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Author Topic: power question  (Read 4025 times)

Gus Housen

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power question
« on: January 05, 2013, 09:00:11 PM »

A venue I work @ has a 3 phase 400 or 200 amp disconect w/ fuses installed. ( I am pretty sure its a 200)
I use a 100 amp distro, can I get 100 amp fuses that will phsically fit the 200 amp or 400 amp fuses.

Other wise I am plugging in a 100 amp distro box into a 200/400 amp service
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Tim McCulloch

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Re: power question
« Reply #1 on: January 05, 2013, 09:10:12 PM »

A venue I work @ has a 3 phase 400 or 200 amp disconect w/ fuses installed. ( I am pretty sure its a 200)
I use a 100 amp distro, can I get 100 amp fuses that will phsically fit the 200 amp or 400 amp fuses.

Other wise I am plugging in a 100 amp distro box into a 200/400 amp service

The quick answer is yes, you can probably find 100 amp fuses for the 200 amp switch, but probably not a 400 amp switch.

The disconnect is fused or otherwise overcurrent-protected for the benefit of your feeder, not the downstream load (unless there is only one load).  If your feeder is rated for 200 amps, you don't need to downsize.

Code also permits using a intermediate overcurrent-protection device between a higher ampacity service and your lower ampacity feeder.  It also permits undersized, short supply conductors (tails) to be connected to a distro (switchboard) or the intermediate OCPD.  See NEC 520.53 for more info.
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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

Gus Housen

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Re: power question
« Reply #2 on: January 05, 2013, 09:42:27 PM »

essentually what I have is 100 amp box with 2 50 amp cs connectors and a 20 amp utility outlet . The box has 2 50 amp 240 breakers and 1 20 amp 120 breaker ( no main breaker, essenually a sub pannel)
So as long as i have the correct feeder i should be good to go. i will look@ the code

an addtional question is my sub pannel is single phase, and the disconect is 3 phase I assume that I can just tie into 2 of the hot.
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Tim McCulloch

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Re: power question
« Reply #3 on: January 06, 2013, 12:40:14 AM »

essentually what I have is 100 amp box with 2 50 amp cs connectors and a 20 amp utility outlet . The box has 2 50 amp 240 breakers and 1 20 amp 120 breaker ( no main breaker, essenually a sub pannel)
So as long as i have the correct feeder i should be good to go. i will look@ the code

an addtional question is my sub pannel is single phase, and the disconect is 3 phase I assume that I can just tie into 2 of the hot.

To answer your last question... ah... ahhemmmmmmm... all I'll say is that I see it done frequently.   ::)
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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

David Parker

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Re: power question
« Reply #4 on: January 06, 2013, 07:19:32 AM »

essentually what I have is 100 amp box with 2 50 amp cs connectors and a 20 amp utility outlet . The box has 2 50 amp 240 breakers and 1 20 amp 120 breaker ( no main breaker, essenually a sub pannel)
So as long as i have the correct feeder i should be good to go. i will look@ the code

an addtional question is my sub pannel is single phase, and the disconect is 3 phase I assume that I can just tie into 2 of the hot.

you probably wont find this on a dedicated disconnect, but quite in service panels often you'll find 3 phase where one of the legs is 220v to ground. Good plan is to meter every time and you wont have to worry about that biting you. And don't expect the wild leg to be signified as per code.

http://en.wikipedia.org/wiki/High-leg_delta

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Vinny DAgostino

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Re: power question
« Reply #5 on: January 06, 2013, 10:58:20 AM »

A venue I work @ has a 3 phase 400 or 200 amp disconect w/ fuses installed. ( I am pretty sure its a 200)
I use a 100 amp distro, can I get 100 amp fuses that will phsically fit the 200 amp or 400 amp fuses.

Other wise I am plugging in a 100 amp distro box into a 200/400 amp service

Going back to the fuse thing....
A 100a fuse will not fit in a 200a rated disconnect without using fuse reducers 200/100a.
Another question....is this disconnect 120/208 or 480/277??
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duane massey

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Re: power question
« Reply #6 on: January 06, 2013, 11:00:54 AM »

Ah, the "wild" leg. I learned about this the hard way in my much younger days. All it took was once....
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Duane Massey
Technician, musician, stubborn old guy
Houston, Texas

Tim McCulloch

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Re: power question
« Reply #7 on: January 06, 2013, 11:14:35 AM »

Ah, the "wild" leg. I learned about this the hard way in my much younger days. All it took was once....

Yep... meter the service side of the switch before connecting.  After, meter the voltage at your cams, and then meter test points or outlets on your distro before connecting any load cables.

That was a lesson I received from one of Dirk Schubert's system techs a very long time ago.  I was stage-managing a small tour with a pair of one-hit wonders, Schubert Systems was the supplier on this nightclub/small theater tour.  We were playing a 600 seat room... I was first off the bus so I scoped out cable runs and power location.  I pointed out the panel to him and went away.  Later he came to get me and said "bring your volt meter."  120-120-208... 208? WTF??  I knew about split phase and 3 phase, but I'd never seen a wild leg service "in the wild."  This was sometime in the mid-1980s, IIRC.

As you point out, Duane, not all wild legs are marked... this one got marked.  I was back in that facility a couple of years ago and the orange tape he put on the service wire was still there.

For the gifting holidays of 2011, I gave new volt meters to several of our employees along with a refresher on how to use them in determining what type of service they're looking at.

edit p.s. - Really, really, folks... go to your library or find the NEC online (there IS a free reference, IIRC) and read NEC 240, 250, 400, 406, 408, 520, 525 and 640.  Those Articles cover *most* of what an on-site inspector will be looking at and considering should he or she decide to inspect a venue or event.  Also remember that there is much left to the interpretation of the inspector and that the locale you are working in may have adopted more or less stringent codes.
« Last Edit: January 06, 2013, 11:21:29 AM by Tim McCulloch »
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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

ProSoundWeb Community

Re: power question
« Reply #7 on: January 06, 2013, 11:14:35 AM »


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