- Location
- Connecticut
- Occupation
- Engineer
Well, no. If you had a 40A MLO panel tapped to your 125A feeder, you would be violating 408.36.
10 foot tap rule
- Thread starter wyboy
- Start date
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Well, no. If you had a 40A MLO panel tapped to your 125A feeder, you would be violating 408.36.
kwired
Electron manager
- Location
- NE Nebraska
What if the 125 A MLO panel with 40 amp tap conductor for supply only has two 20 amp breakers installed in it?
Is more clear with the 25 foot tap rule it that the tap must end at a single overcurrent device.
Is more clear with the 25 foot tap rule it that the tap must end at a single overcurrent device.
- Location
- Connecticut
- Occupation
- Engineer
Doesn't meet the requirements of 240.21(B)(1). Just make it a 40A MCB panel, and the problem is solved.
kwired
Electron manager
- Location
- NE Nebraska
Some of what I am getting at is questionable with the 25 foot tap rule also - that is a single breaker enclosure that still has lugs and plug on buss to land a breaker. Can those not be used at the load end of a feeder tap? The 10 foot tap rule doesn't exactly say it must land in an individual overcurrent device like the 25 foot tap rule does, yet if you consider everything it does say in the 10 foot rule it would be pretty rare not to land in an individual overcurrent device. But with the single (but multipole) breaker enclosures you typically supply the bus instead of the overcurrent device so on technicality you are not landing on an overcurrent device.
This is not ordinarily a problem for over 100 amps because a single breaker enclosure usually holds a breaker with lugs on each end, but for under 100 amps a majority of the time you use a plug on breaker in panelboard with just one space per pole. If you backfeed it you could get around this issue - but they don't make hold down kits for those enclosures that I know of.
Carultch
Senior Member
- Location
- Massachusetts
You are also not "technically landing on an overcurrent device", when you land the tap conductors on the line side of a fused disconnect. Because the tap conductors land on lugs, which are then connected to the blade of the disconnect first, before connecting to the single set of fuses for the load side lugs. Yet this is a very common practice with tap conductors, and that is precisely where you should land the tap conductors when using a fused disconnect.
I think the general idea of landing "on an overcurrent device", is landing in the device that is dedicated to the overcurrent device and disconnecting means.
kwired
Electron manager
- Location
- NE Nebraska
That is how I look at it, but one can pick it apart and say otherwise. A single breaker enclosure with plug on method of supplying the breaker does fit the definition of "panelboard". (I think)
The fused switch situation is a little more complicated because you also are supposed to have a disconnect ahead of fuses in most instances so that you can remove/install the fuses while not energized.
I believe you are hitting on the exact reason the clause "equipment containing an overcurrent device(s)" was used in the latest edition of 240.21(B)(1)(1b).
kwired
Electron manager
- Location
- NE Nebraska
So can a 10 foot feeder tap of 40 amp conductor supply two 20 amp overcurrent devices on a common supply bus?
I believe it does as long as the equipment rating is not greater than the tap conductor ampacity... 40A in this case... and the calculated load is not greater than the equipment rating.
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
I'm inclined to agree with you and kwired. I think the hang-up for most, and as the OP stated, we all just assume that a feeder tap has to end in a single OCPD. The addition of the "s" in parenthesis after the word device in the 2014 edition seems to imply that it would not have to be a single OCPD. While it is common to see and allowed up to six OCPD (the sum total not exceeding the value allowed for a single OCPD) on a transformer secondary, in the wild we don't seem to see this for a regular feeder tap.
Maybe this begs a further question. If multiple OCPDs can be used, would it be limited to 6 as in a transformer secondary? Is that stated? For example, what if you had a 200 amp tap going to a 200 amp MLO panel with 10, 20 amp breakers in it?
- Location
- Connecticut
- Occupation
- Engineer
408.36?
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
Well gee, you're probably right. But seems to raise another question. 408.36, Exception 1 would seem to say that we also could not do this even if we only had 6 or less as 230.71 would not apply here. Using this logic you couldn't use it for the 6 OCPD allowance on a transformer secondary either. Seems like we are back to requiring a single OCPD. ?? The more I think about the original OP question and all the replies the more questions I seem to have.
- Location
- Connecticut
- Occupation
- Engineer
I don't know where you are getting a 6 OCPD allowance for transformer secondary conductors. I think you are confusing the transformer secondary protection in 450.3 with the secondary conductor protection in 240.21(C).
texie
Senior Member
- Location
- Fort Collins, Colorado
- Occupation
- Electrician, Contractor, Inspector
No, I didn't mean to confuse the two as you are right there is a difference. I guess I'm saying that since the same change was made in language to 240.21(C)(2) as was made in in 240.21(B)(1) it would not be possible to use multiple OCPDs due (as I think you pointed out early on) to 408.36.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
I suppose maybe I'm not thinking of some other kind of device besides panelboards, but 408.36 seems to effectively nullify the language in 240.21 B b 1.
What else would apply. Switchboards?
What else would apply. Switchboards?
Panelboard was included in the origin ROP substantiation I qouted earlier, as was a fusible switch, a switchboard, and a motor control center. CMP made no comment on any. FWIW, a switchboard is not required to have line-side protection whereas a panelboard is, and I don't believe an MCC is either.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Ergo, CMP10 not paying attention to what CMP9 has done.
It certainly makes little sense. So I have a tap to a 125A MLO panelboard, conductors sized to 125A, and then I put in a 200A panelboard because the supply house had it cheaper on clearance ... surely I've not created any additional danger.
It certainly makes little sense. So I have a tap to a 125A MLO panelboard, conductors sized to 125A, and then I put in a 200A panelboard because the supply house had it cheaper on clearance ... surely I've not created any additional danger.
Indirectly you have facilitated a potential overloading of the tap conductor. That's not saying you have or will, for you know the limitation of the tap... but the next guy in does not unless he's really on his toes. Notifying durable signage would help...
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
The next guy could just as easily overload a typical 125A panel. Indeed I'd say the ease of that happening arguably depends more on how many extra breaker spaces are in the panel than it does on the panel's rating.
With that said, I'm inclined to grant that your insight speaks to the larger point: having a main breaker in the panel, rather than wiring it MLO, goes much farther towards ensuring that the next guy doesn't overload a tap conductor without thinking about what he's doing three times. So I'll not defend that a tap should be allowed to land in an MLO panelboard with respect to it being just as safe.
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