Design B Motor calc

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aceonskis

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Just finished a masters test and ran across a question about design B motor conductor size. The HP was given 10 and it was 3phase 208. Im not sure where to start with this, i don't see it in the FLC tables. Anyone seen this before?
 
"Design B" is a red herring, it doesn't matter, probably thrown in there to mess you up. It's just a 10HP 208V motor as far as conductor sizing is concerned.
 
Thank you for replying first of all.
Ok well that's good i thought there might be something special about it. the conductors offered in the question stated at #6 and went up from there, this motor should have an FLC of 30.8 so i assume you multiply by 1.25 but that still doesn't get you to #6. Thoughts?
 
NEMA design A, B, C, D essentially determines what amount of inductance is used in the stator vs the rotor. In a Steinmetz circuit it is X1 and X2. It determines the shape of the torque curve and starting current but has no effect on FLC.
 
aceonskis said:
Thank you for replying first of all.
Ok well that's good i thought there might be something special about it. the conductors offered in the question stated at #6 and went up from there, this motor should have an FLC of 30.8 so i assume you multiply by 1.25 but that still doesn't get you to #6. Thoughts?
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“Should have” is irrelevant. You size the conductors based on the FLC listed for your HP and voltage in the appropriate chart in the NEC, section 430, x 1.25. It doesn’t matter if the actual FLC is lower, but it does of it is higher (like on a 12 pole motor or something unusual), in which case you use the higher value.
 
aceonskis said:
Thank you for replying first of all.
Ok well that's good i thought there might be something special about it. the conductors offered in the question stated at #6 and went up from there, this motor should have an FLC of 30.8 so i assume you multiply by 1.25 but that still doesn't get you to #6. Thoughts?
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I question whether they had the right answer if 6 AWG was the smallest conductor in the possible answers. Table 430.250 says 10 HP 208 volt three phase motor is 30.8 A like you said. times 1.25 = 38.5. 8 AWG is minimum copper conductor, even if only 60C insulation.

Unless there were other reasons to make more adjustments that you neglected to read or tell us about.
 
retirede said:
Did it say single or three phase? Single would be about 55 amps.

Can you post the full question?
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Thanks for the reply
Jraef said:
“Should have” is irrelevant. You size the conductors based on the FLC listed for your HP and voltage in the appropriate chart in the NEC, section 430, x 1.25. It doesn’t matter if the actual FLC is lower, but it does of it is higher (like on a 12 pole motor or something unusual), in which case you use the higher value.
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Thank you Jraef
I feel the same way about it. I only missed passing by 2 questions so I wanted to know as much as I could about, that question comes up every time it seems.
 
kwired said:
I question whether they had the right answer if 6 AWG was the smallest conductor in the possible answers. Table 430.250 says 10 HP 208 volt three phase motor is 30.8 A like you said. times 1.25 = 38.5. 8 AWG is minimum copper conductor, even if only 60C insulation.

Unless there were other reasons to make more adjustments that you neglected to read or tell us about.
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Thanks Kwired
I came up with the same answer myself. The question was so vague. No other info. That’s why I thought that the design B maybe had some special note that I haven’t ran across and I have all the new 2020 mike holt stuff and I can’t find anything. I appreciate you time helping me with this.
 
aceonskis said:
Thanks Kwired
I came up with the same answer myself. The question was so vague. No other info. That’s why I thought that the design B maybe had some special note that I haven’t ran across and I have all the new 2020 mike holt stuff and I can’t find anything. I appreciate you time helping me with this.
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I'd ask them their answer and how they come up with it. Maybe let us know what they said. With no more information than given, I don't see why 8 AWG wouldn't be the correct answer though.
 
kwired said:
I'd ask them their answer and how they come up with it. Maybe let us know what they said. With no more information than given, I don't see why 8 AWG wouldn't be the correct answer though.
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Yeah, #6 might be correct if there was along distance and you had to deal with voltage drop, or you are running conductors for 3 motors in one conduit and had to de-rate, or it is aluminum wire, some other mitigating circumstance. But if what you posted was the entirety of the data you had to work with, they are not correct.
 
Jraef said:
Yeah, #6 might be correct if there was along distance and you had to deal with voltage drop, or you are running conductors for 3 motors in one conduit and had to de-rate, or it is aluminum wire, some other mitigating circumstance. But if what you posted was the entirety of the data you had to work with, they are not correct.
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Technically, #6 is not incorrect. Nothing prohibits running oversized conductors as long as it’s done properly.

I know, I’m reaching here
 
retirede said:
Technically, #6 is not incorrect. Nothing prohibits running oversized conductors as long as it’s done properly.

I know, I’m reaching here
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Kind of have to assume the question was asking what is minimum size conductor allowable for the situation, if they don't state that then anything larger is also a valid answer.
 
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