Fuse Block / Fuse Wire Sizing

I see not difference in this installation than a 400 amps main switch with 225 amps fuses. The code only requires you to install a conductor rated at 225 amps. It does not require a 400 amp conductor based on the possibility that some one in the future might change the fuses to 400 amps. Is this what you were thinking?

NEC 240.4 (B)(2), page 70-83 (2008) has a guideline for the conductor protection based on fuse rating, not the fuse block (fuse holder) rating. So, if you use the 225A fuse and conductor ampacity is above 225A or immediate below 225A, it should be fine.

- MS

225A would likely be too small of a fuse for a 205A rated VFD, but that's just me.

The NEC requires that wire be sized for a minimum of 125% of the max amp rating of the drive. Fuses should then be sized for the wire. So you need conductors rated for 256A minimum. Fuses could be smaller but a VFD can draw up to 150% current for up to 1 minute, so make sure that fits under the fuse curve. In other words, look at the fuse curve and look at 308A (150% of 205), then follow across the time axis at 1 minute to make sure that you have not crossed the fuse melt curve.

Personally I would start with 250A fuses and wire before I even start looking at the fuse curve. Copper is not as expensive as down time.

Most major name VFDs such as Siemens are now rated as "Short Circuit Protective Devices" for their LOAD, so the only thing the fuses do now is protect the conductors leading to the VFD and the VFD front-end (converter).

That is my understanding; that motors and drives require the wire to be sized to 125% of the load.

But...I cannot find the specific section that says this.

Rick S.

Your concern is now different. You original question was 'is the wire sized for the maximum capacity of the fuse block or the fuses installed'. Now your concern is about 'motors and drives require the wire to be sized to 125% of the load'.

Here is the NEC reference for 125% cable size for VFD and Motor:

VFD (or Adjustable Speed Drive): NEC 430.122

Motor: NEC 430.22

The previous thread http://cr4.globalspec.com/thread/36311/Sizing-Circuit-Breakers may help for sizing the cable and circuit breaker (or fuse).

- MS

You are correct, my question morphed. It all seems to depend on which direction you are looking at the circuit from, if you start at the motor and work your way back to the feeder fuses you end up with one size of wires and fuses but if you start with a mfg recommendation for the fuses and look at the circuit from the feeder to the motor you end up with another set of sizes.

I guess that is the primary problem here. Our electrical production has "decided" that all cable is underrated by some percentage and that matching the current on a fuse to the current rating of the cable to the "rated" current of a drive is acceptable.

This is why the question asking the location of the standard and why all the help is so greatly appreciated.

Pay close attention to that NEC section. The 125% applies to the MAX AMP RATING of the VFD, not the load connected to it.