feed through lugs and tap rule

[rszimm]

I have a 125A subpanel fed with a 125A breaker in the main panel (200' away). I have a 100A fused disconnect (with 100A fuses) directly adjacent (less than 10') to the sub-panel fed with feed-through lugs. The load side of the disconnect feeds a secondary 100A sub-panel.

So, I believe the tap rule (240.21(B)(1)) applies. So the question is twofold:

1. Can I use a 100A disconnect here. (99.9% sure the answer is yes)
2. Do the conductors between the sub-feed lugs and the disconnect need to be sized at 100A or 125A? (again, about 90% sure the tap rule allows me to use 100A sizing, but I just wanted to be sure)

 

[electrofelon]

Seems a little convoluted, but sounds code compliant to me. Yes you need full 100 amp conductors (no next size up rule, although for 100 amp next size up wouldnt generally be available for both alum nor copper), and remember EGC sized to the 125 amp feeder.

 

[rszimm]

Seems a little convoluted, but sounds code compliant to me.

It's a workshop / guest house combination. Main (non-switched) subpanel feeds the guest house, the HVAC, the lights and a couple outlets in the workshop (for battery charging, etc). The subpanel on the load side of the disconnect feeds all the "dangerous" equipment in the shop (table saws, band saws, welders, lathes, etc). I wanted a single disconnect that I can easily lock out so if the kids get into the workshop they can't cut their arms off.

I'm open to better ways to achieving the same end that's not as "convoluted"...

And thanks for the info on the ECG. Makes total sense and I probably would have forgotten that one...

 

[infinity]

I have a 125A subpanel fed with a 125A breaker in the main panel (200' away). I have a 100A fused disconnect (with 100A fuses) directly adjacent (less than 10') to the sub-panel fed with feed-through lugs. The load side of the disconnect feeds a secondary 100A sub-panel.

So, I believe the tap rule (240.21(B)(1)) applies. So the question is twofold:

1. Can I use a 100A disconnect here. (99.9% sure the answer is yes)
2. Do the conductors between the sub-feed lugs and the disconnect need to be sized at 100A or 125A? (again, about 90% sure the tap rule allows me to use 100A sizing, but I just wanted to be sure)

1. Yes a 100 amp disconnect is fine.
2. Your tap conductors will be 100 amps as you've stated.

 

[electrofelon]

It's a workshop / guest house combination. Main (non-switched) subpanel feeds the guest house, the HVAC, the lights and a couple outlets in the workshop (for battery charging, etc). The subpanel on the load side of the disconnect feeds all the "dangerous" equipment in the shop (table saws, band saws, welders, lathes, etc). I wanted a single disconnect that I can easily lock out so if the kids get into the workshop they can't cut their arms off.

I'm open to better ways to achieving the same end that's not as "convoluted"...

Ok makes more sense. I think loadcenter type panelboards generally do not have the ability to lock out the main, but you could just subfeed off the first panel with a 2 pole 100, or use a backfed breaker as the main in the 2nd sub and get a LOTO or breaker on/off kit for the breaker. Honestly those things are kinda hokey though, so I wouldn't blame you for wanting to use a safety switch.

 

[rszimm]

Ok makes more sense. I think loadcenter type panelboards generally do not have the ability to lock out the main, but you could just subfeed off the first panel with a 2 pole 100, or use a backfed breaker as the main in the 2nd sub and get a LOTO or breaker on/off kit for the breaker. Honestly those things are kinda hokey though, so I wouldn't blame you for wanting to use a safety switch.

That was exactly my thought. I considered that but decided a lockout breaker would be less convenient than a big safety disconnect switch. Especially when switching it on and off multiple times per day. I also don't know how many switch operations a breaker is rated for, but I know disconnect switches are rated high enough so I'll likely not ever need to care.

 

[kwired]

A contactor and single or even multiple E-stops may be what is convenient. Could even have one with a keyed switch if you want to control potential users. If this is at your home, OSHA doesn't really apply, unless you also are employing people in this shop.

 

[rszimm]

A contactor and single or even multiple E-stops may be what is convenient. Could even have one with a keyed switch if you want to control potential users. If this is at your home, OSHA doesn't really apply, unless you also are employing people in this shop.

Oooooo, I like that idea. I have wired e-stops into some of the more dangerous equipment (metal lathe, milling machine), but a couple system e-stops wired around the shop might be nice.

 

[rszimm]

Oooooo, I like that idea. I have wired e-stops into some of the more dangerous equipment (metal lathe, milling machine), but a couple system e-stops wired around the shop might be nice.

Wow. 100A contactors are expensive!!! Maybe I'll stick with just the switch.

 

[kwired]

Wow. 100A contactors are expensive!!! Maybe I'll stick with just the switch.

I can buy a pump panel with 200 amp fuse holders and disconnect, HOA switch, motor overload, and NEMA size 4 contactor all in a NEMA 3R (would be NEMA 4/12 if not for drain holes) enclosure for less than I can buy the contactor as a replacement part:blink:

Consider the fact you likely won't ever need to supply 100 amps at any given time and maybe you can get by with a 60 amp contactor.

Definite purpose contactors are UL recognized but not listed FWIW. and much less than NEMA contactors. This means a UL panel shop could possibly build you a control panel with one but technically you can't.

 

[jap]

For safety's sake, I think your disconnect idea would be the simplest.

Although, if it were me, and the subpanel was 125a rated like the one that's already there, I'd go with a 200 amp non fused disconnect to feed the safety panel full size wire and forget the tap and fuses.

Just my opinion.


JAP>

 

[jap]

Another danger of most non commercial wood working tools is the fact of standard maintained on off switching.

If the power goes out or a branch circuit breaker trips while the machines are running, most don't remember to shut the switches off.

The auto restart when the breaker is reset or when the power comes back on is a significant danger in itself.



JAP>

 

[rszimm]

For safety's sake, I think your disconnect idea would be the simplest.

Although, if it were me, and the subpanel was 125a rated like the one that's already there, I'd go with a 200 amp non fused disconnect to feed the safety panel full size wire and forget the tap and fuses.
>

I would but a neighbor has a 100A fused safety switch complete with fuses that he would give me for $70. A 200A switch will likely cost me 4x that, and I don’t really need more than 100A.