On a substation breaker that is coming off of a 13200v bus down to 7620v through the breaker I would like to know after you get your average amperage per phase on the 7620 side (Distribution Side) say 420 amps for example do you use the 13200 or the 7620 to figure your Mega-Watts? How is the best way to calculate and if you don't mind could you give an example. Thanks!
Calculating Mega Watts
- Thread starter dsmith411
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zog
Senior Member
- Location
- Charlotte, NC
You have not provided enough information to calculate MW. Is this homework?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
As Zog said, there is not enough information given. For starters, a breaker cannot change the voltage from one value to another. The voltage change takes place in a transformer. So at what point in the system (i.e., what side of the transformer) was the current value measured? You use the voltage corresponding to that side of the transformer.
calculating Mega watts
calculating Mega watts
For Ex: you have a load of 428 amps on A Phase,420 amps on B phase and 400 amps on C phase on the 7620 outgoing side of the breaker.The voltage coming into the top of the breaker is 13,200v and the voltage coming out is 7620v.How do I calculate how many Mega Watts is on that breaker?
calculating Mega watts
For Ex: you have a load of 428 amps on A Phase,420 amps on B phase and 400 amps on C phase on the 7620 outgoing side of the breaker.The voltage coming into the top of the breaker is 13,200v and the voltage coming out is 7620v.How do I calculate how many Mega Watts is on that breaker?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
You missed my point. What you are describing is physically impossible. Breakers do not have different voltages on the top and on the bottom. Oh there might be a very small voltage drop within the breaker, perhaps from 13,200 volts to 13,299.93 volts. But you are not describing the situation correctly, if you think a breaker can drop from 13,200 to 7,620.
mivey
Senior Member
You are mixing line voltage and phase voltage. The voltage in and out is the same.
eric9822
Senior Member
- Location
- Camarillo, CA
- Occupation
- Electrical and Instrumentation Tech
That's not possible, as Charlie stated there has to be a transformer somewhere. Can you verify what you have and post a sketch of a one line?
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
OK, I just noticed the relationship between the two numbers 13,200 and 7,620. The one is 1.732 times the other. So you are talking about a measurement of 13,200 volts from phase to phase, and a separate measurement of 7,620 from phase to neutral (or to ground). The 13,200 phase to phase would be the same value on both sides of the breaker, and the 7,620 phase to ground would be the same value on both sides of the breaker.
zog
Senior Member
- Location
- Charlotte, NC
Still need PF.
eric9822
Senior Member
- Location
- Camarillo, CA
- Occupation
- Electrical and Instrumentation Tech
I missed that. Now I understand.
mivey
Senior Member
If you want to know the capacity need, take the highest current, calculate the single phase MW and multiply by 3.
If you want to know what is being used, calculate the individual phases and multiply by 3, or use the average current and directly calculate the 3-phase MW.
I just checked one, it had 480 on top and 0 on bottom :grin:
(could not resist)
mivey
Senior Member
as a tack.
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