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12.5A through 0.75mm flex (just out of curiosity)
by LongRunner on 07/15/16 12:24 AM

I noted in my thread on power boards/strips (in the small text) that my calculations said "if 1.0mm flex can carry 16A under optimal conditions, then 0.75mm could even do about 13A in the same scenario":
  • 13A through 0.75mm flex - 10.5W/metre max. (2 load wires * 0.0267R/m at 20C * TCR 1.16 to 60C * 13A squared)
  • 16A through 1.0mm flex - 11.9W/metre max. (2 load wires * 0.02R/m at 20C * TCR 1.16 to 60C * 16A squared)
BTW, those resistance values are for tinned conductors, for bare conductors they're slightly less (0.026R and 0.0195R respectively).

Confirmed, apparently, with this arrangement (using two heaters with currents/power indicated in the overlay); I ran it for over an hour and the cords still stayed at a tolerable temperature (not too hot to touch, nor getting overly soft). Note the pillow (from my bed) in the background, which I added for 'good measure' cool. (On a side note, the IEC 60320 C13-C18 - rated for 10A, like the base version of AS/NZS 3112 - are also meant to survive testing to this current, from what I've read; and presumably the 16A C19-C24 are meant to survive 20A.)

This sure flies in the face of hyperbole from some (mainly British) sources about the 0.75mm versions of C13-C17 cords being "fire hazards", at least. (The short ones, anyway. Not that I would recommend going over 10A in normal use of these - although the UL/CSA are willing to rate the C13-C18 as high as 15A with 14AWG cordage, but I don't really trust that.) Even the connectors stayed fairly cool (although it should be noted that the 10A and 15A versions of AS/NZS3112 outlets and rewireable plugs/trailing sockets seem all-but-identical anyway apart from the earth pin width, the absence of a shroud on the majority of 15A trailing sockets, and maybe the contact sizing in the outlet switches).

The cord itself, in case you're wondering, was salvaged from an el-cheapo PC speaker set (a bit of an odd choice there, given that those are more usually Class II with a type H03VVH2-F cord; the earth wire was only connected to the transformer frame, and the cord was even squashed into a cord-grip meant for a flat cord!), and measures about 125cm 'net' so is within the '2m limit' for the 'downsized' cords. The plug is the original moulded part, the socket Deta model 6338 (which clamps the cord solidly enough - far better than the HPM versions I used in extension cords I assembled in years past). And it is ordinary-duty (H05VV-F), not light-duty (H03VV-F) (which so far as I know, isn't allowed for any normal extension cords here in Australia).

As an aside, I've noticed that a properly fitted (and in good condition) rewireable plug and trailing socket, mated together, seem to run almost supernaturally cool compared to most of the combinations of moulded-on versions that I've tested with the heaters (including their own plugs!). I presume that the moulded cordsets are manufactured with less stringent methods of terminating the wires to the pins/receptacles (although I'll admit that the common DIY pitfall of trapping the insulation in the terminals doesn't work out much better). (There's significant variance among them, too, so I'll probably be 'cherry-picking' from my stash of IEC 60320 cordsets to use with heavier loads - should I get any high-power appliance that has a C14 or C18 inlet on it in the first place...)
Last edited by LongRunner; 07/15/16 12:58 AM.

Comments
#217778 - 11/19/16 09:04 PM Re: 12.5A through 0.75mm flex (just out of curiosity) [Re: LongRunner]  
uksparx  Offline
Member
Joined: Apr 2005
Posts: 44
Egremont,Cumbria,UK
I've noticed we are getting a lot of 3kW kettles here with smaller flex now. Admittedly, that flex is very short and the boil time is very quick, so I guess they reckon it's okay. It is interesting though, to see what flex will cope with, particularly as the Brits are so obsessed with having 3amp fuses in plugs for lamp flex etc. If it's so bad having less (i.e a 16 amp mcb) protection, why aren't there more fires all around the globe where they don't have this protection? Just a thought....


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#218384 - 04/10/17 06:58 AM Re: 12.5A through 0.75mm� flex (just out of curiosity) [Re: uksparx]  
LongRunner  Offline
Member
Joined: May 2015
Posts: 44
Albany, Western Australia
Since posting this thread, I've found that at least one manufacturer (Dikkan Cable) actually does list a 13A rating for 0.75mm2 (along with 8A for 0.5mm2) on their pages for H03VV-F and H05VV-F. wink (Although their pages for the 90C flat flexes, H03V2V2H2-F and H05V2V2H2-F, still list the very conservative "British" ratings for whatever reason; for the record, their page for H03VVH2-F is obviously unfinished, with no ampacity numbers to see, while their H05VVH2-F page lists the optimal-condition ratings similar to their circular flexes.)

It also occurred to me that many kettles (and toasters, among other items) have cord storage in the base, actively encouraging operation while coiled. Fortunately, the geometry is such that only 1 or 2 layers can be wound into most kettle bases, with their supplied (short) cords. (Even a compact 1L kettle, which I bought just this last Saturday, doesn't quite make it to a 3 layer coil. It's a Breville BKE320 in case anyone's interested; and it is rated for the 2.4kW maximum in Australia/NZ, with an H05VV-F3G0.75 flex.) Of course, if you did replace the cord with a longer one then you might be able to get another layer or two in - not that modern appliances make it easy to replace cords frown (apart from the IEC 60320 types, obviously).

I conclude, then, that having a 0.75mm2 flex for 10A, or 1.0mm2 for 13A, is quite safe for 2 layers (as in typical kettle bases). And, indeed, boiling the BKE320 at full capacity (which took just under 3 minutes) only made the cord lukewarm (if that), as felt by hand (quickly unwound from the base after the boil finished). I did a second, longer test with a bigger kettle (Russell Hobbs Montana 3090, same 2.4kW rating and 0.75mm2 flex but 1.7L capacity), but the effect still wasn't much greater after 4 and a half minutes to boil. (Mind you, I had to wind its cord in reverse to make 2 layers, and then use a short extension cord to be able to connect power to the bit still accessible. grin)

I've also tested before what the effect of coiling up a cord under its rated load is; with 10A (using a 2.4kW room heater) through 1.0mm2 (standard fare for Australian extension cords), 5 layers gets warm but not excessively, and 10 layers gets hot to the touch but still far from melting down. (I'd estimate that above 10 to 15 layers is where the real danger begins, but you'd need a pretty long cord to do that easily; extension reels are a different matter, but as has been mentioned in previous threads, many modern ones include a thermal cut-out to guard against exactly this condition.)

Not that this stops Breville from saying in their manuals (albeit under the "safeguards for all electrical appliances", instead of for kettles specifically) to unwind the cords before use... crazy



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