Motor circuit

[tomito]

Hello guys, I have a question and I hope you can help me, when you are selecting your wires for a motor circuit, you select it by 1.25 of table 430.250.
When you calculate feeders or branch circuits you use 125% of continuous current plus 100% of non continuous or conditions of use (adjustment and/or correction factors), whichever is higher.
The question is: It is the same for motors circuits? the higher of 430.250 *1.25 or adjustment and correction factors? or you have to apply both? continuous and correction?

In a totally unrelated question, I have seen in a lot of mining projects that the steel structure of pretty much everything gets connected to ground (4/0 wire) and they connect the panelboards to ground too.
It is NEC complaint to connect the panel boards to this ground when the feeder already has an EGC? I would say no because that would create parallel ways for the fault current and besides 250.4 (A) (5) says that "The earth shall not be considered as an effective ground-fault current path". What do you think?

Tom

 

[tom baker]

Mines are under the MSHA and may not follow the NEC. The issue with building steel for an EGC is section 300.5 and the increased impedance.
I worked on a pump station where all the motor frames had heavy copper grounds, it was built in 72, maybe it used old specs

 

[Master Nater]

tom from mexico -

when you are calculating your circuit ampacity, you have to take the horsepower and voltage from the tables in the back of article 430. then, you need to apply adjustment factors to that ampacity, such as whether it is a continuous duty usage (by multiplying by 125%). once, you figure your circuit ampacity, then you can begin selecting your conductor sizes. that is done back in article 310. and there are multiple derating factors that may apply, depending upon a number of factors, such as ambient temperature, the number of conductors in a conduit, and conductor temperature rating, as well as termination temperature ratings.

on your unrelated question, we do two things when we're grounding and bonding.

1 is grounding - attaching everything metallic that is non-current carrying, as well as any grounded current carrying condcutors, to the earth, in order to dissipate any static electricity buildup back to the earth. and...

2 - bonding - connecting everything metallic that is non-current carrying together, to provide a fault path back to the source to clear any overcurrent devices in the event of a ground fault, and disconnect power as quickly as possible.

we do both of these nearly at the same time, but they are two entirely separate things.

 

[ramsy]

is the same for motors circuits? the higher of 430.250 *1.25 or adjustment and correction factors? or you have to apply both? continuous and correction?

As Tom Baker notes your jurisdiction may adopt MSHA for mines, which may simplify everything.

If NFPA-70 is adopted, then some cross referencing shows both are required for motors.

210.19 is amended by 430.22 & .24, 125% for motor wiring, per Table 210.3 Specific-Purpose Branch Circuits.

Conductor ampacity adjustments & corrections also apply starting with 310.14, since 310.1 Scope only excludes wiring integral to equipment.

 

[kwired]

n a totally unrelated question, I have seen in a lot of mining projects that the steel structure of pretty much everything gets connected to ground (4/0 wire) and they connect the panelboards to ground too.
It is NEC complaint to connect the panel boards to this ground when the feeder already has an EGC? I would say no because that would create parallel ways for the fault current and besides 250.4 (A) (5) says that "The earth shall not be considered as an effective ground-fault current path". What do you think?

NEC doesn't prohibit EGC bonding jumpers to pretty much anything imaginable, the whole idea is equalizing any voltage potential between bonded objects. In an all steel structure situation wire type jumpers are often redundant as everything is already bonded together by being bolted/welded/etc. to one another.

What NEC doesn't want is having current carrying grounded conductors (typically neutral conductors) bonded beyond the service equipment or the first disconnecting means of separately derived systems which will minimize grounded conductor currents from flowing across non current carrying components and possibly creating "stray voltages"

An EGC is not supposed to carry current other than in abnormal conditions and those conditions generally will be very limited in time as the idea is to allow for high enough current to flow that it will help cause the overcurrent device protecting that circuit to trip.