breaker and wire sizes

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heybud

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Hello I have an electrical engineer that is telling me that I can put a 30A breaker and fuse on #12 wire for a 3phase 208V roof top A/C because of the "unless specifically permitted in (e)through (g)" in article 240-3d. Is this correct. He also has drawn a 35A breaker and fuse on #10 wire for the same purpose (different unit).
The fuses are spec'd as timedelay. :confused:

Thanks
 
Re: breaker and wire sizes

So is it your interpretation that if you use this exception the #12 and # 10 can be loaded to 100% of its rating at 90 degrees. Wouldn't you have to use the 75 degree rating because of the connections? So the #12 could only be landed on a 25A breaker not a 30A? I am having trouble digesting this because in 21 years this is the first time I have seen this. The loads are as follows: FLA 19.1 MCA 22.5 MOCP 30 #12 wire. FLA 24.6 MCA 28.9 MOCP 35 #10 wire. I guess that the #10 is logical but the #12 on a 30 what am I missing. Please send help =-] :D
 
Re: breaker and wire sizes

So is it your interpretation that if you use this exception the #12 and # 10 can be loaded to 100% of its rating at 90 degrees.
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No, the load has to be limited to the ampacity of the conductor, but I am not saying that the OCPD is limited to the ampacity of the conductor. The conductor ampacity is limited by the termination temperature and any adjustment or correction factors that are applied.
So the #12 could only be landed on a 25A breaker not a 30A?
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No, the rating of the OCPD for these types of applications really have nothing to do with the ampacities shown in Table 310.16. There are cases where the #12 branch circuit conductors for a motor circuit are permited to be connected to a 50 amp breaker or a 60 amp non time delay fuse.
FLA 19.1 MCA 22.5 MOCP 30 #12 wire. FLA 24.6 MCA 28.9 MOCP 35 #10 wire. I guess that the #10 is logical but the #12 on a 30 what am I missing. Please send help
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You have to remember two things about conductor sizing for motor and air conditioning type loads: 1) the conductor is sized for the load and not the rating of the OCPD, and 2) the branch circuit OCPD is sized to permit the motors to start and to provide short circuit and ground fault protection only. The branch circuit OCPD does not provide the overload protection for these conductors. The over load protection is provided by the motor overloads.
Don
 
Re: breaker and wire sizes

This may help

You did not indicate the horsepower rating of the two motors but lets say the first is a 5 HP 3 phase 208-induction motor.
Art 430.6A for general motors requires that the entire motor branch circuit (except the running protection) be sized based upon the tables FLA. Table 430.150 lists a 5HP motor with a FLA of 16.7A. I will use that as an example.

430.22 requires the motor branch circuit conductors have an ampacity not less than 125% of the tables FLA 1.25 X 16.7 = 20.875A.

Table 310.16 lists a #12 Cu 75 deg C copper wire with an ampacity of 25A.
This conductor is appropriate for the conditions of 430.22

430.52 States the Branch Circuit and Short Circuit and Ground Fault Protection may be sized as follows
Time Delay 175% of 16.7 = 29.2A select 30A Fuse

Non Time Delay 300% of 16.7 = 50.1A select 50A Fuse

Inverse Trip Breakers 250% of 16.7 = 41.75 select 50A (45A) Fuse
The section allows the next higher sizes OCPD if the calculation does not coincide with those listed in 240.6. That can be tricky and it was debated in the 93 code.

240.4 (G) recognizes several applications in the Code, which can allow small conductors (240.4 (D)) to be protected at ampacities exceeding the general 15, 20, and 30A limitations.

That 12-gage conductor may have short circuit and ground fault protection set as indicated above. Theses devices are set higher to allow the motors to start. They protect the circuit from high current faults. Note that this is short circuit protection.
As Don mentioned the conductors must be protected at their ampacity. 430.6 A requires the running protection to be set per 430.31 and 32, 125% or 115% of the NAME PLATE CURRENT (generally). For example if the running protection for this motor is set at 125% of 16.7 it would protect the entire branch circuit at a level of 20.875 A. the conductor has an ampacity of 25A.

Motor circuit protection is a coordinated effort between the fuse or breaker and the overload device. The code understands and recognizes this. This is a long time practice and it works fine and lasts a long time.

Hope this helps

charlie

I edited the inverse trip breaker to 45 A as noted in the folowing post by Roger thanks again

[ October 05, 2004, 01:16 PM: Message edited by: cpal ]
 
Re: breaker and wire sizes

Cpal, the inverse time breaker selection would be a 45.

Roger
 
Re: breaker and wire sizes

Roger, Thank You It would help if I opened the book.

Charlie
 
Re: breaker and wire sizes

Simply put and yes I am simple :D
 
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