Which temperature rating would you use for multiple motor feeder calculation.

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Angel Electric

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The question-The motor feeder conductor size for 3-15 hp, 3 phase , 208 volt motors, 3-3 hp, single phase, 208 volt motors, and 3-1 hp, single phase, 115 volt motors

The given answer for the test question is 4/0, But how did we determine which temperature rating in 310.15 (B) (16) to use ? The chosen temperature is 75 celsius. I wanted to choose THHN, Which is 90 celsius.. I'm sure there is a standard as to why we chose 75? Any help is appreciated:)......
 
Angel Electric said:
The question-The motor feeder conductor size for 3-15 hp, 3 phase , 208 volt motors, 3-3 hp, single phase, 208 volt motors, and 3-1 hp, single phase, 115 volt motors

The given answer for the test question is 4/0, But how did we determine which temperature rating in 310.15 (B) (16) to use ? The chosen temperature is 75 celsius. I wanted to choose THHN, Which is 90 celsius.. I'm sure there is a standard as to why we chose 75? Any help is appreciated:)......
Click to expand...

Check out NEC 110.14(C).
 
As David said if the rated current is 100 A or less 60 dgr.C; more than 100 A 75 dgr.C. [NEC 110.14 (C) Temperature Limitations.].
That means you can provide any cable insulating rating but the ampacity has to be recalculated for 60 or 75 maximum. [approx. as for 60 dgr.C or 75 dgr.C insulation rating for the same rated voltage].
For information-of ?non-NEC recommendations?-see:]
http://www.energy.siemens.com/us/po...oducts/TechTopics/ANSI_MV_TechTopics90_EN.pdf
 
Angel Electric said:
The question-The motor feeder conductor size for 3-15 hp, 3 phase , 208 volt motors, 3-3 hp, single phase, 208 volt motors, and 3-1 hp, single phase, 115 volt motors

The given answer for the test question is 4/0, But how did we determine which temperature rating in 310.15 (B) (16) to use ? The chosen temperature is 75 celsius. I wanted to choose THHN, Which is 90 celsius.. I'm sure there is a standard as to why we chose 75? Any help is appreciated:)......
Click to expand...

I get 3/0 at 75 degrees, though (71,523VA.) I wonder how they arrived at 4/0.
 
The answer is 4/0 based on two additional details

The answer is 4/0 based on two additional details

The rating for 3/0 is only 200 amps at 75^0 celsius, also the calculations of three phase spread out the load on all phases during the calculation.

Motor FLCs - Tables 430.248 and 430.250
15 hp, 208V, three-phase = 46.20A
3 hp, 208V, single-phase = 18.70A
1 hp, 115V = 16A
L1 L2 L3
3 - 15 hp, three-phase 46.20 46.20 46.20
46.20 46.20 46.20
46.20 46.20 46.20
3 - 3 hp, 208V 18.70 18.70
18.70 18.70
18.70 18.70
3 - 1 hp, 115V 16.00 16.00 16.00
192.00 192.00 192.00
Feeder conductors must not be less than: (46.20A x 1.25) + 46.20A + 46.20A + 18.70A + 18.70A + 16A = 203.55A
4/0 THHN is rated 230A at 75?C
Note: 3/0 THHN is only rated for 200A at 75?C
 
Angel Electric said:
The rating for 3/0 is only 200 amps at 75^0 celsius, also the calculations of three phase spread out the load on all phases during the calculation.

Motor FLCs - Tables 430.248 and 430.250
15 hp, 208V, three-phase = 46.20A
3 hp, 208V, single-phase = 18.70A
1 hp, 115V = 16A
L1 L2 L3
3 - 15 hp, three-phase 46.20 46.20 46.20
46.20 46.20 46.20
46.20 46.20 46.20
3 - 3 hp, 208V 18.70 18.70
18.70 18.70
18.70 18.70
3 - 1 hp, 115V 16.00 16.00 16.00
192.00 192.00 192.00
Feeder conductors must not be less than: (46.20A x 1.25) + 46.20A + 46.20A + 18.70A + 18.70A + 16A = 203.55A
4/0 THHN is rated 230A at 75?C
Note: 3/0 THHN is only rated for 200A at 75?C
Click to expand...

