240.4 and motor circuits

Status
Not open for further replies.
S

SteamDonkey

Member
Hi,

An overcurrent coordination study identified NEC 240.4 violations in circuits from switchboard breakers to VFDs and starters on a project I'm reviewing. The installation meets requirements of Article 430 so believe it complies with 240.4(G). The part that's a little concerning to me is that motor branch circuits are required to have overload and short circuit protection per 430.31 and 430.51, but feeder circuits are only required to have short circuit protection (430.61). Do feeders serving motor loads need both overload and short circuit protection to meet the overcurrent protection requirements of 240.4?

Two examples:
200A breaker - #2/0 CU conductors - Motor Starter - #2/0 CU conductors - 124FLA motor.
400A breaker - #350 CU conductors - VFD (240A input rating) - #350 CU conductors - 240FLA motor.

Does anyone see a violation of 240.4 here?

Thanks
 
I may be overlooking something obvious, but I see no problem.
 
SteamDonkey said:
Hi,

An overcurrent coordination study identified NEC 240.4 violations in circuits from switchboard breakers to VFDs and starters on a project I'm reviewing. The installation meets requirements of Article 430 so believe it complies with 240.4(G). The part that's a little concerning to me is that motor branch circuits are required to have overload and short circuit protection per 430.31 and 430.51, but feeder circuits are only required to have short circuit protection (430.61). Do feeders serving motor loads need both overload and short circuit protection to meet the overcurrent protection requirements of 240.4?

Two examples:
200A breaker - #2/0 CU conductors - Motor Starter - #2/0 CU conductors - 124FLA motor.
400A breaker - #350 CU conductors - VFD (240A input rating) - #350 CU conductors - 240FLA motor.

Does anyone see a violation of 240.4 here?

Thanks
Click to expand...
First things first, 240.4 does not apply here. 240.3 directs you to Art. 430 rules for this application.
Let's look at the first example:
Assuming you got the 124 amp value from Table 430.250 (as required-not the nameplate) this must be a 480 V, 100 HP motor.
The branch circuit conductor has to be 124 X 125% or 155 amp. This would be a 2/0 @75 degree. Good there.
The breaker can be up to 250% (Table 430.52) of the 430.250 table value. 124X 250% = 310 amps. Next Std. size is allowed. This would be an allowed 350 amp breaker. The 200 amp breaker you indicate is installed may, depending on the type of driven load, not be sufficient to start the motor, but compliant if it works.
The overload protection is provided by the starter and is sized to the nameplate current value (430.32 for details).
The key here is that for motors and a number of other applications the breaker and conductor do not have to "match" as the breaker is only protecting the conductors from short circuit and the overload device protects from overload.

Pressed for time here...have to let someone else help with the VFD. There are some different things to consider for that. I'm sure others will check my hasty math for errors.:)
 
texie said:
First things first, 240.4 does not apply here. 240.3 directs you to Art. 430 rules for this application...
Click to expand...

I think they both apply, but protection of conductors (240.4) is the issue I'm asking about. 240.3 pertains to protection of equipment.
 
SteamDonkey said:
I think they both apply, but protection of conductors (240.4) is the issue I'm asking about. 240.3 pertains to protection of equipment.
Click to expand...

Yes, poor choice on my part. My point is that you use Art. 430 as opposed to Art. 240 for this application. I should have said that 240.4(G) directs you there.
As you can see from the calculations, if anything the 200 amp breaker is likely not large enough for most applications. This could be a big deal to change as it may be likely that it will be a larger frame size.
 
Status
Not open for further replies.
Top