using #12 on a 25A breaker for motors - OK, except when VFD is present?

[kwired]

I still think a good example is a drive with single phase in and three phase out. The input will have a higher current level because it is single phase. The output is protected by the internal overload protection function of the drive.

 

[Smart $]

I don't think the new language in '14 changes anything.

430.130 says to provide OCP of the size and type as specified in 430.53(C)...which is in Part IV...which still exempted from the 240.4(D) small conductor requirements.

I still think a good example is a drive with single phase in and three phase out. The input will have a higher current level because it is single phase. The output is protected by the internal overload protection function of the drive.

Under 430.130(2) & (3) [2014], I think 240.4(D) would apply, because the SC/GF is stipulated in the manufacturers instructions or marked on the equipment and not determined under Article 430, Part IV.

 

[david luchini]

Under 430.130(2) & (3) [2014], I think 240.4(D) would apply, because the SC/GF is stipulated in the manufacturers instructions or marked on the equipment and not determined under Article 430, Part IV.

I'm not sure I would interpret it that way...430.130(2) says to size per Part IV, but not to exceed a manufacturer listed maximum.

If you had a 17.5A drive, and used #12, you could use a 45A c/b. If the manufacturer listed the max OCPD as 40A, you'd then have to go to #8 when you can use #12 on a 45A c/b in the first instance. I don't think that is the intent.

 

[augie47]

My read is that the conductor would be sized at 125% of the drive nameplate.
The SCGF device would be per manufacturer instructions or if not given per 430.52
Seems to be kinda hairy.. if the mfg gives you a SCGF size then you followed Part X, a non-exempt section
If you have no mfg. size, you fall back to 450.52 which is exempt.

 

[Smart $]

I'm not sure I would interpret it that way...430.130(2) says to size per Part IV, but not to exceed a manufacturer listed maximum.

If you had a 17.5A drive, and used #12, you could use a 45A c/b. If the manufacturer listed the max OCPD as 40A, you'd then have to go to #8 when you can use #12 on a 45A c/b in the first instance. I don't think that is the intent.

Perhaps not, but I agree with Gus' assessment...

My read is that the conductor would be sized at 125% of the drive nameplate.
The SCGF device would be per manufacturer instructions or if not given per 430.52
Seems to be kinda hairy.. if the mfg gives you a SCGF size then you followed Part X, a non-exempt section
If you have no mfg. size, you fall back to 450.52 which is exempt.

Perhaps a technical correlation oversight when 430.130 was implemented. However, kwired made a good point regarding a single phase input, three phase output drive. The input current would be considerably higher than the output current. I have to assume this would be one instance where manufacturer specifies a maximum OCPD rating. I don't think Code has any considerations for this particular application under Part IV...???

 

[bozo]

Does anyone know why the code makers went to the trouble to note particular sections of Article 430 in Table 240.4(G)? Seems that if the intent was for 240.4(D) not to apply to any motor circuits (including VFD) then it would have been much clearer to just list "Article 430" in the table. That would have eliminated the need for this post. When they specify only particular sections it then forces one to attempt an analysis of each of those sections and their relationship to the other non-mentioned sections as has been done here. Don't know if they were attempting to be user-friendly but I'm convinced that it certainly isn't interpretation-friendly.

 

[Smart $]

Does anyone know why the code makers went to the trouble to note particular sections of Article 430 in Table 240.4(G)? ...

IDK, but I traced it back to the 1999 edition as not new. Part X was added after that, but probably deemed irrelevant at the time. Then perhaps just a technical correlation oversight when 240.130 was added.

 

[don_resqcapt19]

I don't see the supply to the VFD as being the same as a supply to a motor starter. It would make sense to me of the normal (non-motor) overcurrent protection rules applied to the supply conductors to the VFD.

 

[bozo]

Question: Isn't the OP talking about the issue of protection of the motor leads from the drive to the motor? If so, aren't the Overloads providing this protection? If so, would think the discussion of Breaker ratings would be irrelevant, since I believe their only function in a motor circuit is SC/GF protection. So isn't the issue whether or not the code requirements of 240.4(D) apply in this case and require #10AWG because the overload protection is likely sized greater than 20A?

Does anyone know the genesis of 240.4(D)? The reason I ask is because, to me anyway, it seems counter-intuitive to establish what appears to be conservative requirements for protection in 240.4(D) and then in 240.4(G) say don't worry about motor circuit conductors and on top of that in 430 apparently say you can load that conductor up to 140% of nameplate FLA after sizing the conductors at 125% of table FLA, which seems to be the opposite of conservative.

 

[iwire]

The reason I ask is because, to me anyway, it seems counter-intuitive to establish what appears to be conservative requirements for protection in 240.4(D) and then in 240.4(G) say don't worry about motor circuit conductors and on top of that in 430 apparently say you can load that conductor up to 140% of nameplate FLA after sizing the conductors at 125% of table FLA, which seems to be the opposite of conservative.

Consider the loads that 240.4(D) applies to vs the loads it does not apply to.

240.4(D) applies to a general purpose receptacle circuits. The actual connected load is unknown, may vary and has no other source of protection.

240.4(D) does not apply to motors, a known load of predictable needs and has other overload protection.


240.4(D) applies to lighting circuits. The actual connected load is unknown, may vary and no other source of protection.

240.4(D) does not apply to welders, a known load of predictable needs.

