The Electrical Contractor Network

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

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
Calling all Non-US members!! (Non-US only)
by aussie240
Today at 02:39 AM
Photo Upload Tutorial
by DanK
Yesterday at 11:35 PM
Sprinklered equipment 26-008
by bigpapa
12/02/16 04:24 PM
On Delay Relay with Auto Reset
by Potseal
12/01/16 09:59 AM
Wow, that was close!
by jraef
11/28/16 07:06 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 13
HotLine1 9
Texas_Ranger 8
sparkyinak 7
Trumpy 6
Who's Online
0 registered (), 100 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#140464 - 03/12/04 04:39 AM 7671 reference methods
james S Offline
Member

Registered: 05/13/03
Posts: 102
Loc: West England
when looking up current carrying capacities in the 7671 regulations ( IEE ) how would you decide which reference method to use if there where two used in the intallation in question eg if general twin core and earth cable was to run for 10 mtr clipped dirrect and then a further 10 mtr enclosed in a wall?between these two methods the diffrence can be as much as 15 amps

or is it educated guess time?

Top
Test Equipment:

Large Selection of Test Equipment For Electrical, HVAC, Test & Measurement
Large Selection of Test Equipment For Electrical, HVAC, Test & Measurement

#140465 - 03/12/04 07:14 AM Re: 7671 reference methods
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
James,
The ratings are based on the maximum temperature at which the cable should be allowed to run, hence the fact that the "clipped direct" rating is higher because it will lose heat to the surrounding air more quickly than an enclosed cable. (The "clipped direct" ratings are too high in the current Regs., in my not-so-humble opinion, but that's another story. )

So in the situation you describe, you would have to go with the lower current and limit the cable to its enclosed rating.

Top
#140466 - 03/12/04 08:26 AM Re: 7671 reference methods
james S Offline
Member

Registered: 05/13/03
Posts: 102
Loc: West England
just to clear it up then no matter what reference method used always go with the smallest rating method!
ie if there was again two core and earth cable to run 20 mtrs cliped direct and 1 mtr to drop down to fuse board in a insulated wall the insulated wall factor would be the factor used?

sorry to drag this one out but i think the regs fall down on scenarios such as this especialy when there is such a diffrence in carrying capacities involved!

Top
#140467 - 03/12/04 09:14 AM Re: 7671 reference methods
C-H Offline

Member

Registered: 09/17/02
Posts: 1508
Loc: Stockholm, Sweden
James,

The copper leads the heat away along the cable. This means that the current carrying capacity is higher than otherwise if the cable is enclosed or exposed to a higher than normal ambient temperature only for a short distance. I don't know if, and if so how, this is reflected in the IEE regulations, but it is likely that there are special provisions for this case. This doesn't mean you can use the clipped directly/free air value, but you may be able to use a higher value than that of a cable enclosed in an insulated wall for an indefinite length.

Top
#140468 - 03/14/04 12:18 AM Re: 7671 reference methods
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
Yes, as C-H says, where the length of cable in insulation is only very short, it is permissible to use a higher rating.

Here's an extract from the Electrician's Guide:
http://www.tlc-direct.co.uk/Book/4.3.6.htm

Top



ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals