understanding 3 phase contactor sizing

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cjphelps

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Hi

I am trying to understand 3 phase power a little better when it comes to sizing a contactor. I have been given a power supply that has the input line wattage as 6600W, 208V and operating with 3 phase.

It also has been rated to have 29Amps/phase.

I want to know if there is 29 Amps in each of the three lines powering the supply. If so, my Contactor would have to be able to handle that

OR

Is there 29Amps divied evenly amongst all 3 lines, in which case the contactor wold be rated for less Amps?

In sizing contactors there is much calculations involving horsepower but little when dealing with simply wattage,
 
Those numbers do not appear to be consistent with each other. 6600 watts divided by 208 volts, and divided again by the square root of three, gives me a current of 18.3 amps. So I don't know where the 29 amp value would have come from.

But you need to drop from your vocabulary the notion of "amps per phase." It has no meaning. In particular, the word "per" has no meaning. You can have 29 amps flowing in Phase A, at the same time have 29 amps flowing in Phase B, and 29 amps flowing in Phase C. But you don't add 29 + 29 + 29 to get 57 "total amps." Rather, current leaving the source on one phase will return to the source on the other two phases, so it is the same current that is flowing in all three. The total current of the three phases is always zero, at every point in time.

Welcome to the forum.
 
Thank you for the quick reply

Your Reply:
Those numbers do not appear to be consistent with each other. 6600 watts divided by 208 volts, and divided again by the square root of three, gives me a current of 18.3 amps. So I don't know where the 29 amp value would have come from.


I agree with your calculation which is why I am confused
the wording is out of the manual for the power supply.
section 2.4 Table 2-6 describes the maximum AC current rating for my 6.6kW model Power supply as having a input line current of 29 Amps per phase.

http://www.elgar.com/products/P63/Downloads/P60_P80_D3_Operation_Manual_M550001-01_RevF.pdf


If possible I would like to really just understand that if the wattage is 6600watts, and voltage is 208 3 phase, what current the contactor would see in each line supplying power to it.

Thank you in advance!
 
cjphelps said:
If possible I would like to really just understand that if the wattage is 6600watts, and voltage is 208 3 phase, what current the contactor would see in each line supplying power to it.
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You would see 18.3 amps as Charlie said in his post.
 
The current values in the table you referenced (table 2-6) clearly state "amps per phase". I assume that the 6.6 kW rating is output power so the you will not be able to find the input power just using math unless you know the efficiency of the unit, however the input power is so much higher than expected by using the math, I think they may have posted single phase input currents and not three phase.
 
My $0.02: I'd say that number in the table is adequate for sizing your supply circuit at 30a 3ph.
 
So when choosing a contactor I still need some guidance,

Can I convert 6.6 kW directly to hp and then size the contactor for the 3 phase 208V circuit

Thanks everyone
 
cjphelps said:
So when choosing a contactor I still need some guidance,

Can I convert 6.6 kW directly to hp and then size the contactor for the 3 phase 208V circuit

Thanks everyone
Click to expand...

No, horsepower is a measure of output power. There is no method to convert output power to input power without knowing the efficiency of the power supply.

If your device is rated for 29A input then you need a 30A rated contactor (most contactors are rated in Amps, while most starters are rated in horsepower) which is a fairly common and inexpensive size.

Is your power supply rated for 6600W input or output?
 
jim dungar said:
No, horsepower is a measure of output power. There is no method to convert output power to input power without knowing the efficiency of the power supply.

If your device is rated for 29A input then you need a 30A rated contactor (most contactors are rated in Amps, while most starters are rated in horsepower) which is a fairly common and inexpensive size.

Is your power supply rated for 6600W input or output?
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Thanks Jim,

The 6600 watts is the rated output power. I want to be sure I understand everything though,

if the effiency is 80% minimum, Can I assume that the maximum power required to operate the device is 6600*1/0.8?

This would mean that It actually can draw 8250watts during its lowest effiency operation, correct?

Also, If it can draw 8250watts to operate at 80% effiency, Should I then assume that the 29 amps .a.k.a the "maximum AC current rating", is correct?
 
cjphelps said:
Also, If it can draw 8250watts to operate at 80% effiency, Should I then assume that the 29 amps .a.k.a the "maximum AC current rating", is correct?
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I say that the table listing the 29a is what makes the 30a circuit correct. When a manufacturer specifies a particular supply circuit ampacity, that means they've done the calcs necessary for the equipment to perform properly.
 
LarryFine said:
I say that the table listing the 29a is what makes the 30a circuit correct. When a manufacturer specifies a particular supply circuit ampacity, that means they've done the calcs necessary for the equipment to perform properly.
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True, I just wanted more to understand 3 phase powering of this DC power supply. I am trying to buy a contactor to remotely start it up, and I thought that a 30A contactor would not be required, because te current going through the actual lines was only ~18.

Thank for your help
 
cjphelps said:
This would mean that It actually can draw 8250watts during its lowest effiency operation, correct?

Also, If it can draw 8250watts to operate at 80% effiency, Should I then assume that the 29 amps .a.k.a the "maximum AC current rating", is correct?
Click to expand...

That is what I would do, based on what you have provided.
 
LarryFine said:
I say that the table listing the 29a is what makes the 30a circuit correct. When a manufacturer specifies a particular supply circuit ampacity, that means they've done the calcs necessary for the equipment to perform properly.
Click to expand...


Ok I got it, the current given of 29A is after the effiency is taken into account, and then multiplied by a factor of 25% safety.

End redultis good, I now understand the requirements, thanks everyone!:)
 
cjphelps said:
I am trying to buy a contactor to remotely start it up, and I thought that a 30A contactor would not be required, because te current going through the actual lines was only ~18.
Click to expand...
Just looking through the Grainger catalog, at definite-purpose contactors, the 30a 3p unit only costs about $5 more than the 20a one does, so don't sweat it. A burned-out contact is more than a $5 headache.

Also keep in mind the use that the contactor will see. One that switches a full load regularly is much harder on the contacts (pitting from arcing, not just mechanical wear) than one seeing occasional use.
 
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