 

[kwired]

Question: Isn't the OP talking about the issue of protection of the motor leads from the drive to the motor? If so, aren't the Overloads providing this protection? If so, would think the discussion of Breaker ratings would be irrelevant, since I believe their only function in a motor circuit is SC/GF protection. So isn't the issue whether or not the code requirements of 240.4(D) apply in this case and require #10AWG because the overload protection is likely sized greater than 20A?

Does anyone know the genesis of 240.4(D)? The reason I ask is because, to me anyway, it seems counter-intuitive to establish what appears to be conservative requirements for protection in 240.4(D) and then in 240.4(G) say don't worry about motor circuit conductors and on top of that in 430 apparently say you can load that conductor up to 140% of nameplate FLA after sizing the conductors at 125% of table FLA, which seems to be the opposite of conservative.

And to add to what Bob said, when we have a motor, we have a conductor sized 125% of motor full load (intermittent duty motors are an exception). We also have overload protection for that motor. If the motor is not overloaded - neither is the conductor, it is inherently protected by the motor overload, but still has protection by the branch circuit device for short circuits and ground faults.

A VFD also provides overload protection so the conductors between the drive and motor are still inherently protected from overload still assuming they are sized to carry 125% of motor full load. They are typically protected from short circuit or ground fault as a side effect of the drive protecting itself from output side short circuits or ground faults.

 

[iwire]

They are typically protected from short circuit or ground fault as a side effect of the drive protecting itself from output side short circuits or ground faults.

Or the SCRs blow open. :weeping:

 

[bozo]

(Originally posted by iwire) 240.4(D) does not apply to motors, a known load of predictable needs and has other overload protection.

Thanks for the reply. Where is the "other" overload protection if an MCP is used instead of a thermal-mag?

 

[bozo]

And to add to what Bob said, when we have a motor, we have a conductor sized 125% of motor full load (intermittent duty motors are an exception). We also have overload protection for that motor. If the motor is not overloaded - neither is the conductor, it is inherently protected by the motor overload, but still has protection by the branch circuit device for short circuits and ground faults.

However, I might be wrong but it appears that you are allowed to bump the OL setting to 140% of nameplate without any corresponding requirement to increase conductor size (at least I couldn't find any such requirement). If this is true, then it seems possible that the OL setting might even be greater than the 75 deg. ampacity of the chosen conductor based on 125% of the table FLA.

 

[ActionDave]

However, I might be wrong but it appears that you are allowed to bump the OL setting to 140% of nameplate without any corresponding requirement to increase conductor size (at least I couldn't find any such requirement). If this is true, then it seems possible that the OL setting might even be greater than the 75 deg. ampacity of the chosen conductor based on 125% of the table FLA.

Yes you can.

 

[kwired]

However, I might be wrong but it appears that you are allowed to bump the OL setting to 140% of nameplate without any corresponding requirement to increase conductor size (at least I couldn't find any such requirement). If this is true, then it seems possible that the OL setting might even be greater than the 75 deg. ampacity of the chosen conductor based on 125% of the table FLA.

If your conductor is already 125%, then this allows 15% over the conductor ampacity, and that is assuming nameplate is same as Art 430 motor current charts you are supposed to select motor current from. t But in reality those charts are generally pretty conservative from the conductor perspective - I wouldn't worry much about the situation being much of a problem.

 

[bozo]

If your conductor is already 125%, then this allows 15% over the conductor ampacity, and that is assuming nameplate is same as Art 430 motor current charts you are supposed to select motor current from. t But in reality those charts are generally pretty conservative from the conductor perspective - I wouldn't worry much about the situation being much of a problem.

Thanks. I realize that nameplate FLA's are usually significantly less than table FLA's, just didn't know if it's possible that in some cases nameplate FLA might be close enough to table FLA that 140% could be a problem.

 

[kwired]

Thanks. I realize that nameplate FLA's are usually significantly less than table FLA's, just didn't know if it's possible that in some cases nameplate FLA might be close enough to table FLA that 140% could be a problem.

What I tried to say was that 310.15 ampacities are already conservative enough that I am not too concerned about overheating a conductor if it happens to carry 140% of the values in the tables, and in reality you are only 15% max over conductor ampacity because you used 125% of motor current as a base to start with.

example: motor current of 12 amps x 1.25 = 15. You will use at least a 15 amp conductor.

overcurrent protection 12 x 1.4 = 16.8.

I wouldn't be too concerned if the 15 amp conductor actually had 16.8 amp overload protection, whether it is code compliant or not, the 310.15 values are not that close to being that critical. If a particular install has some ventilation involved it may run cooler then one that is at 100% with no ventilation.

 

[bozo]

What I tried to say was that 310.15 ampacities are already conservative enough that I am not too concerned about overheating a conductor if it happens to carry 140% of the values in the tables, and in reality you are only 15% max over conductor ampacity because you used 125% of motor current as a base to start with.

example: motor current of 12 amps x 1.25 = 15. You will use at least a 15 amp conductor.

overcurrent protection 12 x 1.4 = 16.8.

I wouldn't be too concerned if the 15 amp conductor actually had 16.8 amp overload protection, whether it is code compliant or not, the 310.15 values are not that close to being that critical. If a particular install has some ventilation involved it may run cooler then one that is at 100% with no ventilation.

Thanks for the clarification. Appreciate the info.