Sizing wire for 3 phase current - Page 1
Forum section: electronics / beginners

author: made2hack
author_rank: Frequent Contributor
timestamp: June 15, 2019, 01:16:19 pm
content: Hi all,I need help trying to size solid core conductor wiring for 3 phase current.Let us assume the following:1 - A resistive load with a power factor of 12 - 10.4 kW of power3 - P/P Voltage of 380V, P/N voltage of 220V.My kVA in this case is 10.4 since pf is 1. As a result, current per phase is I = Power / (sq. root of 3 x pf x P/P V), so I = 10400 / (1.732 x 1 x 380) or approximately 15.8 Amps, say 16 Amps.Do I size my wire according to the current per 1 phase? Ie 16 Amps? Or do I size it according to all 3 phases, ie 48 Amps? I guess since not all the current flows at the same time through the Neutral, since they are 120 deg out of phase, I would be sizing for something closer to 16 Amps.Thanks,

author: soldar
author_rank: Super Contributor
timestamp: June 15, 2019, 01:31:17 pm
content: 3.5 KW per phase = 16 A per phase.In theory neutral will carry little or no current but you need to size it at least for 16A in case one phase or load fails.

author: made2hack
author_rank: Frequent Contributor
timestamp: June 15, 2019, 01:32:37 pm
content: Ok, thanks

author: bdunham7
author_rank: Super Contributor
timestamp: June 16, 2019, 12:15:07 am
content: You need to specify whether the resistive loads are across the phases or from phase-to-neutral. If across phases, no neutral is needed, balanced load or not. If from phase to neutral, then if the loads are perfectly balanced the neutral current will be zero, if not the neutral current will be no more than the largest phase current--so I'll leave it to you to guess what size wire you'd need.Be careful--if your resistive load assumption isn't true, bad things can happen. Non-PFC SMPS systems can have a neutral current that is 3X the phase current.

author: soldar
author_rank: Super Contributor
timestamp: June 16, 2019, 08:38:58 am
content: Always follow local codes and regulations.

author: Dubbie
author_rank: Unknown
timestamp: June 16, 2019, 11:30:41 am
content: I think Soldars answer is the correct one.There will be a table in your countries regulations. You just need to look up what breaker size you plan on using and there should be two wire size options specified. A free air one, and a covered one. The free air option is thinner and can run warmer. The other option requires that less heat is generated because it may be inside insulation or something, so is thicker.

author: Vtile
author_rank: Super Contributor
timestamp: June 16, 2019, 02:02:15 pm
content: No. It depends what you are after. Also note what local regulations have to say. Most countries today do use harmonized low-voltage standards baded on international work. Also do note that the application can fall under ie. machine regulation etc.Above is mentioned two tables, this is not true. There is more than two based on ie. the insulation material of the used cablesand the conduct type and if the cables are going throug some insulation materials etc. Also note that there might be rules for used insulation material outside of direct electrical ones. Mostly fire related. Etc.So take a local regulation and assosiated standard (or preferably handbook) for basic reguirements.

author: dmills
author_rank: Super Contributor
timestamp: June 16, 2019, 04:37:31 pm
content: Also note that installed wiring is a different set of regs to wiring within equipment...For installed wiring it is generally either the upstream circuit protection or the disconnect time and PSC that drives sizing, so you need to find the worst case for either. For internal chassis wiring it is generally thermal considerations.Regards, Dan.