When you balance your three 3HP 208V single phase motors, you will get 32.39A (18.7 * 1.732), not 37.40A.

46.20*1.25 + 46.2 + 46.2 + 32.39 + 16 = 198.54A.

3/0 is rated for 200A, which is sufficient for the calculated load.
 
Classic error where the affect of three phase power distribution system is misunderstood and where amperes is used instead of KVA.

I concur with the others that the minimum feeder size needed is #3/0 AWG.
 
Better depiction for the answer.

Better depiction for the answer.

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 _____. (a) 2/0 AWG (b) 3/0 AWG (c) 4/0 AWG (d) 250 kcmil

A B c
46.2 46.2 46.2*1.25
46.2 46.2 + 46.2
46.2 46.2 + 46.2
18.7 18.7
18.7 + 18.7
18.7 + 18.7
16. 16. + 16.
= 203.55

Now take the any column A, B,or C, of panel board to add the current total to see what the size conductor will be needed.

I'll Take C... But First I must multiply 1.25 To the highest motor load of my feeder.

46.2*1.25+46.2+46.2+18.7+18.7+16.0=203.55 Which puts the need for a 4/0 into play B/C 3/0 is only rated for 200Amp..

I really appreciate the feedback..
Thanks to all!
 
Angel Electric said:
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 _____. (a) 2/0 AWG (b) 3/0 AWG (c) 4/0 AWG (d) 250 kcmil

A B c
46.2 46.2 46.2*1.25
46.2 46.2 + 46.2
46.2 46.2 + 46.2
18.7 18.7
18.7 + 18.7
18.7 + 18.7
16. 16. + 16.
= 203.55

Now take the any column A, B,or C, of panel board to add the current total to see what the size conductor will be needed.

I'll Take C... But First I must multiply 1.25 To the highest motor load of my feeder.

46.2*1.25+46.2+46.2+18.7+18.7+16.0=203.55 Which puts the need for a 4/0 into play B/C 3/0 is only rated for 200Amp..

I really appreciate the feedback..
Thanks to all!
Click to expand...

OK, I'll try again. The 208V single phase motors (18.7A in the columns) do NOT directly add.

Try figuring just the three 3HP single phase motors, balanced on a 3phase system. In your method, you would get:

18.7.......18.7
..............18.7.......18.7
18.7.....................18.7
...................................
37.4.......37.4.......37.4

So you would expect your three single phase motors connected in a balanced manner to draw 37.4A.

Now try this method:

18.7A @ 208V single phase = 3889.6VA

Three 3889.6VA loads would give a total load of 11668.8VA

11668.8VA @ 208V, 3ph would give a load current of 32.4A.

As you can see, the two methods produce a different load current value for the same motors. The 32.4A is the correct value, the 37.4A is wrong.

Using 32.4A instead of 37.4A in your original problem provides a required minimum feeder size of #3/0.
 
Last edited:
For some it may be easier to switch to KVA, the reason is "KVA is KVA". Voltage or number of phases does not alter the KVA. Using KVA, the voltage and phases can alter the current.

46.2A x 1.732 x 208V = 16.64KVA
18.7A x 208V = 3.89KVA
16A x 120V = 1.92KVA

(2 x 16.64KVA) + (1.25 x 16.64KVA) + (3 x 3.89KVA) + (3 x 1.92KVA) = 71.51KVA

71.51KVA/(208V x 1.732) = 198.5A

This is a simplified approach for this problem due to the fact that all three phases have the same amount of load (KVA). Otherwise you would need to put it in columns and use the highest column to determine amps. But, you need to always work in KVA; regardless.
 
david luchini said:
OK, I'll try again. The 208V single phase motors (18.7A in the columns) do NOT directly add.

