. My focus was only trying to answer his basic questions.
This appears to be small ac unit. On many (commercial) pakaged AC units with humidifier and/or reheat etc, the motor becomes a small part of the overall AC unit load and the relationship gets closer to regular load calcs.
RE: Simple Panel Calculation Question
Simple addition of currents in the real world is not always accurate, but it is safe for panel loading purposes. Also, the codes in North America don't generally address phase angle when using demand factors so simple addition of currents is generally legal as well as safe.
There will always be special cases where a conscientious engineer will do a vector addition of the currents.
Bill
--------------------
"Why not the best?"
Jimmy Carter