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Dear experts over here, I am new to electricity. I have some doubts about the 'balancing' of 3-phase voltage. Assuming there is 3-phase 480V, name as R, S, and T. Now... assume that I have 3 components(3A, 4A, 5A) that is using 240V. Then, I get the 240V voltage supply to these 3 components by tapping JUST from R phase and then to Neutral point. My question is, 1. how much voltage or current, or power or percentage can be tapping this way without affecting the 'balance' of the 3-phase supply? 2. if I just have 1 component with 240V (3A) and just tap from R-phase, then how can I balance back the 3-phase? Well, very "sorry" for disturbing you all over here about this kind of minor problem. Hope you all can explain to me or can just simply show me some websites and just let me browse through myself rather than spending time talking to the 'new' guy. Thank you... :) regards,

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You affect the balance as soon as you tap any non-3-phase loads off a 3-phase supply. In other words, you can take 0% without having any affect. To balance it back, you need to take the same additional amount from the other phases. So, if you have 3 of these loads & they are all the same size, put 1 on each of the 3 phases. Don't put them all on 1 phase. If they are different size loads, then distribute them appropriately. It's not automatically a bad thing to not be balanced. For instance a transformer doesn't care if it's balanced or not. You can draw the maximum phase current from each phase without concern for what the other phases are doing. On the other hand, if you want to use the entire capacity of a 3-phase transformer or run it at it's coolest temperature for longer life, you'll have to use each phase equally. The thing to remember is that the more you draw on a phase, the lower that phase's voltage will be. If you have uneven currents on your phases, you will have uneven voltages. How much current you can draw for a particular voltage drop depends on your system. Depending on your loads, the amount of drop you encounter may or may not matter. Heaters will still work but the heat output drops fast (according to the square of the drop, ie if you divide the voltage by 2 then you just divided the heat by 4). More of an issue are 3-phase motors which are easily affected by unbalanced voltages. It becomes an issue if you have heavy single-phase loads that unbalance things on the same system as 3-phase motors. Motors run hotter than normal on unbalanced lines. A motor can handle a small imbalance and most systems are far from perfect. You calculate it as follows: Measure the voltage on all 3 phases (either phase-to-phase or phase-to-ground). Find the average value (ie take the sum of the 3 values & then divide by 3). Now determine which of the original 3 values is furthest away from the average and divide that one by the average then multilply by 100. This will give you what is known as the "percent imbalance". Ideally, it should be less than 1%. Above 1%, you should derate the motor or fix up the imbalance. At much over 3%, you should shut the motor down before you destroy it.

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grabbing one leg of three phase 480V, will give you 277 volts, not 240. With 3 phase power any one leg is the voltage (480) divided by the square root of 3(1.73) Ken Edited by Ken Moore

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Phase balancing is not an issue if there is no big difference or if not motors are effected by it. You take your DVM and if the difference is little, don't even mention it. My 2 cents.

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hi, Very much sorry for 'missing out' the square root of 3 or 1.73... . Instead of 277V and I get 240V... :) Anyway, thanks for spell out the error. Can I confirm one more thing? That is, is it means that if the 'percent imbalance' after been calculating out is less than 1% then it will not be affecting the system as mentioned by mr. JimRowell? or is it that 1% tolerance is only when there is motor in use? I have one more doubt. Eg, I have 3 components with different loads and in such a way that no matter how I tap from the R, S, T phase and I still get the 'percent imbalance' that is more than 1%. Then, can someone help me out that how to 'balance' it back? That's all for now first. Hope the expertise in this forum can help me out. If someone think that I am too poor in the electrical field, then can someone show me some websites that can be use for my own self-studying? Thanks... :) regards,

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Your question is a good one and your electrical knowledge is OK. Once you read the books on electricity you end up with these type of questions. Now for the reality. It is best practice to allways balance the loads on any multiple phase circuits. Why? Since this balancing of the loads implies that these loads are working on a single phase and if the voltage level is great enough nobody really cares then again why even needing a good balance? BUT ...Next week imagine that you connect a 3 phase motor to this circuit. The operation of your motor could be jeopardized by this imbalance. A 5% drop in voltage could lower the tork by 25%. ...The transformer will have a temperature raise due to the imbalance. Best practices are not always VERY IMPORTANT they give you some limits to prevent a bigger problem in the futur. IF you have a set of lights on one phase and heaters on the next and the light go on and the heater gives out enough heat then you don't have a real problem. To balance the phases there is no real magic. Parse your load a bit more or get a tranformer with taps enabling you to boost up the phases one by one. If you connect a load with specs saying that you need to supply from 100 to 125 volts to it and on this phase you have 110 compare to the other 2 with 117 then you don't have a problem. Just remember that the next load will be used to redress the imbalance.

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Hi, Thanks a lot for your valuable advices... Thank you. I got one more doubt that is bordering my mind. That is, eg, if we just 'plan' of to tap this load to that phase and tap that load to this phase, then without say the real connection and we can't measure the reading to compare phase by phase differences then how we know whether the way we 'segregate' the loads to difference phases is actually balancing or is acceptable or not? Is it some way or something to be calculate to know that the way of distributing the loads to these phases 'will not' cause any imbalancing or the imbalancing is negligible or not? Is just like we can tell directly that shorting 'life' voltage to 'neutral' is not allowed and will cause breaker tripping. Then, is any calculation method or what that can tell us the distributing way will cause the 3-phase becomes imbalancing without have to connect them and measure and then concluding out from there? I knew I ask a lot but I really do not know. Hope you all can bear with me... Sorry... (or can get me some online helps and let me go through first before asking some questions over here) Regards,

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You read the nameplate on the load!

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