Try figuring just the three 3HP single phase motors, balanced on a 3phase system. In your method, you would get:

18.7.......18.7
..............18.7.......18.7
18.7.....................18.7
...................................
37.4.......37.4.......37.4

So you would expect your three single phase motors connected in a balanced manner to draw 37.4A.

Now try this method:

18.7A @ 208V single phase = 3889.6VA

Three 3889.6VA loads would give a total load of 11668.8VA

11668.8VA @ 208V, 3ph would give a load current of 32.4A.

As you can see, the two methods produce a different load current value for the same motors. The 32.4A is the correct value, the 37.4A is wrong.

Using 32.4A instead of 37.4A in your original problem provides a required minimum feeder size of #3/0.
Click to expand...
Please note that by using the 37.4 amp figure he will not have a code violation, he just will have larger conductor than what the minimum requirement is, and since we are hovering so close to 200 amps maybe will end up with larger than needed overcurrent device also, but also consider that since we are dealing with motors it is still possible to have overcurrent protection higher than the conductor ampacity though.
 
kwired said:
Please note that by using the 37.4 amp figure he will not have a code violation, he just will have larger conductor than what the minimum requirement is, and since we are hovering so close to 200 amps maybe will end up with larger than needed overcurrent device also, but also consider that since we are dealing with motors it is still possible to have overcurrent protection higher than the conductor ampacity though.
Click to expand...

I didn't say that using the 37.4A figure will result in a code violation, just that it resulted in a larger than the minimum required feeder conductor size.

I don't think it would make a difference in the OP example, but using the larger figure in calculating the motor feeder SC-GF protection device could result in a code violation by resulting in a larger than the maximum allowed OCPD device.
 
david luchini said:
I didn't say that using the 37.4A figure will result in a code violation, just that it resulted in a larger than the minimum required feeder conductor size.

I don't think it would make a difference in the OP example, but using the larger figure in calculating the motor feeder SC-GF protection device could result in a code violation by resulting in a larger than the maximum allowed OCPD device.
Click to expand...
I know you didn't say it was a violation - just wanted to point out that it wouldn't really hurt anything to be oversized other than maybe it will cost a little more than something that is otherwise code compliant.

Overcurrent protection is a little more complex when dealing with motors. The OP could very well have a 200 amp conductor supplying the motors mentioned with more than 200 amps of protection on it and still be code compliant. But when calculating max allowable values he could be too large if using the 37 amp figure instead of the 32 amp figure. Kind of depends what the figures are and if rounding up or down with any of the figures. Rounding down to next standard size device could still end up being legal when someone checking it does their own calculation (correctly), especially when only talking a difference of 5 amps for the starting values.
 
kwired said:
I know you didn't say it was a violation - just wanted to point out that it wouldn't really hurt anything to be oversized other than maybe it will cost a little more than something that is otherwise code compliant.
Click to expand...

Of course, any feeder size larger than the minimum allowed would also be code compliant. But, as this is an exam type question, I would assume that they are looking for only one answer.

Angel Electric said:
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 _____. (a) 2/0 AWG (b) 3/0 AWG (c) 4/0 AWG (d) 250 kcmil
Click to expand...

(b), (c) or (d) would be an adequate feeder size. Only (b) would be the minimum required feeder size.
 
david luchini said:
Of course, any feeder size larger than the minimum allowed would also be code compliant. But, as this is an exam type question, I would assume that they are looking for only one answer.



(b), (c) or (d) would be an adequate feeder size. Only (b) would be the minimum required feeder size.
Click to expand...

And if question posted is the actual wording of the question - it does not contain the word "minimum". If that is the case it is a poorly worded question.

Nothing like adding confusion to people that are trying to learn by not wording a question properly. Any answer that is above the actual minimum allowed is technically not incorrect if they don't include the word minimum in the question.
 
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