NEC Test Question Help

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jms53

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This question has me stumped. Wanted to run it past you folks that have more code and calculation expertise than me. I understand everything except how the single phase 208 motor loads are added. Answer only shows value from 2 motors, and not 3. Any thoughts on why it's this way. It may have something to do with the way FLC's of all other motors on the same line are recognized. Any thoughts

Thanks for any help you may be able to provide.

[FONT=&quot]Question:[/FONT][FONT=&quot]8. • The motor feeder conductor size for three 15 hp, three-phase, 208V motors; three 3 hp, single-phase, 208V motors; and three 1 hp, single-phase, 115V motors will be _____.

[/FONT][FONT=&quot](a) 2/0 AWG (b) 3/0 AWG (c) 4/0 AWG (d) 250 kcmil[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]Answer:[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot]8. (c) 4/0 AWG[/FONT]
[FONT=&quot][430.24(A) and Table 310.16][/FONT]
[FONT=&quot]The feeder conductor must be sized not less than 125% of the largest motor FLC, plus 100% of the FLCs of all other motors on the same line. [/FONT]
[FONT=&quot]Motor FLCs - Tables 430.248 and 430.250[/FONT]
[FONT=&quot]15 hp, 208V, three-phase = 46.20A[/FONT]
[FONT=&quot]3 hp, 208V, single-phase = 18.70A[/FONT]
[FONT=&quot] 1 hp, 115V = 16A[/FONT]
[FONT=&quot]
[/FONT]
[FONT=&quot] [FONT=&quot]L1 L2 L3[/FONT][/FONT]
[FONT=&quot]3 - 15 hp, three-phase 46.2 46.2 46.2[/FONT]
[FONT=&quot] 46.2 46.2 46.2[/FONT]
[FONT=&quot] 46.2 46.2 46.2[/FONT]
[FONT=&quot]3 - 3 hp, 208V 18.7 18.7[/FONT]
[FONT=&quot] 18.7 18.7 [/FONT]
[FONT=&quot] 18.7 18.7[/FONT]
[FONT=&quot]3 - 1 hp, 115V 16.0 16.0 16.0[/FONT]
[FONT=&quot][FONT=&quot]
[/FONT][/FONT]
[FONT=&quot]Feeder conductors must not be less than: (46.20A x 1.25) + 46.20A + 46.20A + 18.70A + 18.70A + 16A = 203.55A[/FONT]
[FONT=&quot]4/0 THHN is rated 230A at 75ºC[/FONT]
[FONT=&quot] Note: 3/0 THHN is only rated for 200A at 75°C[/FONT]
 
The 208v single phase motors will only used 2 phases of the three phase system.

So to balance the load they would be connected:
A-B
B-C
A-C

Each phase is only used twice, so that is all you have to account for in the calculation.
 
packersparky said:
The 208v single phase motors will only used 2 phases of the three phase system.

So to balance the load they would be connected:
A-B
B-C
A-C

Each phase is only used twice, so that is all you have to account for in the calculation.
Click to expand...

There are
3- 15 hp 3 phase motors
3- 3 hp 208 v single phase
3- 120V single phase


So there are 6 loads per phase not 2. I assume you were just doing the 208V loads
 
Dennis Alwon said:
There are
3- 15 hp 3 phase motors
3- 3 hp 208 v single phase
3- 120V single phase


So there are 6 loads per phase not 2. I assume you were just doing the 208V loads
Click to expand...

I should have been more clear. Each phase is only used twice for the supply of the three 208v single phase motors.
 
Mike shows it very clearly in his book

enhance
 
Note that the line currents for the individual 208V single phase motors and the 120V single phase motors will not be in phase, but the NEC calculation ignores that.
Another way of looking at the same thing is that the currents from the two 208V motors that share a line conductor will actually add vectorially, not algebraically.
 
GoldDigger said:
Note that the line currents for the individual 208V single phase motors and the 120V single phase motors will not be in phase, but the NEC calculation ignores that.
Another way of looking at the same thing is that the currents from the two 208V motors that share a line conductor will actually add vectorially, not algebraically.
Click to expand...


So why is the wrong way the correct way :?:
 
One way to think about connecting three (balanced) L-L single phase loads to a three phase system is that they are effectively connected in a delta configuration and then connected to the three phase system, thus the three phase system sees a total current increase of 1.732 times the single phase current.
 
electrofelon said:
Im confused. Post number 5 shows a method that is wrong but seems to be accepted as correct. In post 6 Goldie states that the nec ignores the vector addition. Do they say that in 220?
Click to expand...

The vector addition he speaks of is why one should always use VA.

For L-L loads on a 208Y/120 system the correct wattage is 208V x I, split the total and assign half to each hot leg. Post 5 can easily not convey this. Bad methodology IMO.

Adding amps leads to mistakes.
 
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