Feeder to MCC

[wyboy]

Regarding a feeder to an MCC on a set of plans. The feeder is 350 kcmil, the overcurrent is 400 amps. According to 430, the feeder may be sized at 125% of the FLC of the largest motor and the sum of the FLC of the remaining motors. The overcurrent device protecting the feeder is sized at the largest protective device feeding a branch circuit for a motor, and the sum of the FLC of the other motors. So depending on these calculations, this could be a legal install even though 350 is rated at 310 amps on table 310.16?

 

[markstg]

I thought MCC feeders and overcurrent protection was sized per NEC section 430.94

 

[don_resqcapt19]

The rule in 240.4(G) that lets you use an OCDP with a rating higher than Table 310.16 ampacity only applies to parts III, IV, V and VI of Article 430. MCCs are in part VII. You will have to use a conductor with a ampacity of 400 amps for this application.

 

[weressl]

The rule in 240.4(G) that lets you use an OCDP with a rating higher than Table 310.16 ampacity only applies to parts III, IV, V and VI of Article 430. MCCs are in part VII. You will have to use a conductor with a ampacity of 400 amps for this application.

So what would you do if your connected load and calculated demand factor only requires 250A, but the minimum standard size MCC is 400A?

Which rule do you use that dictates that you can't provide a 250A OCPD and wire sized accordingly?

 

[raider1]

So what would you do if your connected load and calculated demand factor only requires 250A, but the minimum standard size MCC is 400A?

Install a 250 amp breaker and appropriate sized conductors for the 250 amp breaker

Which rule do you use that dictates that you can't provide a 250A OCPD and wire sized accordingly?

There is none, you did not read the original post and Don's response very close.

The original poster was using a 400 amp breaker and wanted to use 350 Kcmil conductors with an ampacity of 310 amps which would be a violation of 240.4.

Don was pointing out the 240.4(G) does not apply to motor feeders in part II of Article 430.

Chris

 

[petersonra]

So what would you do if your connected load and calculated demand factor only requires 250A, but the minimum standard size MCC is 400A?

I would run bigger wire unless there was some really serious cash crunch or other reason not to.

You can never have enough juice. Eventually it will be all used up. It's like spare control wires. Fill the conduit up with them.

 

[weressl]

I would run bigger wire unless there was some really serious cash crunch or other reason not to.

You can never have enough juice. Eventually it will be all used up. It's like spare control wires. Fill the conduit up with them.

I routinely open up junction boxes that were installed 20+ years ago with 30% spare wires and not a single is used up, so it is not necessarily true.

We routinely oversize large distribution points yet install 1/3 or 2/3 of the required wire ampacity and set the feeder protection accordingly. Some were never expanded, some went from 1/3 to 2/3, so we pulled an additional parallel feeder. It is called managing your capital. If you let underutilized capital sit there in the form of installed wire cost, it means that you could not use that cash to install something that actually earns money on your spent money.

 

[don_resqcapt19]

...
We routinely oversize large distribution points yet install 1/3 or 2/3 of the required wire ampacity and set the feeder protection accordingly...

But in the original post that was not being done. A feeder with an ampacity of 310 amps was being protected at 400 amps.