fused safety switch

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shaw0486

Senior Member
Location
baltimore
Hey Guys,
Would it be illegal to run a 2 # 8 and 1 # 10 MC Cable protected at 35 amps to a 2p/30a/208v fss fused at 30amps. If so what is the code article.

Thanks,
 

nakulak

Senior Member
I don't understand the question. what is the second #8 MC cable doing ?

If you have a 35A 2 pole ckt breaker, and are running a #8/2 MC to feed a disconnect, then fuse a disco at 30A feeding a #10/2 MC cable, you have not violated the code (conductors are protected at their ampacity 240.4) . I don't know if that is what you were saying in the original question though.
 

shepelec

Senior Member
Location
Palmer, MA
No it is not illegal, you can feed a 30A disconnect with a higher ampacity.

The breaker is there to protect the feeder not the disconnect.
 

ASG

Senior Member
Location
Work in NYC
Occupation
Electrical Engineer, PE
If you think about it, you probably only want to use 30A of your 35A CB anyway.
 

iwire

Moderator
Staff member
Location
Massachusetts
...I don't think so...equipment needs to be protected, too.

Fused disconnects are not required to be protected at their ampacity, the fuses contained inside the disconnect will ensure the switch cannot be overloaded.

It is very common to run a large feeder to a bank of fused disconnects.

You might have a 400 amp feeder tapped with a 60 amp disconnect.
 
It's Legal and Sometimes Necessary

It's Legal and Sometimes Necessary

We had a project a few years ago that had loads that appeared to need a 208V 1-phase 30A Branch Circuit for each machine (machine came UL Listed with 6 Ft 10-3 SO Cord and L6-30P Twistlock Plug). After installing 12 circuits for these, they started nuisance tripping the 2P 30A CBs (Cutler-Hammer BAB2030 Bolt-On CB) on a frequent basis. After checking circuits, relocating machines (I had 12 to play with for deductive elimination) the problem wouldn't go away. So, I installed a 2P 30A Fused Switch next to the souce panel, with 30A Time Delay (Rk5) Fuses. Then changed the 2P CB to a 3P CB, and left the (+/-) 6 Ft. of #10 from CB to FSS, and used the (E) #10 branch circuit on the load side of the FSS. No more tripping! The fuse time delay solved the tripping problem. So I did all 12 ckts. this way.

Note: Later that year, customer added another 12 machines. So we made the feeder CBs 3P 100, with #2 into a gutter, and tapped 6 2P FSS per 100A Feed. Was able to fit all into the existing 400A Panel, and no nuisance trips.
 
We had a project a few years ago that had loads that appeared to need a 208V 1-phase 30A Branch Circuit for each machine (machine came UL Listed with 6 Ft 10-3 SO Cord and L6-30P Twistlock Plug). After installing 12 circuits for these, they started nuisance tripping the 2P 30A CBs (Cutler-Hammer BAB2030 Bolt-On CB) on a frequent basis. After checking circuits, relocating machines (I had 12 to play with for deductive elimination) the problem wouldn't go away. So, I installed a 2P 30A Fused Switch next to the souce panel, with 30A Time Delay (Rk5) Fuses. Then changed the 2P CB to a 3P CB and left the (+/-) 6 Ft. of #10 from CB to FSS, and used the (E) #10 branch circuit on the load side of the FSS. No more tripping! The fuse time delay solved the tripping problem. So I did all 12 ckts. this way.

Note: Later that year, customer added another 12 machines. So we made the feeder CBs 3P 100, with #2 into a gutter, and tapped 6 2P FSS per 100A Feed. Was able to fit all into the existing 400A Panel, and no nuisance trips.

Oops--meant to say changed 2P 30A CB to 2P 40A CB.
 
Reasons

Reasons

So, the problem was really that the 30a breakers were too small. Couldn't you have used the 40a breakers without the fuses?

(A) Equipment was a "robot" used to move heavy test heads among CHIP testers. It came with a UL Label, stating 208-240V 1 Phase 24A, Max OCPD 30A (I guess because it was a combination of a large pump, smaller motors and a lot of control circuitry), and as I stated came factory wired with 10/3 SO cord and L6-30P T-L Plug.After lots of testing for max inrush, it became apparent that when the robot was lifting weight very close to it's max. load rating, the inrush to the pump (if the other small motors were activated simultaneously) exceeded the trip curve of the 2p 30a CB, but not the 2P 40A CB or the RK5 30A Fuses.
(B) Design was orginally a 2p 30 CB, 3#10 (2 phases plus ground) to a 2P 30A NFSS (req'd by customer's own Lock-Out Tag Out Safety Regs) and then to a L6-30R T-L recept below it. Customer wanted to be able to plug in 3-4 different types of equipment on this circuit, depending on what was being tested in the adjacent test bay.
(C) The cost of the 2P 30A FSS, 2 RK5 fuses, and a 2P 40A Bolt on cb, plus labor to wire it in, was about $250.00 (2002 prices). The cost of new 8/3 SO Cord, strain relief, 2P+grd matching pin and sleeve plug, receptacle, and back box, new 2P 60A NFSS, plus labor was a lot more, and wouldn't interface with all the other L6-30P connections on their other equipment.

Yes, you can wire a motor circuit with a 2P 40 and use #10, but I don't think all the NFSS/Recept./Plug (and is 10-3 OK for this use?) downstream devices would pass inspection on a 40A circuit.

I originally posted just to have an example of a situation where a larger CB can protect a 2P 30A FSS. I guess I gave too many details but not enough of the reasons why the choices were made.
 
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