ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Shout Box
Recent Posts
Safest socket system for wet pottery studio?
by Jules69. 02/21/18 05:12 PM
diazed fuses
by Texas_Ranger. 02/21/18 10:22 AM
110.26
by gfretwell. 02/18/18 12:14 AM
New in the Gallery:
Housebilding DIY wiring
SE cable question
Who's Online Now
0 registered members (), 22 guests, and 10 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
NEC 450.4(A) & Autotransformer overcurrent protection #98810
06/08/06 08:43 AM
06/08/06 08:43 AM
S
SteveFehr  Offline OP
Member
Joined: Mar 2005
Posts: 1,213
Chesapeake, VA
NEC 2005, Article 450.4(A) has a confusing clause regarding autotransformer overcurrent protection:

An overcurrent device shall not be installed in series with the shunt winding (the winding common to both the input and the output circuits) of the autotransformer between Points A and B [Output taps] as shown in Figure 450.4.

What, exactly, do they mean by this? It sounds almost as though they're probihiting overcurrent protection on the output, but that makes no sense and this is directly contradicted by the table referenced earlier in the paragraph. Do people actually physically open up these transformers to break the windings so they can put a breaker in the middle, is that what they're prohibiting?

2017 / 2014 NEC & Related Books and Study Guides
Re: NEC 450.4(A) & Autotransformer overcurrent protection #98811
06/08/06 10:18 PM
06/08/06 10:18 PM
W
winnie  Offline
Member
Joined: Sep 2003
Posts: 649
boston, ma
Yup, they are prohibiting an overcurrent device in the middle. If such an overcurrent device were to operate, then the output voltage would rise to the input voltage.

Since the current rating of the shunt winding is less than that of the transformer and the series winding, I can see why someone might want to put an overcurrent device in that location. This would be a case of a safety improvement in a component of a system reducing the over-all safety of the system.

-Jon

Re: NEC 450.4(A) & Autotransformer overcurrent protection #98812
06/09/06 12:54 PM
06/09/06 12:54 PM
L
Larry Fine  Offline
Member
Joined: Feb 2005
Posts: 693
Richmond, VA
Steve, I doubt they're concerned with someone opening the transformer to insert a breaker. Referring to the diagram again, I believe they don't want a breaker between the bottom of the winding and point B; t should be in the line to the left of point B.


Larry Fine
Fine Electric Co.
fineelectricco.com

Featured:

2017 Master Electrician Exam Preparation Combos
2017 NEC Electrician
Exam Prep Combos:
Master / Journeyman

 

Member Spotlight
akmaster
akmaster
alaska
Posts: 70
Joined: June 2012
Show All Member Profiles 
Top Posters(30 Days)
Admin 8
Popular Topics(Views)
244,048 Are you busy
181,039 Re: Forum
171,281 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.6.1
(Release build 20180101)
Page Time: 0.027s Queries: 14 (0.015s) Memory: 0.9514 MB (Peak: 1.0743 MB) Zlib enabled. Server Time: 2018-02-22 02:47:04 UTC