ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals

ECN Shout Chat
Top Posters(30 Days)
Admin 16
Recent Posts
Another Forum Update
by Admin. 03/22/17 03:04 PM
Dining room plugs
by watersparkfalls. 03/21/17 10:31 PM
TRUE POWER
by jraef. 03/21/17 09:13 PM
WEG CFW-11 Frequency Inverter
by jraef. 03/21/17 08:50 PM
fuse rejectors
by HotLine1. 03/20/17 08:14 PM
New in the Gallery:
SE cable question
Popular Topics(Views)
231,218 Are you busy
165,932 Re: Forum
160,462 Need opinion
Who's Online Now
0 registered members (), 57 guests, and 10 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#85090 - 05/28/03 11:56 PM x-fmr feeds equipment  
Steve T  Offline
Member
Joined: Feb 2001
Posts: 306
Oak Park, IL, USA
A building has a 120/240 high leg three phase service.

The contractor wants to install a delta-wye 240/480 volt x-fmr that will directly feed a piece of equipment (industrial lathe, in a commercial building).

I am not sure how far away the equipment will be from the x-fmr, but lets say more than 25 feet for this question.

First off, can this be done?

Looking at 240-21 (c)(2) and (3), over 25 feet would not be allowed.

But even if it is not over 10 feet 240-21(c)(2)(c) says the conductors must terminate at an enclosed switchboard, panelboard, or control device, or back of an open switch board. Is a fused disconnect a control device as used in this section?

If so, does the neutral need to go to the disconnect? To the machinery?

Does the secondary transformer winding need to be grounded to all available grounding electrodes on site? What size does it have to be.? (15KVA x-fmr)


2017 / 2014 NEC & Related Books and Study Guides

#85091 - 05/29/03 09:17 AM Re: x-fmr feeds equipment  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
For a 15kVA 3-wire [no neutral] 240V-delta 480V-delta 3-wire [ungrounded] transformer, per 99NEC240-3(f), one could use the primary-side overcurrent protection to also protect the secondary side. Note that a delta-Y transformer will require 240V- and 480V-winding overcurrent protection and tap-rule application.

Per 99NEC450-3(b) suggest 45A 250V dual-element fuses for primary {with no secondary} overcurrent protection. 1.25∙15kVA/240V/sqrt3 = 45 amperes

{Small aside: Instead of a 150°C-rise drytype, suggest a 115 or 80°C-rise unit be obtained. Under load, expect the higher-temp unit to run very hot.}




[This message has been edited by Bjarney (edited 05-29-2003).]


#85092 - 05/31/03 08:44 PM Re: x-fmr feeds equipment  
WebSparky  Offline
Member
Joined: Feb 2003
Posts: 138
Cleveland, Ohio
Hi Steve,
Quote
But even if it is not over 10 feet 240-21(c)(2)(c) says the conductors must terminate at an enclosed switchboard, panelboard, or control device, or back of an open switch board. Is a fused disconnect a control device as used in this section?


Yes

Quote
If so, does the neutral need to go to the disconnect? To the machinery?

In the application you describe, there is only a 3-phase load of 480V, so there is no need for a neutral.

Quote
Does the secondary transformer winding need to be grounded to all available grounding electrodes on site? What size does it have to be.? (15KVA x-fmr)

Yes. The "XO" terminal at the xfmr needs to be bonded to the enclosure of the xfmr, and to the load side disconnect as a grounding conductor. The xfmr also needs to be bonded to the equipement grounding of the supply side (240V system).
The size of the bonding conductor to the load side is based on 250.66 and the equipement grounding conductor on the supply side needs to be sized according to 250.122.

If I did the math correctly, in both cases an 8AWG conductor will be adequate for both.

Hope this helps!
Dave


Dave

#85093 - 06/01/03 11:56 AM Re: x-fmr feeds equipment  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
Dave,
Quote
Yes. The "XO" terminal at the xfmr needs to be bonded to the enclosure of the xfmr, ...

The 480 volt secondary of this system is not required by the code to be a grounded system. You can leave XO floating. If you choose to have a grounded 480 volt system, then you must tie XO to a grounding electrode system.
Don


Don(resqcapt19)

#85094 - 06/01/03 01:40 PM Re: x-fmr feeds equipment  
WebSparky  Offline
Member
Joined: Feb 2003
Posts: 138
Cleveland, Ohio
Hi Don,

You are correct. Thanks!
IMO
It seems safer to me to bond the secondary for possible ground fault currents.


Dave

#85095 - 06/02/03 11:43 AM Re: x-fmr feeds equipment  
Bjarney  Offline
Moderator
Joined: Apr 2002
Posts: 2,527
West-Southern Inner-Northeast ...
No personal offense intended, but there’s a slight terminology issue here. By ANSI definition, a lower-voltage winding is designated as having “X” terminals and the higher-voltage winding is identified by “H” terminals. For the described transformer, there would be no X0 terminal—a 480V wye-point connection would be labeled H0.

Connecting the 240V “neutral” to a winding midpoint would serve no purpose. [Small aside: based on IEEE C57.12.70, the 240V delta midcoil tap would be labeled X4 or X6.]

If there are no 277V loads to be served, then a 480Y/277V winding is unnecessary, and may have a longer leadtime and higher price. In this case, the 480V system is so small that system grounding gains very little. A delta-delta transformer may be more easily obtained.

An economical approach could be http://www.jeffersonelectric.com/pdf/3-phaseencap.pdf page 7 for a 240-to-480V autotransformer configuration, using two 5kVA 1ø transformers to serve up to a 17.3kVA 480V 3ø 3-wire load.




[This message has been edited by Bjarney (edited 06-02-2003).]


#85096 - 06/02/03 09:55 PM Re: x-fmr feeds equipment  
WebSparky  Offline
Member
Joined: Feb 2003
Posts: 138
Cleveland, Ohio
Bjarney;

No offence taken, ever!
You are correct. I am too used to the step down that I forgot there is a difference!
Seems like the best solution is to go with a delta/delta.
Thanks for reminding us [Linked Image]


Dave


Member Spotlight
Posts: 362
Joined: April 2003
Show All Member Profiles 
Featured:

2017 NEC and Related
2017 NEC
Now Available!

Shout Box
Powered by UBB.threads™ PHP Forum Software 7.6.0
Page Time: 0.019s Queries: 14 (0.004s) Memory: 0.7856 MB (Peak: 0.9357 MB) Zlib enabled. Server Time: 2017-03-23 12:20:52 UTC