[Electrical wiring question] AC wires sizing for current rating and distance. - Page 1
Forum section: electronics / beginners

author: aramosfet
author_rank: Contributor
timestamp: May 25, 2014, 11:59:17 am
content: Hi All,I have a basic question on Electrical wiring. I've been setting up a small electronics lab at my home. Unfortunately, there aren't any electrical outlets at the place. So I set out to do the electrical wiring myself. My requirements were fairly simple. Enough outlets for a desktop PC, Laptop, an oscilloscope, a power supply, Few outlets for soldering station, lighting etc. I calculated that the maximum power drawn shouldn't exceed ~1.2KW (230V 50Hz here).While doing the wiring, i ended up buying a 70m spool of 1Sqmm multicore (~19 strand ??) local brand wiring. I didn't see any safety certifications or labels. I completed the wiring and connected all the outlets. At the end I checked for shorts between line/neutral and ground, everything was fine. Then for some reason I decided to check the resistance. I was surprised to see 2ohms for the roundtrip length of 20M on Line and neutral. The total length of the wiring is ~10M. That would mean, if i'm pulling the estimated 1.2KW through the wires, I'd be dissipating a 54W in the wires alone!!!This seems far too much. Not to mention the fire hazard.I compared the resistivity datasheets for well known branded cables and they specify ~20 ohms/km for 1sqmm multistrand cables. Compared to that, the ones I've got have about 5 times resistance for the same length.The question I have now is, Should I rip out all the wiring and redo with branded cables. Are there any other options? How should I size the wires for the application I have.Also, why do all the sizing suggestions for cables only talk about current capacity alone? Shouldn't the length of the cables also be considered?Thanks,

author: SeanB
author_rank: Super Contributor
timestamp: May 25, 2014, 12:08:31 pm
content: It might be a mislabelled roll of 0.75mm wire or even a copper coated steel wire ( check with a magnet) or even thinner wire sold falsely. 1.2kW is around 5A, so a 10v drop is within the limits of a mains cable, as you typically can have a max drop of 10% from nominal mains voltage. Yes the cable will get warm, but if you want a cooler cable you have to have more copper, like 1.5 or 2.5mm cable. Preferably you should have a breaker of around 5A to protect the cable, or do it properly and use a 2.5mm cable so as to have the same rating as the wiring in the wall.

author: Rerouter
author_rank: Super Contributor
timestamp: May 25, 2014, 12:12:08 pm
content: I agree 2.5mm cable is really recommended for a dual outlet (on 230V mains) as who knows one day you could want to vacuum the lab and poof, fire starts off in the wall, (2.5mm is rated for 16A according to my countries regulations)also another point is that the earth return (if your country uses one) is recommended here to be under 2 ohm to ensure your circuit breakers will go off fast and reliably,

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 25, 2014, 12:24:48 pm
content: Also how good is your multimeter? you can short the leads and subtract the amount shown from your 2 ohm figure.How did you get 54watts? 5A x 2 ohms is 10 watts.oopsAnyway (in order)first you decide what your load is.work out max load current.choose a fuse or circuit breaker to let this amount of current through.then choose a cable that will carry more current than the circuit breaker.the current carrying capacity is dependent on the conditions of the cable run as well as the CSA, and also maximum ambient temperature, local rules and temperature rating of the cable.but 1mm csa is only good for approximately 5amps.You need to check the local rules and conditions.Current carrying capacity is basically trying to measure the amount of current that it can carry before overheating.Voltage drop is another separate limitation on cabling systems.In Australia we cant use less the 2.5mm csa for power outlets.

author: aramosfet
author_rank: Contributor
timestamp: May 25, 2014, 12:44:51 pm
content: Quote from: HackedFridgeMagnet on May 25, 2014, 12:24:48 pmI have taken the relative readings from the multimeter, So the probe resistance is not included.P=I^2 * R isn't it?@SeanB , I checked the cable again and it's not copper coated steel cable. Should most likely be 0.75mm. no easy way to check that?I should probably change them to atleast 1.5sqmm cables.Thanks,

author: SeanB
author_rank: Super Contributor
timestamp: May 25, 2014, 01:05:27 pm
content: Most likely a 0.75 cable with a false rating. You can run 2 cables and have the outlets shared on them, so that you have half the load on one and half on the other if you want, or with lights on one and all on the rest. That way you can switch off all except lights fast.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 25, 2014, 01:19:43 pm
content: Yeah lol brain fade.

author: Simon
author_rank: Unknown
timestamp: May 25, 2014, 02:26:43 pm
content: Take the cable back out and take it back to the shop and wrap it round his/her neck for selling you such garbage.I agree that you want 1.5-2.5sqmm, if you are doing it do it properly as you WILL run out of capacity sooner rather than later