how do i calculate the load on a three phase panel?

[Kmansker]

hi everybody! i'm new here but have been pursuing a career in the electrical field for a number of years. i have come across a problem that for the life of me i can't figure out or maybe i just don't know how to word my question. so here it goes.......

i have a 225 KVA DELTA transformer that feeds a three phase panel. the voltage is 240 volt. it is a true DELTA. no neutral.
the panel is rated for 800 amps but is protected by a 600 amp fused disconnect.
the name plate on the transformer and my math both come up with 540 amp capacity. ( 225000/240*1.73) so far everything is good.
the problem is that i want to add another circuit to this panel but cant figure out the theory on how it is loaded now and how much load can still be added safely. the breakers are as follows

quantity 1 3 phase 80 amp breaker
quantity 8 3 phase 60 amp breaker
quantity 6 3 phase 30 amp breaker

all these loads are continuous duty as they run machines that never shut down and if i calculate for worse case scenario i would add all the breakers and multiply by .80 coming up with 592 amps.
when i take an actual amp reading on the feeders it is but a fraction of the capacity, when i do the math it would suggest that something should be on fire!

please help :?

 

[charlie b]

This is simple. The ratings of the breakers tell you nothing about the load that they carry. You can have a 60 amp breaker and a 1 amp load. If you want to add load to an existing installation, I would use load measurements as the method of determining whether the installation has capacity. This usually means taking KVA measurements as a 15 minute average, and doing that for a total of 30 days. See 220.87 for more information.

Welcome to the forum.

 

[Kmansker]

thank you for the direction!! i did look up 220.87 and this is the method i will use to prove this out.

i would like to take this a step further though and ask how would i go about loading the panel if this were a new installation?

 

[Kmansker]

part 2

part 2

i have a similar case in the same plant except this is a new installation and it was done by an outside contractor.

the panel is again an 800 amp, this time protected by an 800 amp circuit breaker.

277/480 although all the breakers are 3 phase. my breakers are as follows

1-200 amp
1-150 amp
3-125 amp
3-100 amp
5-80 amp
1-50 amp
1-40 amp
1-30 amp

all these add up to 1545 amps. i do understand the above post that says the size of the breaker tells me nothing about the load it is actually carrying. BUT! being as this is a new installation i am lead to believe that somebody sized the breakers for the loads they carry then decided that an 800 amp breaker would be big enough to protect the panel that houses the breakers. How did "they" come up with this number? is there a formula or some derating factor that i am missing??

thanks

 

[charlie b]

There could be demand factors that you are missing. There could also be a non-coincident loading condition (e.g., you have three pumps, but only two of them can possibly be running at the same time). The size of the breaker not only tells you nothing about the amount of load the breaker will supply, but it also tells you nothing about the component that will be drawing that load. For example, one of the breakers might be supplying a point-of-demand water heater in a bathroom. Those things operate for a few seconds at a time, and really don't contribute to the average load the panel will see. Or perhaps one of the breakers might have been sized to prevent a trip during the normal starting of a large motor. Once that motor is running, the current it will draw will be much lower than the breaker rating. The correct approach is to perform a load calculation, per article 220.

 

[Kmansker]

thanks for your time, i have a power analyzer in my hand! lol

keith

 

[kwired]

If you are supplying motors - the breakers will throw you way off if you think you can just add the settings.

A motor with a full load amp rating of 20 amps may need to have a 40 or 50 amp breaker just to ensure it doesn't trip while starting, but the actual load may only average around 17 amps if you were to measure it. You are looking for what the actual load is when trying to find out why that 600 or 800 amp breaker holds on what otherwise looks like could be much higher load is connected.

Even in a simple dwelling look at how many 15 and 20 amp breakers are typically installed in a 100 amp panel. Should all of them be loaded to their max setting - you would trip the 100 amp main, but the reality is they are almost never all loaded at same time or to max setting.