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#25716 - 05/16/03 04:07 PM Small transformer application  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
I have been asked to install a 0.5 KVA transformer with a 480/240 primary and 240/120 secondary.
A small, ground level sign with 3 recessed, incandescent fixtures (120 volt) was installed, along with the HID parking lot lighting (240 volt) for a new shop/office.
The lighting conduits include stubs to feed the sign. It would be much more labor-intensive to run a new 120 volt ckt. than to install this xfmr. and have the sign come on with the lot lights.
Assuming proper fusing and disconecting means, is this an acceptable plan?


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#25717 - 05/16/03 05:46 PM Re: Small transformer application  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
That should be OK… 0.5kVA = 500VA = 500 watts If you could serve everything 120V, your could get by with 2-pole primary overcurrent protection--ideally using dual-element fuses per 450-3. 500VA/480V x 125% = 1¼-amp fuse would work. At 240V, secondary full load is ~2.1 amps, or at 120V, secondary full load is ~4.2 amps. The secondary needs a local ground-electrode connection/bonding jumper to X2.



[This message has been edited by Bjarney (edited 05-16-2003).]


#25718 - 05/17/03 10:29 PM Re: Small transformer application  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
OK,
Just for fun..
240.21(C)(1) permits primary OCPD to protect the secondary conductors providing the OCPD doesn't exceed the value of the conductors ampacity multiplied by the secondary to primary voltage ratio.
In this case...
Primary current = 2.08 amps @ 240 volts. 2.08 X 1.67 per Table 450.3(B) = 3.47
Primary OCPD = 3 amp fuse.
Secondary conductor ampacity (#12 THWN) = 25 amps.
Secondary to Primary Voltage ratio = 0.5 (1 to 2).
25 X 0.5 = 12.5.
3 is less than 12.5, therefore secondary protection is not required. (I'll probably install secondary protection, anyway.)


#25719 - 05/17/03 10:44 PM Re: Small transformer application  
caselec  Offline
Member
Joined: Apr 2002
Posts: 558
San Jose, CA
Oops! I thought you said the existing circuit was 480 volt.

I deleted the rest of my post.

Curt


[This message has been edited by caselec (edited 05-18-2003).]


Curt Swartz

#25720 - 05/17/03 11:34 PM Re: Small transformer application  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Sorry Redsy — Completely missed the 240V source/primary… old habits die hard. That simplifies things a bit. I’ve steered away from the 167/300% multipliers on smaller 150/115°C-rise drytypes. They run {UL-labeled} quite hot, and it just seems to be pushing things. Things have a way of getting scabbed on to small lone transformers—just being gun shy. With 55°C-rise machine-tool transformers, there’s some headroom. If you want to protect the two-wire grounded secondary, might consider putting in a 3-pole fused switch for the primary side, and use the middle pole for a secondary fuse.


[This message has been edited by Bjarney (edited 05-17-2003).]


#25721 - 05/18/03 06:50 AM Re: Small transformer application  
zapped208  Offline
Member
Joined: Feb 2003
Posts: 196
Somewhere in USA
Redsy,- Is it easyier to pull in the 120v circuit ,than to go through all the baloney to install a transformer for this sign? More info would help. Zapped208


Shoot first, apologize later.....maybe

#25722 - 05/18/03 09:43 AM Re: Small transformer application  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Due to the location of the sign (at the end of a run of about a dozen or more light poles)it is a great deal easier to install the transformer than to run a new ckt. There is already a conduit at the sign from the last pole (about 20' away). Installing a new ckt. would entail multiple pulls through each handhole of each pole. And the panel is about 300 ft. away.
My main concern is whether or not this application of the transformer is acceptable, assuming all fusing, grounding, and disconnecting requirements are observed.


#25723 - 05/18/03 06:26 PM Re: Small transformer application  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
Redsy — That should work fine. With the incandescent load, voltage is not critical, but consider that at rated load a drytype will have about 3-6% voltage drop in the transformer alone.

{It’s also nice to put in a NEMA 5-15 receptacle out of open view so the toothless guy with his shopping cart and tinfoil hat can recharge his laptop PC after hours. 40VA of spare capacity should be adequate. ;-) }




[This message has been edited by Bjarney (edited 05-18-2003).]



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