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| Subject - Panel Schedule Questions!? | |
| skudwoofer |
When creating a panel schedule, do I use the formula (volts * amps)/1000 = KW or KVA? Specifically, for, lets say an A/C unit that is rated at 30A and 220V. First do I use 220 or 240 in my calculations on a single phase panel? So would the figure be (30 * 220)/1000 = 6.6 KVA OR this (30 * 240)/1000 = 7.2 KVA Then, do I add the 25% of 125% for continuous loads into the total kva figure or to the demand kva figure? (same question for the largest motor) Finally, on a 220 circuit, do both the "A" and "B" legs see that 220 volts or do they see 1/2 that figure so they would see 110? Example: the same A/C unit above, does the "A" leg see 6.6 KVA or 3.3 KVA (for the KVA totals for each leg and thus the total KVA for the panel, leading to the Demand KVA figures. E-Mail me for the file...  |
| Scott Vickrey | I'm assuming you are talking about a dwelling calculation since you mention the largest motor.quote:Check out this section in your code book 220-2(a). In here it tells you what voltages to use. In your case 240. 240 is a nominal voltage 220 is not. quote: You are actually calculating KVA. KW includes the power factor. quote: I'm not sure exactly what you mean. When you find the largest motor load apply the 125% immediately. (largestmotorload x 1.25) quote: The voltage will be 120v to ground and 240v between the two. Since at it's peak one leg will be +120v and the other -120v. The amperage will be 7.2kva on the entire circuit from one leg through the load and at the other leg. ImportantNote This message refers to the 2002 NEC. The national electrical code may or may not be adopted by the authority having jurisdiction where you are working. It's possible they have not adopted the latest edition of the code. Also any local rules or ordinaces will supersede the NEC. |
| David Hyatt | I was researching this topic and it asked about the same question I have but don't understand fully. Can someone tell me how to create a 3 phase panel schedule with total VA per phase and total amperage per phase? Also on a 200amp service if you put a amp clamp on each leg could they read 100amps each and not trip the 200amp main? |
| JimmyDee | quote: You will have used up half of your 200 amps if each leg reads 100. Just like a single phase service that is 200 amps. It allows 200 amps of 120 vlots for each leg or 400 amps total of 120 volts. |
| David Hyatt | How about that panel scedule, Jimmy, can you take 5 loads of your choice and show us how to make a balanced panel schedule? Use (2) three phase loads, (1) 2 pole load, and two single phase loads. 120/208 3 phase panel 200 amps. Thanks |
| JimmyDee | I've never thought of a ballanced panel as having anything to do with total amps of the panel. I'll give it a try. A. B. C. 35 35 35 3 phase 35 35 35 3 phase 30 30 2 pole 20 1 pole 15 1 pole __________________ 100 100 100 Total You are making it too hard. Jim |
| David Hyatt | Jimmy, I know its Christmas so don't worry yourself too much about this right now but what I really need is VA per phase (show your work) how you determined VA per phase, just like you have your amperage now, then show the formula to how to find total connected amps and total connected VA or KVA. Thanks. The reason I am doing this is because I am going against the architect and going over his head to the owner. I need a new schedule. Any help will be appreciated. I tried to use his format and none of the calculations worked out, either I am figuring wrong or he is. I really think he is but without a second opinion I can't afford to be wrong. Merry Christmas. My book from Tom Henry says if you have a 240 volt 3 phase motor with 6000watts each phase will see 2000watts. This is where I am confused. Where did he get the 6000va in the first place? I x E X 1.732? And is it really 2000 per phase or 6000 per phase? In this formula watts and VA would be the same thing right? I think you have the right idea of what I am looking for now, please help! |
| David Hyatt | Somebody help me!!!!!!!!!!!!!! Its the holidays but whats more important family or the local brotherhood of electricians. LOL Just kidding. |
| JimmyDee | I'm not sure what you are looking for. But the book you read about the 6000kv for the motor I can follow. Where did the 6000 kv come from? It came from the formula of volts X amps X sq root of 3 (1.732). So this motor to find the amps per phase, you would divide 6000/1.732 and that would = 3464/ voltage. Since the voltage is 220 volts, you will divide 3464/220 = 15.75. So far that probably makes sense. Now if you multiply 220 X 15.75 to get VA you will come up with 3464 VA. That probably don't make sense because you were given that it should have been 2000 va and the 2000 is correct. What to do. We now have to revert to the fact that in a 3 phase system the calculated actual amps and volts don't add up to 1/3 of the total VA. Because of a whole bunch of stuff that I don't understand about phase angles and stuff like that the line amperage and voltage multiplies together have to be multiplied by the sq root of 3 or 1.732. Lets go back to our problem of this motor. 15.75 amps X 220 volts X 1.732 will = 6000 va, sooooooo one third of that on each leg will be 2000 va per leg. Also if you divide the 3464 by 1.732 you will find it will = 2000 va. Hope you can follow this. Jim |
| David Hyatt | I think I got it now. Thanks Jimmy 5 hp 3 phase motor 120/208 volt Per TBL 430.150= 16.7amps 5 hp single phase 208 motor Per Tbl 430.149= 30.8 amps 2-1500watt lighting circuits= 25amps A********B********C 2005*****2005*****2005 3203*****3203***** ***************** 1500 ***************** 1500 5208va 5208va 5005va Aphase Bphase Cphase Total= 15,422VA/1000= Total Connected KVA= 15.42KVA Total connected amps= 15422/208/1.732= 42.81amps???? Using the KVA/voltage/1.732 gives 42.81amps. But the actual connected amps is= 72.5amps???? Why is there a difference?  I thought I had it? |
| JimmyDee | quote: No it isn't. You are trying to ascribe the single phase amperage to the 3 phase motor and you can't do that. Notice the 2 motors and their total connected wattage or kva. The same aren't they? A 5hp motor is going to draw close to another 5hp motor in wattage, be it single or 3 phase. The line amperage is going to be considerably less with the 3 phase because it is sharing the VA load with 2 other phases. Look at it like you have a 5hp gas engine with a single cylinder. All 5 hp is coming from that one piston and rod. If you had a 5hp engine that was a 3 cylinder engine, each cylinder and rod would be contributing 1/3 of the 5 hp. It is just like that in a 3 phase motor. Jim |
| Mike Delaney | Darn I'm glad I'm not in your shoes...your confusing the heck out of me.
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| Ryan_J | David: I'll give you a call when I get home, like I promised, but for right now, where are you getting your 72.5 amps from? |
| David Hyatt | Ryan, I am glad you called, I wasn't thinking clearly because I had been doing taxes for hours, but I now see where I get the 72.5 amps from. I added the amp load per motor from the table: 16.7 + 30.8 + 25 = 72.5? If I then divide by 1.732 I am close to the 42amps. Check it out again please, I highlighted the values in above formula in red. |
| Ryan_J | David: I think you need to choose whether you want to work in VoltAmps or in Amps...choose one and stick with it. Look at your panel schedule one more time...notice how phase A will never even see 25 of those amps? Notice how phase B will never see 25 of those amps? Notice how phase C will never see 26 0f those amps? Right...so you can add up all the amps if you really wanted to, but then you could just erase 25 amps right off the top because at some point, all three of the phases are 25 amps lighter than the sum total of the amperage...That is why we balanced the panel in the first place. Does that make sense? |
| David Hyatt | Yea, I think I got it now. I just sent you an email let me know what you think. |