O.T. (U.S.)NEC question

[Ken Moore]

Thanks for all the replies. After most of you folks agreed with me, I consulted with the head company E.E., I checked here first, to make sure I was correct.
The headquarters guy agreed with what y'all said, so I'm going to have another meeting with the contractor on Monday.



Again thanks for the help.

 

[Lancie1]

From NEC Table 430.150: 7.5 HP 3-phase 460 volt motor Full Load Current is 11 Amps.

From NEC Table 310.16: 12 AWG 75-degree C wire Allowable Current Ampacity is 25 Amps, BUT is reduced to 20 Amps for safety reasons.

From NEC Table 310.15(B)(2)(a): Adjustment factor for 10 to 20 current carrying conductors in one conduit: 50%

Therefore, 0.50 X 20 Amps = 10 Amps per #12 wire.
Therefore, since motor full load amps is 11, wire must be upgraded to next larger size, which is #10 in standard stocked sizes. Check: #10 is rated for 30 Amps, reduced by 50% = 15 Amps. We need about 125% of FLA for motor startup, so 1.25 X 11 = 13.75 Amps, which looks okay.

From NEC Annex C, Table C1, twelve #10 THHN phase wires and a ground wire will require a 1" EMT conduit (at 40% fill) instead of four 1/2" conduits that could be used for separate feeds. Because it is a long run, you better go up to the 1.25" conduit size to avoid skinning off the insulation when you pull in 13 wires.

Bottom line: One 1.25" conduit with thirteen #10 wires is much cheaper to install than four 1/2" conduits each with four #12 wires, probably not a 75% cost reduction but maybe 50%, because it will take some extra time to arrange 13 reels of wire and make the one pull.

 

[Ken Moore]

thanks Lancie1

 

[Jim Dungar]

You always start at the maximum capacity of the insulation. Then you make adjustments for # of conductors and ******t temperature. Then you check to see if your final ampacity is within the range of other code sections. #12 wire is prevented from being protected at more than 20A after (not before) correction factors (NEC 240). Then the final check is to see if the corrected ampacity does not exceed the rating of the termination (there is no UL Listed starter or protective device rated for more than 75C terminations).

NEC Table 310.16 is used as the starting point. (i.e. #12 THHN = 30A)
Table 310.15(B)(2)(A) has adjustment factors. (i.e 12 conductors = %50)

The adjusted ampacity of (12) #12 THHN in a single conduit is 15A. This circuit must be protected at not more than 15A which is less than 20A so we meet the conductor protection requirement. The adjusted ampacity of this circuit is compared to the 75C column of table 310.16 of 20A.

 

[Tark]

Seems to be some confusion as to which amperage rating for 12AWG is to be used in the calculation.

The amperage rating to be used for correction factors on a 12AWG conductor connected to a starter having 75C terminations is –

A. 20 amps
B. 25 amps
C. 30 amp
D. None of the above

The correct answer is (B) 25 amps. (A) is incorrect because 240.4(D) states that the overload protection cannot exceed x after correction factors have been applied. (C) is incorrect because 310.10, if the conductor is connected to terminations that are rated for 75C you cannot use the 90C rating of the conductor for correction factors, you need to use the 75C rating of the conductor.

This being the case you are back to having to use a 10AWG conductor. Although, if the conduit is run on top of a roof you should also adjust for the ambeit temperature as well, I’m sure it gets hotter than 30C on top of a roof in the middle of summer.

 

[Jim Dungar]

The correct answer is 30A.

All adjustments may be made starting at the base temperature of the insulation. After adjustments have been made the final ampacity of the conductor must not exceed that of the terminations temperature per NEC 110.14(C). Unless provided elsewhere in the NEC (i.e. for motor circuits) #14, #12, and #10 AWG need to be protected based on limits imposed by 240.4(D).

NEC 110.14(C)(1)(a)(2) says in part "Conductors with higher temperature ratings, provided the ampacity is determined based the 60C(140F) ampacity...". There is no actual verbiage in the NEC preventing you from starting at the 90C column nor requiring you to use the 75C column.

 

[Tark]

Thanks for pointing out that Article number Jim, you are correct.

 

[MikeW]

The correct answer is 30A.

All adjustments may be made starting at the base temperature of the insulation. After adjustments have been made the final ampacity of the conductor must not exceed that of the terminations temperature per NEC 110.14(C). Unless provided elsewhere in the NEC (i.e. for motor circuits) #14, #12, and #10 AWG need to be protected based on limits imposed by 240.4(D).

NEC 110.14(C)(1)(a)(2) says in part "Conductors with higher temperature ratings, provided the ampacity is determined based the 60C(140F) ampacity...". There is no actual verbiage in the NEC preventing you from starting at the 90C column nor requiring you to use the 75C column.

I might be confused (not the first time and definitely not the last related to a NEC interpretation).

The very next section referenced by Jim, NEC 110.14(C)(1)(a)(3) states “Conductors with higher temperature ratings if the equipment is listed and identified for use with such conductors.” may actually be the verbiage that excludes using 90C ratings, correct? If the 90C wire is connected to a circuit breaker or terminal block with 75C terminal ratings, you have to then derate the wire to 75C, correct?

If that’s the case, Ken needs to look at the load and final connections prior to sizing the wire.

 

[Jim Dungar]

No. Technically you do not derate the wire, instead you "correct" its capacity.

For example, using the data in the OP:

#12 AWG THHN is rated for 30A based on the 90C column.
The correction factor for (12) conductors is 50%, so now our #12 conductor is rated 15A.

The 7.5 HP motors draw 11A (we must use the NEC amperage when sizing conductors. If these are continuously operated motors we need to adjust their amps requirement by 125% - so they will need conductors rated a minimum of 13.75A

Our conductors have a higher ampacity than that of the load so we have chosen them correctly.

Now we look at the device terminations which are only rated as 60C (for NEMA starters) or 75C (for UL listed IEC contactors). A 13.75A load requires a minimum conductor size of #14AWG based on both the 60C and 75C columns.

Our #12 conductor is larger than the require #14 for the termination, so again we have chosen correctly.

Finally we check 240.4 to see how to size our overcurrent protection for these conductors. Because, these are motor power circuits the overcurrent protection limitation of 20A is not appliable (but even if it were our conductors are still okay as their adjusted ampacity requires them to be protected at 15A).

There are no UL Listed breakers, fuse holders, starters, receptacles, or switches that are rated for use with conductors sized solely on the 90C column. This means that conductors must always be the minimum AWG as shown in the 60C or 75C column.

 

[MikeW]

Jim,

Thanks for the clarification, I had been misinformed by another engineer on what to do in this case. It didn't seem correct to me but I could not get exactly what applied.

Mike

 

[mahjoby]

i wont to know about how we use rating of contactors for motor 37kw

 

[Honga]

Start a new thread

 

[troutbum76]

I agree that there is no reason for the separate conduits provided that the wiring is derated properly. My only suggestion would be to pull the motor sets from bucket to peckerhead without any joints, so that in the future, when maintenance is performed, there won't be any reason for an undertrained individual to mess with multiple joints. Its been my experience that, when given the option, people tend to panic, break a bunch of joints, panic some more, and then inevitably call in their electrical contractor to sort out the mess. Not to mention, you could really hose up some motors if you got your sets crossed up.