Abbreviation for Low Voltage AC Power Rails - Page 1
Forum section: electronics / projects

author: calzap
author_rank: Frequent Contributor
timestamp: April 21, 2014, 03:53:23 pm
content: For DC, we have + and -. For AC mains single phase, we have hot (or phase) and neutral (or grounded conductor). For AC mains 3-phase, we have L1, L2, L3 and neutral. But there seems to be nothing generally agreed upon for low voltage AC. If there are official or semi-official abbreviations, I would like to know what they are. The leads coming out of a transformer or inverter are essentially the same to everything in the universe except each other. Exceptions are autotransformers and cases where one of the leads is grounded to the chassis or earth.It's not much of a problem for schematics or ladder diagrams; the low voltage AC rails are usually easy to see and if they are given any designation, it's usually just numbers or letters, like other components, or simply the volts as a number followed by AC or a squiggle. The problem comes when verbal descriptions need to be given of the circuit or wires need to be labeled in the physical product. Generally, it's done be referring back to the arbitrary designations on a schematic or ladder diagram.So what would be a good choice? I'm already using LA1 and LA2, but am open to suggestions. It's useful, for example, to make a list of what's connected to LA1 and LA2 as well as other verbal descriptions. If one of the rails is connected to ground or is the secondary lead from an autotransformer that's connected directly to its primary, it's automatically LA1. If I ever had to deal with low voltage 3-phase (hopefully never), the third leg would be LA3.It should be a combo of regular letters and digits. Other symbols, like the squiggle, are harder to use in a word processor or spreadsheet and may not be available in some label-makers. Symbols found on the top row of most keyboards would probably be OK, but some label-makers lack some of these.Mike in California

author: Zero999
author_rank: Super Contributor
timestamp: April 21, 2014, 05:21:51 pm
content: Technically mains and most three phase is low voltage AC.If you're talking about extra low voltage, i.e. 24VAC, I just use the same as with mains but will put the voltage on to avoid confusion, if it's in the same box as mains circuits.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 21, 2014, 06:48:24 pm
content: In HVAC, the usual convention is R for 24V "hot" and C for 24V "neutral".

author: calzap
author_rank: Frequent Contributor
timestamp: April 21, 2014, 07:14:16 pm
content: Interesting. Where did R and C come from? Is one grounded and called neutral for that reason, or is it just arbitrary? I do a lot of 24 VAC work, but it's for water pump control circuits and solenoid valves.Mike in California

author: Clear as mud
author_rank: Regular Contributor
timestamp: April 21, 2014, 08:55:48 pm
content: R is for Red, because it conventionally attaches to a red wire if one is used. 4-wire thermostats don't use it. C is for common. The conventional 4-wire thermostat has the common wire connected, and three other wires switched to it, to activate heat, cool, and blower only. There are standard colors for all of them, but I don't remember what they all are. Originally, thermostats were mechanical, with no electronics in them, so there was no need to run an unswitched power wire (the red one).Neither wire is grounded, generally. I think NiHaoMike was referring to it as neutral just because it's 'common.' It serves as a reference voltage.In my schematics, I've sometimes used LV1 and LV2 to indicate the secondary of a transformer converting 120 volts to a lower voltage. But it would be nice if there were a standard.