×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Simple Panel Calculation Question

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.

RE: Simple Panel Calculation Question

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.

RE: Simple Panel Calculation Question

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

RE: Simple Panel Calculation Question

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.

RE: Simple Panel Calculation Question

(OP)
Thanks, Sparkies.  I appreciate it.

RE: Simple Panel Calculation Question

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.

RE: Simple Panel Calculation Question

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.

RE: Simple Panel Calculation Question

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.

 

RE: Simple Panel Calculation Question

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

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources