Ampacity of main distribution panel

pablo r. reyes · Oct 9, 2005

What is the correct formula to calculate the ampacity of main distribution panel to select the main circuit breaker size? Given that the loads are 10kVa on phase A-N, 10kVa on phase B-N and 10kVa on phase C-N, these loads are fed from 480V-380Y/220V system. What if the loads are fed from Line to Line (say 480V-220Y/127V). Is the formula be changed in order to find the panel ampacity and OCPD?

jtester · Oct 10, 2005

Re: Ampacity of main distribution panel

Pablo

Is your main panel on the low side of a stepdown transformer, and does the wiring on low side of that transformer already have overcurrent protection? Your panel main may need to be sized for that.

Normally a panel should be sized to carry all known loads plus any future that may arise. The known loads include any multipliers such as 1.25 for continuous loads, etc.

In your case, the load is 30 kva. You can turn that into amps in any system by dividing kva by (kv*1.732). In your 480 system amps=36.08; in your 380/220 system amps =45.58; in your 220/127 system amps = 78.78

Hope that helps.

Jim T

charlie b · Oct 10, 2005

Re: Ampacity of main distribution panel

We generally don't use the word "ampacity" in connection with a panel. That word applies to conductors (e.g., wires and busways). The panel has to be "rated" for the total amount of amps that it will be called upon to carry. Jim T. gave the formulas for that calculation.

It does not matter if you are placing your loads line-to-line versus line-to-neutral. Add up the total KVA, and use the formula that Jim posted.

pablo r. reyes · Oct 11, 2005

Re: Ampacity of main distribution panel

Charlie,

By the calculation posted by jtester, is shows that the Panel Main OCPD rating depends on how the loads are connected. When the 3 identical loads each rated 10kVa 220V are connected across 220V of 480-220Y/127V transformer secondary, the current flows to the main OCPD is 30kVa/(1.732*220) = 78.78Amps, we size the OCPD 80AT. When these are connected 220V Line-Ground of 480-380Y/220V, the line current is 30kVa/(1.732*3800 = 45.58Amps, we may size the OCPD 50AT. So, it is a matter if we place the loads line-to-line versus line-to-neutral, also depends on transformer secondary connection. Then calculation of main panel OCPD depends on how the loads are connected.

pablo r. reyes · Oct 11, 2005

Re: Ampacity of main distribution panel

Correction to my reply above:

By the calculation posted by jtester, is shows that the Panel Main OCPD rating depends on how the loads are connected. When the 3 identical loads each rated 10kVa 220V are connected across 220V of 480-220Y/127V transformer secondary, the current flows to the main OCPD is 30kVa/(1.732*220) = 78.78Amps, we size the OCPD 80AT. When these are connected across 220V Line-Ground of 480-380Y/220V, the line current which is also equal to phase current is 30kVa/(1.732*380) = 45.58Amps or 10kVa/220V = 45.58Apms, we may size the OCPD 50AT. So, it is a matter if we place the loads line-to-line versus line-to-neutral, also depends on transformer secondary connection. Then calculation of main panel OCPD depends on how the loads are connected.

jtester · Oct 11, 2005

Re: Ampacity of main distribution panel

pablo

The reason your current changed is because you did calculations in two different voltage systems. As the system voltage goes up, the current will go down.
Within a given system voltage, it doesn't matter if you connect your loads L-L or L-N, although you will have to change the voltage rating of the load from 127 volts to 220 volts or whatever.

Jim T

charlie b · Oct 11, 2005

Re: Ampacity of main distribution panel

Originally posted by pablo r. reyes:


When the 3 identical loads each rated 10kVa 220V are connected across 220V of 480-220Y/127V transformer secondary, the current flows to the main OCPD is 30kVa/(1.732*220) = 78.78Amps, we size the OCPD 80AT.



When these are connected across 220V Line-Ground of 480-380Y/220V, the line current which is also equal to phase current is 30kVa/(1.732*380) = 45.58Amps or 10kVa/220V = 45.58Apms, we may size the OCPD 50AT.

So, it is a matter if we place the loads line-to-line versus line-to-neutral, also depends on transformer secondary connection. Then calculation of main panel OCPD depends on how the loads are connected.

