Simple Panel Calculation Question

[fiberstress]

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.

 

[wareagle]

The sum of the breaker sizes has nothing to do with the load on the panel. Some of the ckts will be in use while others are not in use. I would take a clip on amp reading on the 480 volt size of the transformer during peak load and determine the 208 loading on the panels. The breakers used for A/C usually are sized by 1.75 x the full load amps of the A/C. Use the name plate FLA to calculate the actual kva.FLA x Volts. Same for the 3 phase unit which would be FLA x volts x 1.73.

 

[itsmoked]

Be very careful with breakers for A/C. You must use the highest quality breakers you can find,(HVAC rated), and often if the exact size breaker is not provided the warranties will be voided. That means if the equipment states that it needs a 47A breaker a 50A breaker will not do.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rbulsara]

fiberstress:

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.

 

[fiberstress]

Thanks, Sparkies. I appreciate it.

 

[EEJaime]

Depending on the type of HVAC equipment to be fed, an exhaust fan, an air handler or a packaged gas-electric a/c unit the sizing is slightly different. Units with multiple motors in them have three important sizing thresholds. For example, an a/c unit with a compressor, condenser fan, and a power exhaust fan/economizer, will have a "Minimum Circuit Ampacity, (MCA), equal to 125% of the compressor amperage,plus the sum of the other motors. This is the minimum value that the feeder and breaker are to be sized. The second is the "MOCP", or the "Maximum Overcurrent Protection" size, which is the largest the feeder overcurrent device may be and is equal to 225% of the largest motor plus the remaining motors. Any device between these two values is acceptable by code. The third value is the "RLA", or "Running Load Amps", that is the load of the unit when it is running at steady state and this is the actual load you would add to 125% of the panel load to see if the load can be fed by the panel.

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.

 

[wareagle]

Quote "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."
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.

 

[rbulsara]

wareagle:

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.

 

[waross]

Remember that there is a phase difference between line to line loads and line to neutral loads. The actual current will be less than the sum of the line to line and line to neutral currents. The actual current drawn by equipment with different power factors will be less than the arithmetic sum of the currents of the equipment.
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