calculating 120 and 240 volt loads

[xxiiiliv]

I've been in the trade for some time but have never had to calc loads. I am trying to figure the load for several portable buildings which will be joined into one. There is kitchen equipment, all of which is 120v, lighting, and an a/c unit on each portable. I have most of the data I need but am confused on a couple points. I have added all the nameplate ratings, figured the va for lighting and receptacles, but now I'm stumped. Do I convert everything to va then to amps? is there a difference in the 240 and 120v calcs? ALL help is appreciated

 

[infinity]

Convert all nameplate ratings to VA. Since you mulitply the volts times the amps the voltage does matter, for example:
10 amp load @ 120 volts = 1200 va
10 amp load @ 240 volts = 2400 va

 

[bob]

Convert everything to VA and divide by 240 volts. Put half the load on 1 leg and the other half on the other leg 2. Check the balance. Sometimes you can't get it balanced exactly even.

 

[xxiiiliv]

Great...that was along the lines of my thinking. One more thing if I may, I came up with 489 amps, now, do I add 20% and that's the breaker size ? or is it simple a 500 amp breaker

 

[infinity]

Great...that was along the lines of my thinking. One more thing if I may, I came up with 489 amps, now, do I add 20% and that's the breaker size ? or is it simple a 500 amp breaker

That's it. Your calculation has already taken into account the extra 25% for continuous loads and motors. Now if you want to add some room for future expansion that's a differennt issue.

 

[Smart $]

... Your calculation has already taken into account the extra 25% for continuous loads...

Really? Are you sure?

If you follow Article 220 to a tee, motors will be covered... but there is nothing about an extra 25% for continuous loads...

 

[xxiiiliv]

I did figure in the extra 25% for the lighting and the equipment I figured would be continuous. The 489 amps I got reflects that. So, at this point, am I done, or do I need to add 20% of this load to get the breaker size? Forgive me if I didn't see the answer in these posts, but this has been driving me batty. Thanks again for not only the help, but for being so to the point and making it understandable

 

[Little Bill]

I did figure in the extra 25% for the lighting and the equipment I figured would be continuous. The 489 amps I got reflects that. So, at this point, am I done, or do I need to add 20% of this load to get the breaker size? Forgive me if I didn't see the answer in these posts, but this has been driving me batty. Thanks again for not only the help, but for being so to the point and making it understandable

You don't need anything else that I can see. Since your calculations came to 489A, you will need to use the next size nominal OCPD, which would be 500A.

 

[infinity]

Really? Are you sure?

If you follow Article 220 to a tee, motors will be covered... but there is nothing about an extra 25% for continuous loads...

You're right. 125% of the largest motor and just about everything else at 100% unless you can apply some of the other reductions.

 

[xxiiiliv]

Awesome...many thanks

 

[jrohe]

I did figure in the extra 25% for the lighting and the equipment I figured would be continuous. The 489 amps I got reflects that. So, at this point, am I done, or do I need to add 20% of this load to get the breaker size? Forgive me if I didn't see the answer in these posts, but this has been driving me batty. Thanks again for not only the help, but for being so to the point and making it understandable

With all due respect, I believe you may be thinking about this a little backwards as you are using the load to size the breaker with no mention of using the load to size the feeders. When sizing circuits, it is important to remember that you calculate the loads to size the conductors, not the OCPD. You then size the OCPD as required to protect the conductors.

In this specific case, if the 489 amps is the calculated load (based on Article 220 requirements for continuous loads, permitted demand factors, etc.), you would then select two sets of #250 kcmil conductors (assuming terminals are rated at 75 degrees C, copper conductors are being used, and derating is not an issue). The parallel sets of #250 kcmil conductors are rated at a total ampacity of 510 amps. You would then select the OCPD based on this ampacity. I, personally, would go with the 500 amp breaker, but 240.4(B) may technically let those conductors be protected by a 600 amp OCPD if the conditions within that section are met.

 

[kwired]

Depending on what you have, you could possibly apply some demand factors and reduce the needed service or feeder size. You only need to count larger of loads that will not run simultaneously like heating and cooling.

Don't know how you came up with your load calculation but keep that in mind - read through art 220 and see what you can do.