Sorry, but no. You used two different transformers for your comparison. That is like comparing apples to walnuts, and we are not making Waldorf Salad here.

Your two configurations are not just different by virtue of the way the loads are connected (L-L versus L-N). They are also different by virtue of having a different operating voltage. If you want to push out 30 KVA of load with a 220 volt system, it will take more current than would be drawn by a 380 volt system.

If you want to compare L-L connections with L-N connections, you need to use the same transformer for both calculations. Let's take the 480-220Y/127.

L-L (loads get 220 volts): 30 KVA / (1.732 * 220) = 78.7 amps.

L-N (loads get 127 volts): 30 KVA / (1.732 * 220) = 78.7 amps.

In your examples, you get different OCPD sizes because you used different transformers.

pablo r. reyes · Oct 12, 2005

Re: Ampacity of main distribution panel

Guys please be patient with me, sorry for my confusion, but please dont look at 30kVa/(1.732*380) = 45.58Amps.

Each 10kVa load require 220V. What I mean is, I should utilize the 220V Line-Ground of 480V-380V(LL)/220V(LG) system, what current will I get and if I use 220V Line-Line of 480V-220(LL)/127V(LG) system, what current will I get?

Now, please tell me what OCPD rating should I select for both?

Actually, it is a very hot discussion with our subcontractor who made the calculation. He arrived at the calculation wherein each phase AN, BN, CN current is 45.58Amps, using 220V line-ground of 480V-380Y/220V. Then he multiply 45.58Amps by 1.732 to size the main OCPD. Should I allow him to do this way? I think 1.732 is not necessary in the calculation since phase current is also the line current here, 45.58Amps is enough reference in the selection of OCPD.

Your replies, charlie b and jtester are very very much appreciated.

jtester · Oct 12, 2005

Re: Ampacity of main distribution panel

pablo

You don't need to worry about whether your loads are connected L-N or L-L. You have 30 kva of load. Calculate amps from that for each system voltage. The previous posts have shown you how. Be sure to include any future loads if appropriate.

The 380/220 system will have lower amperage, and correspondingly lower breaker size. That is because it operates at a higher voltage.

What is your system voltage, 380/220 or 220/127? If the transformer hasn't been purchased and you have a choice of voltages, usually the higher voltage is more efficient.

Jim T

charlie b · Oct 12, 2005

Re: Ampacity of main distribution panel

Originally posted by pablo r. reyes: Now, please tell me what OCPD rating should I select for both?

First tell us what transformer you will be using. I think this is the key point that you are missing. You have to pick the transformer, and then you can calculate the current.

I should utilize the 220V Line-Ground of 480V-380V(LL)/220V(LG) system, what current will I get and if I use 220V Line-Line of 480V-220(LL)/127V(LG) system, what current will I get?

I don't have to tell you that, because you have already said it yourself (and you were correct):

Originally posted by pablo r. reyes: 

When the 3 identical loads each rated 10kVa 220V are connected across 220V of 480-220Y/127V transformer secondary, the current flows to the main OCPD is 30kVa/(1.732*220) = 78.78Amps, we size the OCPD 80AT.



When these are connected across 220V Line-Ground of 480-380Y/220V, the line current which is also equal to phase current is 30kVa/(1.732*380) = 45.58Amps or 10kVa/220V = 45.58Apms, we may size the OCPD 50AT.

He arrived at the calculation wherein each phase AN, BN, CN current is 45.58Amps, using 220V line-ground of 480V-380Y/220V.

The results of that calculation are correct. But to get those results, he would have had to divide 30KVA by (380*1.732), and ignored the value of 220 volts line to ground.

Perhaps this will help: When calculating three phase current from given values of power and voltage, you never, never look at the phase-ground voltage. You always use the phase to phase voltage.

Then he multiply 45.58Amps by 1.732 to size the main OCPD.

That is nonsense. You are correct about this as well. You can protect the secondary of a transformer with an overcurrent device as much as 125% of its rated current. The factor of the square root of three does not enter into the selection of the overcurrent device.

[ October 12, 2005, 10:35 AM: Message edited by: charlie b ]

Ampacity of main distribution panel

pablo r. reyes

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jtester

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charlie b

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pablo r. reyes

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pablo r. reyes

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jtester

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charlie b

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pablo r. reyes

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charlie b

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