A recent fire in an electrical cabinet has prompted me to look at the conductor sizes for the circuit in question. The circuit has a full load demand of 23Amps, is protected with a 25Amp circuit breaker and was built with 12AWG (4 sq. mm) wire. According to NFPA, a 12AWG conductor is good for 25Amps in a panel, and only 20Amps in a raceway or cable. My problem is that when I question the machine manufacturer, an anonymous German vendor, they tell me that I am an idiot, because 12AWG wire is good for 42Amps on their side of the pond. With just half of that load on the circuit the 12AWG wire has a temperature of 100F. Can someone explain why there would be such a vast difference between their code and ours? Thanks, Cory Schaeffer

 

First, as far as I can see the in my books, this cable should have a diameter of 2.05mm and therefore an area of 3.31mm^2, not 4mm^2 as you say below. Well, however, I have seen a recommendation to have at most some 9-10A/mm^2 for short and freely laid out wires. if there is extra insulation around you should go down to about something like 3-5A/mm^2 (depending on how hard it is for the heat to get away I suppose). 42A would give 12.7A/mm^2, a little bit on the high scale I suppose but if the wire could be cooled enough I suppose it would handle this. 23A would give 6.9A/mm^2, that is it should be ok for those short and freely laid out wires, but a little high for a cable. Pure power: 3.31mm^2 means around 5.5mohm/m this give: 42A give you 9.7W/m 23A give you 2.9W/m The temperature a wire reaches does depend on this power and some more things like: - Temperature around the wire - Insulation material and thickness - Flow of air around the cable The heat generated in the cable have to go somewhere, heat go somewhere when there is a difference in temperature, this effecively means: higher difference in temperature (between cable and air surronding it) the higher power transport. Some certain difference in temperature gives the power transport needed and that is the difference in temperature you get. Cooling the air around a wire with some number of degrees will cool the wire with the same number of degrees. Air that are not moving is an quite effective temperature insulation, this means that if the air can't circulate it gets heated around the wire, and the wire will be hotter. Note that a current twice as high will give you a temperature differece about four times as high I know this don't answer your initial question, but I hope this gives you some of the answers of "why?" I would like to know what the temperature is around the wire (like some dm or so from it is). /Johan Bengtsson ---------------------------------------- P&L, Innovation in training Box 252, S-281 23 H{ssleholm SWEDEN Tel: +46 451 49 460, Fax: +46 451 89 833 E-mail: [email protected] Internet: http://www.pol.se/ ----------------------------------------

 

I can't explain the wire rating, but I ran into a similar problem with a switch (but without the fire) a few years ago. According to the specs, in Germany it was a 25 amp switch, in the US it was a 20 amp switch, in Canada it was a 16 amp switch. The clue though was to read the fine print. The 25 amp rating in Germany had to be de-rated according to the type of load. The full 25 amp capacity was only achievable under certain circumstances. It seems though that the equipment designers didn't bother to read the fine print and since they could apparently self-certify their CE approval, no-one else noticed either. In Canada, the rating was simply 16 amps across the board. To go back to your main problem, I would take a close look at the circuit breaker. If it is one of the common DIN rail or panel mount miniature circuit breakers you will find that most of these are not very good at providing overload protection. In Canada, most of these products are considered "supplimentary protectors" and have no application in overload or branch circuit protection. In other words, you can only use them provided they are not needed to protect the wiring or the load (e.g. you can use them to keep someone from blowing fuses when they plug their drills in where they don't belong). The reason for this, is that the manufacturing tolerances on most of these are so broad that there is no guarantee that they will trip on overload before your wiring burns up. A couple of years ago, we received a new machine from Germany which had several dozen of a very well known brand of miniature circuit breaker in it. It failed electrical inspection on this point. We simply replaced all the circuit breakers with 'CC' style fuses in flip-out type fuse holders which fit quite nicely in the same space. The electrical inspectors here began enforcing this rule very vigorously a number of years ago. A "real" circuit breaker has a slightly different CSA standard number than a "supplimentary protector". I don't have the information handy, but if you are interested I can get you the CSA numbers and you can inquire with the manufacturer as to which standard they meet. Considering that the wire had a 25 amp rating, but burned up even though it was protected with a 25 amp breaker, I would investigate whether the breaker is adequate to the job. If this explains your fire, then the easiest solution would be to replace it with a fuse or a "real" breaker. ********************** Michael Griffin London, Ont. Canada [email protected] **********************

