tap rule for sub-panel

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bbanzai

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Clearfield Utah
Can I feed a sub panel from the unused main lugs on a 100 amp 120/240 panel that is being protected by a 2 pole 100 amp breaker? Do I use the 10 foot or 25 foot tap rule? How large of a sub panel can I use? Help!
 
infinity

infinity

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Re: tap rule for sub-panel

If you were to use a 100 amp bus, MLO subpanel with 100 amp conductors you wouldn't need either tap rule since you wouldn't have a tap. Otherwise the rules of 240.21 would apply.
 
iwire

iwire

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Re: tap rule for sub-panel

You can use the unused lugs t feed a sub panel.

You have some options.

Using the tap rules you could install a sub panel with a main breaker with any rating under 100 amps, but the sub panel would need to be close to the main panel.

Say 6 awg copper feeding a 60 amp main breaker sub panel.

Or you could install a 100 amp or larger main lug only sub panel feed it with 100 amp minimum rated conductors as far away from the main panel as you want.

As far as the maximum rating sub panel you can install, there is no limit.

You could install a 1000 amp sub panel on a 100 amp service. :p
 
B

bbanzai

Member
Location
Clearfield Utah
Re: tap rule for sub-panel

Thanks guys, sounds like the smartest thing to do is just feed a 100 amp mlo panel off the unused lugs, this will eliminate having to replace the existing panel. This is for a addition to an older home with a small 12 space 100 amp panel.
 
LarryFine

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
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Re: tap rule for sub-panel

The smallest sub-panel I'm familair with is rated at 125 amps. If you use cable capable of the 100 amps, such as #2 AL, then you have no concern, except cost, which is where distance comes in.

How much lower than 100 amps are you considering? As long as you stay within 1/3 of the OCP, which means a 30-amp feeder is too small, tap rules may apply.

Tap rules say "240.21(B) Feeder Taps. Conductors shall be permitted to be tapped, without overcurrent protection at the tap, to a feeder as specified in 240.21(B)(1) through (5)."

Notice it says "Conductors shall be..." I don't know if that applies to lugs in a panel.
 
iwire

iwire

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Location
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Re: tap rule for sub-panel

Originally posted by georgestolz:
If you're pulling in conductors anyway, might as well size it to the OCPD, unless there's a compelling reason not to, IMO. :)
Click to expand...
I agree with the Honored Member from Fort Collins. :cool:
 
M

MasterMike

Member
The Unknown

The Unknown

OK Guys, I was recently turned down on a design build where I installed a 200-amp OCP and used the 10' tap rule where I tapped 4/0 awg from the main lugs of a 400-amp panel downsteam of a 400-amp disconnect. There were a spare set of lugs for this and we used them.

The point is I am still questioning this application, the city is ruling this as a feed-thru in lieu of a tap.

Please help me either confirm this issue or point me into the right direction.
 
infinity

infinity

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MasterMike said:
OK Guys, I was recently turned down on a design build where I installed a 200-amp OCP and used the 10' tap rule where I tapped 4/0 awg from the main lugs of a 400-amp panel downsteam of a 400-amp disconnect. There were a spare set of lugs for this and we used them.

The point is I am still questioning this application, the city is ruling this as a feed-thru in lieu of a tap.

Please help me either confirm this issue or point me into the right direction.
Click to expand...

IMO it would only be a feed through if you didn't have the 200 amp OCPD.
 
M

MasterMike

Member
Mr Infinity,

I agree with the consept but the issue I have is "where in the code" can I find this. There is no mention of Taps not being able to be made on the main bus of a panelboard, switchboard, and disconnect. It seems that in fact a tap can be made in just about any situation.

I have study this issue for the last week and have sought input from others, we have all come to the same conclusion that the tap rule would apply.
 
H

hillbilly

Senior Member
Just a hunch. If the panel that you are supplying with the tap is a lighting and appliance panel, it has to have overcurrent protection on the supply side. 408.36
steve
 
infinity

infinity

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hillbilly said:
Just a hunch. If the panel that you are supplying with the tap is a lighting and appliance panel, it has to have overcurrent protection on the supply side. 408.36
steve
Click to expand...


Don't almost all taps require an OCPD?
 
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H

hillbilly

Senior Member
Yes they do. The point that I was trying to make is that if he was feeding a lighting and appliance panel board, it has to be protected (on the line side) by breakers or fuses having a rating not greater than that of the panel board. If he tapped off of 400A main lugs to a lighting panel, the lighting panel has to be rated at and fed with 400A, OR has to have OC protection (on the line side) equal to or less than the rating of the lighting sub-panel.
steve
 
infinity

infinity

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New Jersey
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hillbilly said:
Yes they do. The point that I was trying to make is that if he was feeding a lighting and appliance panel board, it has to be protected (on the line side) by breakers or fuses having a rating not greater than that of the panel board. If he tapped off of 400A main lugs to a lighting panel, the lighting panel has to be rated at and fed with 400A, OR has to have OC protection (on the line side) equal to or less than the rating of the lighting sub-panel.
steve
Click to expand...


I agree with one exception, a properly sized connection to a 400 amp rated panel would not be a tap.
 
M

MasterMike

Member
Still Unsure

Still Unsure

Hillbilly,
The Tap is made from the main lugs of a 400-amp panel which has a 400-amp main service disconnect as PROTECTION.

The Tap was made on the 400-amp panel main lugs and feed a 200-amp rated Breaker in a enclosure. The load of the 200-amp breaker feeds a transfer switch then on to feed other panels.

The point was that the wire is not fed thru the bus, thus it would not be a feed-thru installation, it would be a tap.

???
 
J

jim dungar

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Wisconsin
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PE (Retired) - Power Systems
Article 2140 has a very specific definition of what it considers a tap. Not all "junctions" of conductors are taps (for example, #12 AWG pigtails for 15A or 20A receptacles would not be considered taps according to 240.2 ).

Basically, if the conductor is rated at least as large as the upstream OCPD the connection is not subject to the tap rules.

The NEC does not address terms like feed-through or sub-feed, it only cares about connections and taps. Panelboard bus bars are fully rated so they always have the same rating even if they are of a feed-through design.
 
radiopet

radiopet

Senior Member
Location
Spotsylvania, VA
The TAP is not the problem.....The problem is the 4/0 AL used to tap to the main lugs....in the 10' tap rule the wire must be sized to meet the size or rating of the device the taps are going to...in this case 200A panel so the TAPS would need to be 3/0 CU or 250 KCMIL

Again the wire that is the TAP must be able to handle the ampacity of the rating of the device...
 
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