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#72284 - 11/22/06 12:28 AM A little help  
HVAC Tech  Offline
Junior Member
Joined: Nov 2006
Posts: 6
Can someone explain to me a little about KVA? I saw a transformer today at work rated 1,000 KVA going to the primary of the switch gear. Where did they get that from? I saw a box rated by the switch gear at 2,700 VAC, is that coming from the transmission yard outside? This also had to larg capacitors - Im guessing that's to keep the power-factor correct? Any advice would be great, thanks.


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#72285 - 11/22/06 04:55 AM Re: A little help  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
It means DONT GO NEAR IT! (2,700 VAC)

If you can envision a white ball of light enveloping your body as portions of it vaporize and 3rd degree burns if you live - thats what it is. Well, maybe not that extreme - but it will killify you! Voltages of that range and above should only ever be approached by trained and qualified personel - you aint one of them....

KVA: Kilo Volt-Amperes
Ref: https://www.electrical-contractor.net/ubb/Forum15/HTML/000089.html
Kilo = thousand, so multiply the number X's 1000
Volt-Amperes is a referance to a formula for power (watts), and mitigated by power factor, and phase relationship.
Ref: http://www.abrconsulting.com/Conversions/elec-con.htm#Volt-Amps
Ignoring the Power Factor that roughly a million watts of power. (1000X1000) A mega-watt.


VAC as in 2700 VAC would be 2.7KV or 2700 Volts in an AC system. (all voltages can be harmfull - and 277 and above, and this one is far above, can be deadly. Not user friendly...) The box you saw may or may not have that voltage in it, but don't try to find out.... But probhably in the nieghborhood of 2000+, or 2Kv.

Anyway, it was probhably coming from the transmission yard outside.

And the capacitors were probhably, as you guessed, for power factor correction.


[This message has been edited by e57 (edited 11-22-2006).]

[This message has been edited by e57 (edited 11-22-2006).]


Mark Heller
"Well - I oughta....." -Jackie Gleason

#72286 - 11/22/06 05:14 AM Re: A little help  
Luketrician  Offline
Member
Joined: Jul 2006
Posts: 272
Sale Creek, TN USA
Those capacitors are used when you have a lagging phase angle caused by inductive circuits. The capacitors have a leading phase angle, which will cancel out the inductive lag.

I was taught this acronym: "ELI THE ICE MAN"
Current lags the voltage in an inductive circuit.

E=voltage L=inductance I=current

Current leads the voltage in a capacitave circuit.

I=current C=capacitance E=voltage


Luke Clarke
Electrical Planner for TVA.


#72287 - 11/22/06 05:29 PM Re: A little help  
HVAC Tech  Offline
Junior Member
Joined: Nov 2006
Posts: 6
"trained and qualified personel - you aint one of them."

Thanks guys. Well, we shut down switch gear today and they made me "touch the 2,700" them. Infact, they're teaching me everything inside out now about this. After going through all this with my forman and such today, I found its pretty easy stuff - not difficult at all. My journeyman and forman say IM doing outstainding. So the job is going well.


#72288 - 11/22/06 05:54 PM Re: A little help  
SteveFehr  Offline
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
Contrary to popular believe, kW is not whats you get when you multiply Volts x Amps, kVA is!

kVA stands for kilovoltamperes as you probably suspected. For DC circuits and purely resistive circuits, Watts = VA.

The fun part comes in when you deal with AC circuits and power factor. The power factor directly relates VA to Watts; An 800 Watt circuit with a pf of 0.8 will be drawing 1000VA.

Watts = VA * PF
Watts = Amps^2 * Ohms
Watts = Volts^2 / Ohms

To try to explain PF gets a bit into calculus and geometry, but the simple explanation is that in circuits that contain capacitors and inductors, the peak current draw is NOT occuring the same time as the peak voltage. So, if the peak current of 20A occurs when the voltage is 96V and drops to 16A when the current is 120V (PF=0.8), you may be supplying 120x20=2400VA, but you're actually only supplying 1920 Watts. If this circuit was inductive, adding capacitors would help counteract the inductance and move the power factor closer to PF=1.0, which is more efficient.


