Which is a good official AWG table to use? - Page 1
Forum section: electronics / beginners

author: eeguy
author_rank: Regular Contributor
timestamp: October 28, 2020, 08:34:07 pm
content: Hi, I came across different tables that seem to suggest different AWG valued wires for the same current value. Isn't it standardized internationally? Which is a good table to use?

author: tooki
author_rank: Super Contributor
timestamp: October 28, 2020, 08:41:17 pm
content: What you’re inquiring about isn’t an AWG table (“what size is a given AWG?”), but what’s called “ampacity”.The reason you’re seeing many values is that ampacity is a subjective thing: how much temperature rise can you tolerate in your application? That’s subject to debate. Meanwhile, the temperature rise itself (which is the only limiting factor, ultimately) is not only a function of wire cross-section (which AWG is a proxy for), but of the insulation thickness and material, ambient temperature, the available cooling, and how many wires are bundled together, just to name some important factors.So as to rule out unsafe opinions and back-of-the-envelope calculations, many disciplines and/or jurisdictions have established guidelines (or directives/laws/code) that specify what size wire to use in a particular situation.

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2020, 08:41:28 pm
content: Wire ampacity depends mainly on the permissible temperature rise i.e. some insulation is rated 90C, others 105C and then there is the heat when you have many connections/conductors in a junction box.So electrical codes such as NFPA-70 will have different ratings because they assume so many watts of heat is to be safely dissipated in the junction box and along the wire's run with other conductors present.It all depends on your project's configuration- how hot can the conductors get, what is ambient temperature, how many conductors etc.

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2020, 08:49:02 pm
content: The usual safety standards in the US are published (at a substantial price) by the NFPA in the NEC. Non-downloadable access is available:https://www.nfpa.org/NEC/About-the-NEC/Free-online-access-to-the-NEC-and-other-electrical-standardsThe current values differ for house wiring in conduit, wires bundled in cables, transformer windings, and appliance wiring. Other tables in engineering handbooks show the current for 2% voltage drop as a function of length. What is your application?

author: eeguy
author_rank: Regular Contributor
timestamp: October 28, 2020, 08:52:25 pm
content: I want to order some wires from Digikey to make circuits for maker-type projects like controlling motors, filtering signals from sensors, making amplifier, etc. To do that, I need to look up the AWG value based on the current I want the wire to be able to handle. What table suggestion do you have?

author: TimFox
author_rank: Super Contributor
timestamp: October 28, 2020, 09:17:01 pm
content: For safety, look at the appliance ratings in the NEC. Are you running more than, say, 2 amperes?The usual safety ratings are concerned with mains voltages.For low-voltage applications, you should probably concentrate on the resistance of the wire, which is tabulated everywhere, and is not a regulation question.

author: floobydust
author_rank: Super Contributor
timestamp: October 28, 2020, 09:40:48 pm
content: I think theWikipedia AWG tablesare fine for hobby stuff as well as official electricalExample is #22AWG rated for ampacity 3A 60°C or 7A 75°C, but I would use the 3A value.If your wire runs are long then you have to consider losses and possibly upsize the conductors.It's also the whole picture - the connectors, pcb traces etc. that also have to be suitable for the currents expected including a fault or overload.I've seen many robot projects bump into something and melt an element that acted as the fuse by accident. Fuses typically need 2-10X rated current to blow in a reasonable time, and you don't want your wires smoking until that happens. So #22AWG on a say 3A motor that jammed and is now using 30A, the wire can handle that for a very short time and the fuse better clear fast.

author: wizard69
author_rank: Super Contributor
timestamp: October 29, 2020, 08:42:23 am
content: Quote from: eeguy on October 28, 2020, 08:34:07 pmSince you are in the USA, the NFPA is the source of the NEC (National Electrical Code), which can be law. The ampacity of wire is based on a number of factors including temperature rise that the insulation is capable of handling and the packing density in a conduit. Here is a quick review of some of the considerations:https://www.ecmweb.com/national-electrical-code/article/20888271/top-50-nec-rules.But there is a catch, for some gages (14,12 & 10ga) the ampacity is defined by the code, in that there are max values for overload devices.. So if you are working in any area were the NEC applies you really need to know and understand the regulations.Now you will find all sorts of tables for wire ampacity for areas where the NEC does not apply. The NEC however covers just about any building or dwelling in the USA so if you are asking this question I'm almost certain you need to know the code. Frankly anything beyond trivial branch circuits requires a bit of engineering to determine proper operating conditions of the installation.Beyond all of that you have the UL to consider. If you need to get UL approval for whatever, you then have to consider all UL related regulations / standards. Most locations require a UL stamp on just about everything in an installation.Oh one more catch. The NEC is updated every three years. However not all jurisdictions adopt the latest revision of the NEC. While this generally is not a huge problem, it pays to understand what the the local inspector will be using as a reference. The NEC has been adopted just about everywhere in the USA so is more or less law, it is just that the version can vary.