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kcor

current load

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have a stupid question. if you have a three phase 220 volt circuit, and you have 10 heaters that are 3600 watts each, what would the total current load be to use the size of the main power cable. I understand the p=v*I, so the current would equal the power divided the voltage times 1.73 or I = 3600 / 220 * 1.73 = 9.45 amps. Where i am confused is since there are 10 heaters, would you size the wire and protection circuit for 10* 9.45. thanks

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Are you saying that you have 3 phase heaters? Since most heaters are a resistive device, they are usually connected line to neutral or between 2 phases. Is the 3600 watt rating for 220 V? Is your 220 Volts a phase to phase voltage or the phase to neutral voltage (assuming that you have a Y system and not a delta system). If you are using phase voltage, you will have a slightly unbalanced system since you have 10 of these. If you could give a little more information, I might be able to help.

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They are three phase heaters, 220v between each phase. Yes it is 3600 watts for each heater on a Y system, each leg of the heater is tied to one of the phases. I have not actually seen the machine, it is in honduraus, but will be going next week to try and get it running. It was built in the early 70's. The best that I know from the info that i have is that there is a high leg according to the electrican that I have spoke to that can speak some english. I was wrong about there being 10 heaters, there are actually 16 heaters. From the picture that I saw the heaters are assembly on a press type unit, with 8 heaters on top, and 8 on bottom. For example the 8 heaters on top are controlled by 4 very old watlow type power modules, and the 8 heaters on the bottome are controlled by 4 power modules, the output of a power module goes to one of the heater elements, then branching over to the other heater element. So each power module is controlling two heaters. The electrican in honduraus says that since each heater is pulling approx 10 amps, then you multiply the 10 * 16, and come up with a total load of 160 amps. Which is what I also thought was true, but I spoke to a electrican that is doing some work at our plant, and he told me that since there are two heaters (three phase) that are controlled by each power controller, and is you take the wattage of one of the heaters, that is 3600 and use the formula I=3600/ 220*1.73, which is equal to approx 10 amps, that you would then take that value and multiply by a factor ( which I do not remember) and the toal current load for the two three phase heaters being controlled by a power module is somewhere around 12 amps, not 20 amps that you do not add the current for the heaters when they are on a three phase circuit. If this is true, since there are 8 of these power controllers controlling 2 heaters each, then you would multiply 8 * 12, which is 96 amps, so the total current load is 96 amps not 160 amps. The reason why I am so confused, is that the incoming power connections for the machine is fed into a breaker panel that is mounted on the machine that is rated for 200 amps, but the folks in Mexico ( where the machine was shipped from), said that the machine was running on a 100 amp service, which would make sense, if the electrican that told me that you would not add the currents together for the heaters since it is a three phase Y circuit. Thanks

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The only way that would make sense is if the heaters are hooked in series instead of parallel. If they were in series, then the heaters would actually be running on 1/2 of the 220 Volts and the heater output would be approximately 1/4 of the rated output (3600/4=800 watts). The wattage varies as to the square of the voltage. This would give a total system running of about 40 amps. Since I'm not sure about this, I hesitate to say anything for sure. If you want to make sure that you don't have anything wrong, then the safest bet is to size the main power cable for the 160 amps, you won't be undersized in any situation. If there is any kind of demand factor, you can add that in (not sure if all the heater come on at the same time). I think that you are on the right track on your calculations. You won't know what you have until you see the machine. If you feel like posting what you find, I at least would be curious on how it is wired and what the current draw is. brucechase

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