circuit breakers and wire sizing from distribution blocks

[Brent S in Cincinnati 0]

My question is, even though I know it is acceptable in an enclosure to size the conductors to the load protection (motor protectors here) isn't there a minimum ratio ( 10:1 ) of the main fuse size to the current capacity of the conductors coming off of distribution blocks? I haven't been able to find this in the code. I usually build 1 and 2 door PLC panels with servos and vfds, so I'm trying to be careful with the 8 door vfd enclosure I'm putting together. I have a main disconnect for 3 phase 460VAC with 350 Amp fuses. This feeds a distribution block where the conductors step down to #1 AWG. From there I supply each of 4 distribution blocks, 1 for each pair of doors. From there I have #10 AWG feeding groups of 5 motor protectors through bus bars. In this setup the #10s are fed directly from 350 Amp fuses. It seems a little extreme to me. There are 154 Powerflex 40 vfds, mostly 1 hp. Each Powerflex is fed from its own Motor Protector, AB 140M-C2E-B40 for the 1hp vfds.

[ssommers 20]

For my own safety & sanity's sake, I'd put in some intermediate fusing. Since you have multiple panels, why not have a fuse set before each distribution block? This will give an electrician an isolation point for troubleshooting too. HTH! Susan

[BC Steve 0]

I agree with Susan. It will also prevent those #10 wires from melting if there is a problem with one of those motors. BC Steve

[TConnolly 13]

The problem is that you have a distribution block feeding a distribution block. NEC 240.21 appears to prohibit that unless there is an overcurrent protection device between the distribution blocks. The tap rule is intended to let you use smaller feeder wires to secondary protection devices instead of running full sized wire to the secondary protection (which would be unreasonable in some cases) but it was not intended for tapping multiple distribution blocks to a master block without protection. NEC240.21(B) governs the use of feeder taps without overcurrent protection devices however NEC240.21 also states "No conductor supplied under the provisions of 240.21(A) through (G) shall supply another conductor under those provision, except through an overcurrent protective device..." For example, if your PDBs and breakers are rated at 75C, you may feed a 350 A distribution block with 500 MCM wire and use #1 wire from the PDB to feed 125A breakers (so you don't have to run 500MCM to the breakers, which would be ludricous because the breaker terminal is probably not large enough). Then each of those 125A breakers could attach to a secondary PDBs using #1 wire and each of the secondary PDBs might feed the motor protection with #12 wire instead of #1 wire. I attached a picure of the relevant section (its large so you may have to save it off to view it legibly) Edited 19 Mar 2008 by Alaric

[paulengr 44]

I fail to see how you are protecting anything. If I understand you right, you are using 350 A fuses to protect #1 wire, which is rated to 100 A even with a 75 C rating. So far so good assuming that you are protecting motor circuits (350% rating) EXCEPT that first off you're not (there are VFD's in there), and second, you didn't derate everything by the 80% rule since it's a motor system. Now you go to #10 wire. This is only capable of about 24 A at most depending on which chart you are looking at (my good one is at work). You have ZERO protection for your #10 feeders. The only way that this would be legal is if you fed a SINGLE downstream source with 5 of them as parallel feeders. A motor protector is a secondary protection device. It is totally illegal for the purpose of protecting the VFD. A real protective device is required as primary protection. The proper device is either a real circuit breaker or a real fuse. You don't even need motor protectors in this system because the VFD itself provides motor protection (built-in overcurrent limiting and overload tripping). If you had fused the #10 wire correctly in the first place, this could serve as your VFD protection and you'd be done. No need for expensive secondary protection devices that are not doing you any good at all.

[TimWilborne 108]

We might be saying the same thing here Susan, it's been a long day... I prefer to have the fuses right at the VFD, especially in these multi-panel enclosures. Your diagnosing a drive problem...no power...let's check the fuses...I think it's this block...no this one...wait, it's behind this door, not that one.................

[ssommers 20]

Amen TW! When it comes to diagnosing which motor has shorted/died/blown up, fusing as close to the drive/motor is indispensable. Alaric - Thanks for the code references. I knew there had to be something in writing since the kind of wire reduction that Brent S was talking about could end up creating a fire. Brent - Another thing to consider... How will your electrician know which VFD controls which motor and is in which cabinet? If this was my project, I'd be putting a placard on the outside of each door so I didn't have to guess whether VFD 40 for MTR 40 was behind door # 2 or door # 3. A simple block diagram showing the fusing scheme would also be a good thing to laminate & post on the inside of each panel door. This way, an electrician just chasing fuses or circuit breakers won't have to go find the entire drawing package first.

[HiGhVoLtAgEgUrU 0]

We usually fuse each device with proper rating of the motor. It makes it much easier to troubleshoot. Also if you have to replace the drive, you can just pull the fuses for just that drive instead of doing it live or shutting 20 others devices so you can work on one. So I usually try to fuse each device.

[stickman47 0]

Keep in mind that the VFD primary amperage is generally not the same as the motor it is driving. In my experience I had a 10 hp Woods drive that required 25 amp primary fuses but the motor was FLA at 6.3a. Stickman