When does wire guage derating come into play? - Page 1
Forum section: electronics / beginners

author: drakejest
author_rank: Regular Contributor
timestamp: September 25, 2023, 03:05:51 pm
content: Hello !At what length do you start increasing you wire length due to distance?? This question might differ my country-country but im interested in the USA code, none the less i would still like to know how it is done on your country.So assuming i have a 100meter run on a 3phase 3 wire 380V 200kW system. How thick the wire would be for it to be up to code (lets say for a THHN wire)?Question 1: The 200kW system would be outputting 302.36 Amps, Do you gauge each of the 3 wires for that current? or you split it up?Question 2: If it were a 4 wire system (with neutral), should all 4 wires be the same gauge, and should each wire be rated for the full 300A of current?Question 3: Based on the answer on questions 1 and 2. I have read that you derating depending on the voltage drop , what is the allowable voltage drop for the system? (5% , 3%, or 1%, or something else).

author: themadhippy
author_rank: Super Contributor
timestamp: September 25, 2023, 03:18:14 pm
content: 1) you base it on the current that conductor is carrying2) yes,no,maybe.Theirs sevral factors that come into play,but generally as a rule of thumb it should be the same size as the phase conductor3)here in the uk its 3% for lighting circuits and 5% for anything else

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2023, 04:33:51 pm
content: The percentage voltage-drop limits in note 3) above are regulatory, and I believe are for the specific cases mentioned.If not subject to regulation, the physical limits have to do with temperature rise in the conductor (especially when compared to the insulation service temperature).I worked on one project in the UK, and there was a question about where the "boundary" lied between the site power and the "appliance", where the supply current flowed through a long flexible cable through cable-handling fixtures.If I remember correctly, the UK contractor insisted on the 5% derating value for the entire cable run from site to moving load.

author: drakejest
author_rank: Regular Contributor
timestamp: September 25, 2023, 05:17:03 pm
content: Quote from: themadhippy on September 25, 2023, 03:18:14 pm3 phase is a bit tricky for me to grasp, because ideally all 3 wires share the same load right so at 300A each would be carrying about 100A is that correct? But i do think there are scenarios that it isnt balanced so how does code take that into consideration? at what current should i rate the wire. Online calculators say at 100meters 300A 380V with 3% drop say the wire should be 3/0 AWG. this wire is rated 225A in the Codebook table. something weird though is if i reduce the length to 1 meter the guage drops to 18 AWG .. isnt that weird... can a wire that thin really handle that much amperage even at 1 meter?

author: Swake
author_rank: Frequent Contributor
timestamp: September 25, 2023, 05:42:26 pm
content: if you tell me it's 3 phase 100A, then I assume each phase can carry 100A. and not 33A or whatever else.

author: Swake
author_rank: Frequent Contributor
timestamp: September 25, 2023, 05:44:58 pm
content: QuoteNo, way of speaking the amps come in from one wire and the same amount of amps has to go back on one or more other wires.

author: TimFox
author_rank: Super Contributor
timestamp: September 25, 2023, 05:52:02 pm
content: If you place three ammeters in series with the three phases, and three voltmeters connected between each pair of phases, you will see:300 A flowing in each wire (L1, L2, and L3).380 V across each pair of wires (L1 to L2, L2 to L3, and L3 to L1).Including the phase relationships, this delivers 200 kW of power to the (balanced) load.My somewhat klunky mnemonic is to convert the 3-phase delta to wye, where each wire has 380 x (3-1/2) V = 220 V to neutral, and connect three light bulbs from each phase to neutral, then calculate the total power in the three bulbs.

author: bdunham7
author_rank: Super Contributor
timestamp: September 25, 2023, 07:01:05 pm
content: Quote from: drakejest on September 25, 2023, 03:05:51 pmYou wouldn't discuss a 200kW system, for electrical wiring you mean kVA. Also, circuits are typically in even, round current values, not kW or kVA because the wiring doesn't care if it is at 380V or 400V--it's the current that matters when sizing it. A 3-phase circuit with 300A on each leg and 380V phase-to-phase would be 300 x 380 x 1.732 (square root of 3) or about 197kVA.The size of the neutral depends on the situation, but the most common situations require it to be the same gauge as the phases. In rare instances it needs to be a lot larger.The allowable voltage drop could vary quite a bit and it depends on whether you are talking about regulatory or practical considerations. For regulatory concerns, consult your local AHJ. For a very long run, you could have a huge voltage drop and as long as the ampacity of the wire wasn't exceeded and the voltage is still sufficient for the load, there's no issue other than wasting a bit of power. For intermittent uses where you don't care too much about power loss, you can add a boost transformer on one end if your load would be undervoltage. In any case, if you need the voltage drop to be under a certain amount for long runs, you have to upsize.One additional issue is that ampacities are given with a temperature rating. All components connected to the wires must be rated for that temperature to use the full rating. THHN is rated at 90C, but many circuit breakers may be only rated for 60 or 75C. In that case, you have to assign a lower ampacity to the THHN if you use it. Also, if multiple wires are placed in conduit, they may need to be derated. From this chart you can see that you would need 300MCM (kcmil) wire if you can tolerate 90C, otherwise 350MCM. That would apply no matter how short the branch is.https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/Using this handy online calculator, you can enter your values and see that even with a 100-meter run (330 feet) with the minimum 300MCM cable in steel conduit, 380V @ 300A, your voltage drop is only 3.1%. Voltage drops are absolute, only resistance and current matter. The higher the voltage, the more (absolute) voltage drop is permissible. A 208V 3-PH 300A circuit with the same wires would be 5.7%, which might be unacceptable.http://www.mcgowanelectric.com/etools/voltage-drop-calculator

author: drakejest
author_rank: Regular Contributor
timestamp: September 26, 2023, 10:04:15 am
content: Quote from: bdunham7 on September 25, 2023, 07:01:05 pmOkay so let me try to test if i understand it correctly, Ill be using a different setup, In this scenario i need to determine what gauge of wire is needed for a 120kVa 380V 3 phase system with a run of 80meters THHN open air ( conductor in a conduit is not greater than 3)Determine the amerage needed by the system which in this case x * 380 * 1.732 = 120kVA, which is aproximatly 182AUse thecalculator(material:copper, wire:THHN, temp 87-95F, conductors: 1-3) to determine what guage wire is suitable, the calculator resulted in 3/0 AWG. each of the wire of the 3phase should be that size? or the total equivalent guage of the 3 wires should be 3/0 AWG?.check if the tolerable voltage drop is within specs if not increase the size using thiscalculator(temp set to 50C,conduit type: no conduit, length: 80m or 263ft, load 182A) resulted in 1.8 Voltage drop if the local regulation is 3% then 3/0AWG is the correct conductorIf 3/0AWG is indeed the correct conductor, why does using thiscalculator(Voltage: 380 3phase AC, Vdrop: 3%, material: copper, curent: 182, wire length: 80m, temp:50C) will yield me 1AWG wich is veery far from the 3/0 AWG.

author: Zero999
author_rank: Super Contributor
timestamp: September 26, 2023, 10:23:28 am
content: Quote from: bdunham7 on September 25, 2023, 07:01:05 pmA smaller neutral is also quite common. Machines often have a three phase motor, which draws the lion's share of the current and a small single phase control circuit, which draws a negligible current and is connected between phase and neutral.