-------------------------
For most practical purposes, the VA and Watts are pretty close. But if you have to make any calculations for ampacity or OCP, it's important to know that the VA are going to be larger than the Watts and use PF in your calculations.

[This message has been edited by SteveFehr (edited 11-22-2006).]


#72289 - 11/22/06 06:27 PM Re: A little help  
Celtic  Offline
Member
Joined: Feb 2005
Posts: 361
NJ
No offense e57:
Quote

(all voltages can be harmfull - and 277 and above, and this one is far above, can be deadly. Not user friendly...)


...but this statement NEEDS to be addressed.

This is one of the common misconceptions about working around electricity. 120v WILL kill you just as dead as any other voltage you stumble across.

There is more to the story than volts...amperage, path across the body, time energized, etc. The amperage from a single 60w light bulb will make you miss your next birthday - plain and simple.

Our HVAC Tech needs to be aware of this.

Here is a 13 minute long video the graphically illustrates the point: http://www.electrician2.com/safety/electricalsafety_01.htm


Here is a document that lists Electrocution Fatalities: http://www.cdc.gov/niosh/injury/traumaelface.html


Let's not downplay the dangers involved in a circuit that's "only 120v"...it takes very little CURRENT to kill a person

Saftey first ~ unlace your own damn boots every night.

[rant]END[/rant]

I hope you take no offense e57, but it needed to be said....and like a jackass, I lit myself up yesterday because I wasn't paying attention to the circuit I was working on!


~~ CELTIC ~~
...-= NJ =-...

#72290 - 11/22/06 07:06 PM Re: A little help  
steve ancient apprentice  Offline
Member
Joined: Mar 2005
Posts: 169
west springfield,mass
i AGREE WITH CELTIC. My only time I was ever shocked was with 120. Hooking up a fixture in a bathroom. With the switch off it should be dead right. I even stuck the tick tester on it. Well some one had cross polorized the leg before it got to the bathroom. I hooked up the ground and when I connected what I thought was the neutral i got locked on. If it was not for the painter I may not still be here.Next time ill check both the black and the white wires. All the time in the navy, around rocket launchers,gun mounts and high voltage nothing. With a lousy fixture I almost got hurt. Check everything and I do mean everything.I may no longer be in the field, but im sure some of my experiances can help.


#72291 - 11/22/06 07:25 PM Re: A little help  
Celtic  Offline
Member
Joined: Feb 2005
Posts: 361
NJ
That's the problem with having "experience" - you think you know it all...until you get bit.


~~ CELTIC ~~
...-= NJ =-...

#72292 - 11/22/06 09:39 PM Re: A little help  
e57  Offline
Member
Joined: May 2003
Posts: 2,876
S.F.,CA USA
No offense taken... Note first words... "It means DONT GO NEAR IT!" Remembering HVAC guy, just joining ECN a week or two ago, and seeing red flags of 2700V from a guy who just got into a program, obviously near gear out of his league unattended. The wording was on the fly, (late at night from my usual bad back throbing alarm clock) and kinda roughly meant to deter attempts at exposure to voltages that 'act' much differently than the generic 120/240 that some are used to. At about the 277+ mark it has a tendancey to 'reach out and bite you' with little or no contact at all - depending on potential. So in retro-spect, there should be an 'especially' before that 'deadly". [Linked Image]

HVAC Tech I hope these guys that have you putting your hands on stuff went through the Whens, Hows and Nots (safety) of the gear you were in. At that voltage, one mistake could be your last. Off, AND grounded as well as with proper PPE should be the only way to approch that gear.

Steve, there ought to be a whole thread called "Nailed by Neutral" [Linked Image] I could go on, and on and on....


Mark Heller
"Well - I oughta....." -Jackie Gleason

#72293 - 11/22/06 10:01 PM Re: A little help  
Celtic  Offline
Member
Joined: Feb 2005
Posts: 361
NJ
Thanks for the understanding e57 - I didn't want you to take it the wrong way .

I am kinda PO'ed that I nailed myself....what a ma'roon!


~~ CELTIC ~~
...-= NJ =-...

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