ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
Top Posters(30 Days)
Potseal 11
Recent Posts
Permit Snafus...AHJs and Contractors Jump in
by HotLine1. 04/29/17 10:26 AM
201 volt equipment?
by bigpapa. 04/29/17 12:15 AM
Schneider LC1D09
by jraef. 04/28/17 11:05 PM
Electrode boilers question
by annemarie1. 04/27/17 01:40 PM
Why cables look like they do
by LongRunner. 04/26/17 09:36 AM
New in the Gallery:
SE cable question
Popular Topics(Views)
234,940 Are you busy
169,768 Re: Forum
162,977 Need opinion
Who's Online Now
2 registered members (Admin, Ruben Rocha), 78 guests, and 18 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate This Thread
#91389 - 01/14/05 11:39 AM when? - 800A limit on next higher overcurrent device  
greg2468  Offline
Junior Member
Joined: Jan 2005
Posts: 7
Houston
section 240.4(B) permits you to use the next higher standard rated fuse or circuit breaker when the conductor rating doesn't match the standard sizes (with certain restrictions). However, 240.4(C) limits this to 800A or less. I have heard that the 800A limit was not always there. My question is when did the 800A limit start? My oldest Code book is 1981 and it is still there in 240-3 exc. 1.


2017 / 2014 NEC & Related Books and Study Guides

#91390 - 01/14/05 12:31 PM Re: when? - 800A limit on next higher overcurrent device  
Creighton  Offline
Member
Joined: Jan 2002
Posts: 59
Hayward, CA
First appeared in the 1962 NEC in Section 240-5,Exception No.1
There was no published report on proposals at that time, and there was no comment made in the NEC Handbook, so where the proposal came from is not known.
Creighton


#91391 - 01/14/05 12:47 PM Re: when? - 800A limit on next higher overcurrent device  
greg2468  Offline
Junior Member
Joined: Jan 2005
Posts: 7
Houston
So then the restriction was added in 1962. When did the NEC start allowing you to use the next higher standard fuse or circuit breaker setting when your max conductor ampacity did not correspond to a standard size?


#91392 - 01/30/05 11:56 AM Re: when? - 800A limit on next higher overcurrent device  
Elzappr  Offline
Member
Joined: Dec 2001
Posts: 270
Oregon
The same 240.5.e1 rule also existed in the 1959 code book.


#91393 - 01/30/05 12:46 PM Re: when? - 800A limit on next higher overcurrent device  
Elzappr  Offline
Member
Joined: Dec 2001
Posts: 270
Oregon
Also, at that time, the largest "standard ampere rating for circuit breakers" was 800 Amps. Fuses had standard ratings up to 6000A and there was no restriction in the '59 code for going to the next higher standard rating...
Then, in the '62 code, there was a change that required an 800A limitation on going to the next higher standard rating, and they combined the wording so that both breakers and fuses were dealt with in the same exception.
Considering what Creighton has said about there not being any comment about the change in the '62 edition, I infer that the code change was done to clean up the disparity in language between the two types of overcurrent devices. Since it doesn't seem that the device technology (breakers vs fuses) was at issue, then I am thinking that it must have been a practical issue of putting a limit on the amount of space taken up by parallel wires. They might have noticed that switchboards weren't allowing enough wire bending space for a larger parallel feeder. In the '59 code, the ampacity of 500MCM THWN wire was 380 amps. Putting them "in multiple" (as they used to refer to parallel conductors then) would provide an allowable "current carrying capacity" (ampacity) of 760A..which would be just fine for that standard 800A overcurrent device.



Member Spotlight
Tom
Tom
Shinnston, WV USA
Posts: 1,044
Joined: January 2001
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: 0.012s Queries: 14 (0.002s) Memory: 0.7682 MB (Peak: 0.9059 MB) Zlib enabled. Server Time: 2017-04-29 15:38:43 UTC