gjs2.power4
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Are the actual ammeter reading per phase in a three phase system is the three phase current? or it will be multiplied by square root of three to get the three phase current in computing size of interrupting device.
Three phase current
- Thread starter gjs2.power4
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This has been the subject of some debate here. Not the math, just the manner of describing current.
My view is that the notion of "per phase current" is meaninless. A notion with even less meaning is that of "total current." If you put an ammeter on one phase of a three phase system and you measure the current, and (presuming the system is balanced) you get the same reading on the other two phases, then the system's current is that number. You can call that number the "line current" if you wish, or even the "phase current." But to say that value is the amount of "current per phase" is (in my view) nonsense, and to multiply it by three to get the "total current" is flat out wrong.
Welcome to the forum.
My view is that the notion of "per phase current" is meaninless. A notion with even less meaning is that of "total current." If you put an ammeter on one phase of a three phase system and you measure the current, and (presuming the system is balanced) you get the same reading on the other two phases, then the system's current is that number. You can call that number the "line current" if you wish, or even the "phase current." But to say that value is the amount of "current per phase" is (in my view) nonsense, and to multiply it by three to get the "total current" is flat out wrong.
Welcome to the forum.
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Line (or phase) current is the current that flows through a "phase conductor", and is what you measure with a clamp-on ammeter. When sizing overcurrent protective devices this is the only value that is of concern. No multiplier is required regardless if the system is single or three phase.
Multipliers rarely come into play. One time is when transformers are involved and you are trying to calculate the difference between a line current and a winding current. Another time is when you are mathematically adding L-N and L-L currents.
Multipliers rarely come into play. One time is when transformers are involved and you are trying to calculate the difference between a line current and a winding current. Another time is when you are mathematically adding L-N and L-L currents.
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- PE (Retired) - Power Systems
Then you are making a calculation. The OP was about a measurement.
Once you have your line amps (which may be different than the individual heater amps) there is no additional multiplier required in order to use the NEC to size your OCPD.
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- Lockport, IL
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The heater is not going to be able to tell the difference between its source being delta connected or being WYE connected. The OCPD is downstream of the source, and it just has three wires to deal with. The current in any wire is going to be the same as the other two, presuming again that the load is balanced. So you take the total three phase watts of the heater, divide by the voltage of 208V (presumably you are talking about a 120/208 volt system), and divide again by the square root of three. Then you multiply by 125% because this is a continuous load. Finally, you select the next higher standard size OCPD (unless you are over 800 amps, but I doubt that that is what you are asking about).
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Meaningless to your work, very helpful in my work.
Customer complains that this 100 amp three phase breaker has been tripping. The first thing I am going to do is place an amp clamp on one phase at a time, if any one phase shows more then 80 amps it will make me curious and if an one phase is over 100 amps then I know I have a problem. At this point I really do not care about the KW, KVA or even the 'total current' only the current per phase.
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- Lockport, IL
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- Semi-Retired Electrical Engineer
Please allow me to suggest that what you are interested is the current "in each phase." The word "per" carries with it the connotation of multiplication, and that is the part that is meaningless. For example, you get the total price of 3 dozen lemons at $4.00 PER dozen by multiplying 3 times $4.00.
OK, so I am being a nit-picker. But do you know what a "nit" is? It is the larvae stage of a flea or other nasty insect. So if I don't pick the nits, then the fleas are going to come back and bite me some day. :grin:
As others have said, it is the current in each phase that is used for sizing the interrupting device. Normally, however, you would not use measured current, but would calculate the maximum current.
So by that explanation "per" means "each" and can be used to accurately describe the current flowing in "each" leg or phase (if it is the same in the others) even if we are not going to add them together.
Roger
Thanks Charlie. This is how I have been doing it but I caught this thread and thought I would double check my work.
.........and my sources. Thanks again.
weressl
Esteemed Member
A balanced three phase load will read the same current in each phase, and unbalanced one does not. Computing the load - and ignoring the phase angle - one would average the currents, average the voltages and use the SQRT(3) multiplier. That's it.
The interrupting device rating refers to the rating of the protective element in each phase. The size is determined by the load type and necessary multipliers as guided by the NEC, but not as actual current reading.
Should you have a three phase motor protection issue with somewhat unbalanced phases, you may use the highest value of the three phases to set your alarms and trips differently than the nameplate, but this type of protection is not electrical, or guided by electrical protection standards. You would choose to do this in case of sensitive mechanical equipment, say a centrifuge where a slight overload indicates serious mechanical trouble. (Of course this is usually the last resort since you would have other, more sensitive, direct and accurate means of detecting such mechanical troubles and should do so.)
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