Understanding these wire temperature and current ratings? - Page 1
Forum section: electronics / beginners

author: incf
author_rank: Frequent Contributor
timestamp: January 30, 2025, 06:37:13 pm
content: Attached is a PDF spec sheet for a popular wire product.It is '16AWG 2 Conductor SPT-2 Lamp Wire rated for 105C and 13A'• CSA Spec. C22.2, No. 49; CSA LL224650• UL Spec. No. 62; UL E46194Two questions:Can this cable carry 13 amps when the external air temperature is 105C? (If not, what does 105C specify the temperature of? the insulation? the conductor?)Can this cable carry 13 amps on both conductors simultaneously at some max rated temperature? (Air/insulation/conductor temperature of 105C?)I'm fairly sure all of the relevant standards are behind paywalls.edit: I searched through all ~500 pages of an old copy of UL62 and UL 1581 several times for words like "temperature", "amperes", "rise", "current", etc. and am none the wiser.edit2: NEC has some ampacity tables "90C" cables at different ambient temperatures and numbers of conductors, but none for 105C.This calculatorseems to suggest that a 90C cable can conduct 50% of its rated current at an ambient temperature of 70C.

author: benj38
author_rank: Frequent Contributor
timestamp: January 30, 2025, 09:27:25 pm
content: I have no access to these standards either, but I can tell you the following based on similar information sources (e.g., the national electric code):1. No. It can not carry 13 Amp at 105C ambient. In fact, it can probably carry basically no current at this ambient temperature (as the current will heat the wire further) since this is the maximum temperature the insulation is supposed to ever be at during continuous operation.2. The way I would interpret the spec is that it can carry 13A in both conductors at ambient temperature of 30C while being enclosed (like in a lamp post). In this case its insulation temperature may rise due to self heating up to 105C.For plastic insulated wires the maximum allowed temperature is determined by the maximum continuous temperature the insulation can tolerate, and in this case it is 105C. Obviously, the maximum current that would push the wire to this temperature depends on the ambient temperature and how the wire is cooled. As far as I know, the national electric code usually assumes an ambient temperature of 30C, and a conservative assumption that the wire is not cooled well due to being enclosed in some conduit or a piece of equipment.My interpretation above of 13A in both conductors at 30C ambient is in good alignment with the national electric code which specifies about 18A at a maximum temperature rise to 90C for a single 16AWG wire. While your insulation can tolerate 105C, the rating is only 13A for both conductors at the same time since the insulation between the wires is heated from both sides, while having only an indirect cooling route to the ambient air.

author: incf
author_rank: Frequent Contributor
timestamp: January 30, 2025, 09:40:01 pm
content: Is there a formula to translate a NEC "temperature correction factor" up to different temperature? I suppose it might be possible to use physics to predict the relationship between temperature rise, current, and temperature rating using data from the table below.In one forum thread (which I cannot find now) I recall reading that the square root of the ratio of two different ratings (I do not know which) was used to adjust the ampacity for temperatures not listed in their tables.Edit: empirically it looks like tables values are calculated with sqrt( new_delta_t / original_delta_t_at_30C ). For a 90C wire, original_delta_t_at_30C = 90C - 30C = 60C. To calculate the correction factor for an ambient temperature of 75C the value new_delta_t = 90C - 75C = 15C and that gives sqrt( 15C / 60C ) = 0.5 which matches the table.To answer my own question, at 70C with 105C rated wire original_delta_t_at_30C = 105C - 30C = 75C and new_delta_t = 105C - 70C = 35C. This give the ampacity correction factor sqrt( 35C / 75C ) = 0.683. I suppose this means that my "13 amp and 105C rated" cable can conduct 8.88 amps at an ambient temperature of 70C.

author: benj38
author_rank: Frequent Contributor
timestamp: January 30, 2025, 10:02:13 pm
content: Sure you can do the physics/math, but I would argue that this is not the best way to go. All these standards and specs make broad and crude assumptions about the heat dissipation from the wire, which depends on the exact installation, and are thus necessarily over-conservative, and bare only a lose connection with the reality of any specific application.Why don't you just measure the temperature of the wire in your exact installation environment? A tiny thermocouple will do nicely.

author: incf
author_rank: Frequent Contributor
timestamp: January 30, 2025, 10:07:47 pm
content: Quote from: benj38 on January 30, 2025, 10:02:13 pmIs there a guide to this?I feel like accurately making that measurement of the temperature inside the insulation would be hard.I feel like one would have to slice the insulation lengthwise, attempt to wedge the sensor under the insulation without damaging it, and then tape up the damaged insulation in a way that wouldn't significantly affect the measurement. For 16 and 18AWG wires and smaller that would seem hard.Slicing the insulation will affect the measurement, as will taping it back up.

author: IanB
author_rank: Super Contributor
timestamp: January 30, 2025, 10:19:43 pm
content: Quote from: benj38 on January 30, 2025, 10:02:13 pmAnother point is that such wire specifications should be considered absolute maximum conditions. You would be wise not to design any system that approaches these values. They are supposed to give you a safety margin for security. For example, maximum current in a lamp wire about 5 amps. Maximum operating temperature about 50°C (that is, sensible values, well below the limits).Quote from: incf on January 30, 2025, 10:07:47 pmSee my previous comment. The specifications are not targets to aim for, they are values to stay below. So you just tape a thermocouple probe to the outside of the cable and wrap with a couple of turns of tape. If the temperature even get close to 100°C, it is running too hot.

author: incf
author_rank: Frequent Contributor
timestamp: January 30, 2025, 10:44:13 pm
content: All of this is for worse-case calculations. I don't have the luxury of hand-waving away high ambient temperatures in my application. I have to figure out exactly "what it can take" so I can be confident that I am not exceeding it.