Article 455 - Phase Converter MOCP + Conductor Sizing

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Npstewart

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Im helping someone with a design that is uncomfortably un-familar for me. We have some fixed load bakery equipment that we are required to run through a phase converter to power a few pieces of equipment. The phase converter is feeding a panel and the panel feeds the equipment. I have used a phase converter on more then one occasion but for a panel.

Input to the phase converter is 78A @ 240-1 or 18.72 kW

Output voltage is 208-3

The loads are "fixed" in my opinion.

Conductor Sizing:

According to 455.6 (A)(2)

"The conductors shall have an ampacity of not less then 250% of the sum of the full load" (Im assuming this is single phase full load)

So 78A * 2.5 = 195 A so the conductors must be sized for 195A, therefore 3/0 CU will be OK.

NEXT

MOCP:

455.7 (B) requires the conductors to be protected in accordance with their ampacity therefore a 200A CB will be adequate. The problem is that the single phase panel that is feeding the converter is a 100A panel!



Does all this seem correct?
 
don_resqcapt19

don_resqcapt19

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(2) Fixed Loads. Where the phase converter supplies specific fixed loads, and the conductor ampacity is less than
125 percent of the phase converter nameplate single-phase input full-load amperes, the conductors shall have an ampacity not less than 250 percent of the sum of the full-load, 3-phase current rating of the motors and other loads served where the input and output voltages of the phase converter are identical. Where the input and output voltages of the phase converter are different, the current as determined by this section shall be multiplied by the ratio of output to input voltage
 
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Npstewart

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Ok so the current needs to be multiplied by 0.87 so that would put me at 170A or 175A CB with 2/0 conductors.
 
david luchini

david luchini

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Npstewart said:
Maybe I am skipping over something here. How would a 100A CB with #3's be justified?
Click to expand...

455.7(B): The overcurrent protection shall not exceed 125% of the phase converter nameplate single phase amps.

..................The input amps is 78. 78*1.25=97.5. 100A c/b is the next higher standard rating (as allowed by 455.7.)

455.7(B): the conductors shall be protected in accordance with their ampacity.

..................The ampacity of #3 is 100, so the conductors are protected in accordance with their ampacity by the 100A c/b.

455.6(2): and the conductor ampacity is less than 125% of the phase-converter nameplate single phase amps...

..................The ampacity of #3 is NOT less than 97.5. You do not need to consider the 250% of the sum of the full-load, 3 phase currents at all.
 
N

Npstewart

Senior Member
david luchini said:
455.7(B): The overcurrent protection shall not exceed 125% of the phase converter nameplate single phase amps.

..................The input amps is 78. 78*1.25=97.5. 100A c/b is the next higher standard rating (as allowed by 455.7.)

455.7(B): the conductors shall be protected in accordance with their ampacity.

..................The ampacity of #3 is 100, so the conductors are protected in accordance with their ampacity by the 100A c/b.

455.6(2): and the conductor ampacity is less than 125% of the phase-converter nameplate single phase amps...

..................The ampacity of #3 is NOT less than 97.5. You do not need to consider the 250% of the sum of the full-load, 3 phase currents at all.
Click to expand...


455.6(2) is where my misunderstanding was. Thanks so much Dave.
 
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