 

Cory: I can't explain the NFPA vs DIN differences, but did come up with some other points. According to the Belden Wire & Cable catalog, a single 12 AWG conductor in a 25 degree C still air environment will experience approximately 35 degrees C temperature rise at 23 Amps. More factors come in with multiple conductors, conduit or raceway, moving air, etc. The single conductor scale puts the temperature around 140 degrees F so your 100 degree F temperature isn't a problem. Some insulations, however, are only rated to 60 degrees C (140 F), so running at 23 Amps in a 12 AWG conductor might be courting disaster, depending on the other factors mentioned above. Extrapolation from the Belden nomograph shows roughly another 25 degree C rise at 40 Amps in a 12 AWG conductor, which dictates using some of the better insulation types rated 90 degrees C or better. Your 100 degree F temperature still isn't a problem, but it does add confusion. How did the wiring get hot enough to cause a fire if the circuit was limited to 25 Amps? That's not enough to cook even the cheapest insulation. Did the circuit breaker fail? What else could have routed too much current through the cooked conductor(s), possibly bypassing protective circuitry? So it doesn't sound as much like a DIN vs NFPA disparity as a "what went haywire and found the design loophole" problem. Carl Ramer Protective Systems Design Space Gateway Support Kennedy Space Center, Florida Unsponsored personal posting

 

Cory, The electrons are better organized in Germany, so more of them can go through the wire at the same time. Just take a ride on the subways in the US and Germany, you'll see what I'm talking about and notice the difference immediately. It could also be because they're on the metric system - that 100 degree wire would only be at 38 degrees anywhere in Europe. Regards, Willy Smith Numatics, Inc. Costa Rica

 

So the moral of the story is... When doing calculations, don't forget to apply the temperature derating and choosing the correct wire/cable type. Derating involves multiplying the wire/cable ampacity by a number <1 appropriately chosen from the derating table so that you end up with a larger size conductor than at room temperature. The Ontario Hydro code book table says "Based on Ambient Temperature of 30=B0C*" and "*See Table 5A ... for ambient temperatures over 30=B0C". I'm quite sure that DIN and NFPA would have equivalent instructions. Anthony Kerstens P.Eng.

 

This may sound smartnick, but it is not. Europeans in general have lousy electrical systems because systems are not so well developed as they are in US/Can and do not have a long history of use. NFPA has a lot of good data on electrical fires and Europe, you should pardon the expression, is Balkanized in this respect as each country has its own bad data, including Germans. I have never dealt with a European supplier on a project who understood electricity very well and, if they got it at all, they were unfamiliar with the NEC. I had a Brit proudly say that electricity wasn't very important to him. Electrical distribution has come lately to Europe and been retrofitted into equipment as an add-on.

 

In general American and Canadian Standards are more 'pessimistic' than Europeans. Normally Euro equipment, wires, etc. are closer to the real physical limits than on this side of the pond, besides the testing conditions are different. However, I've found several cases where the conductor sizing wasn't the problem, but loose terminals or lugs. Conductor resistance is negligible compared to that in fairy poor contact areas. The generated heat is proportional to I2R and and temperature to I2t and conducted to the wire. I've seen many burnt conductors this way. Check tightness in all your connection points, specially if they are screw-type, since they sometimes get loose during transportation and in some cases by the thermal expansion due to the heat it generates.... A typical positive feedback issue.... In some other cases cable section is reduced at the conection point when stripped, creating a higher resistance spot. Maybe you can check other conductors the same size under the same current regime Tell us about your findings Alfonso Padilla QPS Control & Automation [email protected]