Tap Rule Interpretation

[dscottbrown]

A colleague and I are having a disagreement concerning the tap rule (see 2002 NEC 240.20). We are working on a project where we have a distribution panel with a 225 Amp branch breaker. From this breaker we are feeding a 225 Amp MLO Panel (three phase, four wire) with #4/0 feeders. At the first panel we are tapping off and feeding a small panel beside the 225 amp panel. This second panel is a 100 amp MLO panel. My colleague argues that the tap rule allows him to have size #3 feeders (taps of less than ten feet) to the 100 amp panel with no overcurrent protection other than the 225 amp breaker in the distribution panel that is protecting the large feeder. My argument is that the 100 amp panel needs to be equipped with a main circuit breaker to protect the taps per NEC 240.21.B.1.1.a (sorry that I haven't used proper punctuation in the reference, but they keep changing it on me). Am I wrong or do I need to insist on a change in the design. Let me thank all who answer in advance.

Scott

 

[iwire]

Re: Tap Rule Interpretation

Change the 100 MLO panel to a 100 amp main breaker panel and you will be OK.

The way you describe it you have a couple of violations.

225 amp OCP for a 100 amp panel.

225 amp OCP for 100 amp conductors.

 

[don_resqcapt19]

Re: Tap Rule Interpretation

240.21(B)(1)(1)(b) permits the MLO panel. The panel is a device and the #3 is rated at 100 amps. The only other question is that the computed loads in the 100 amp panel is 100 amps or less.
Don

 

[iwire]

Re: Tap Rule Interpretation

OK I stand corrected.

I would have thought they where taking about a device you could not draw more than the rating from, like a receptacle.

[ November 07, 2003, 05:42 PM: Message edited by: iwire ]

 

[websparky]

Re: Tap Rule Interpretation

I agree with Don and Bob!

Please read:
240.21 (1)
(1)The ampacity of the tap conductors is
(a.) Not less than the combined computed loads on the circuits supplied by the tap conductors

FPN:For overcurrent protection requirements for lighting and appliance branch-circuit panelboards and certain power panelboards, see 408.16(A), (B), and (E).

 

[bob]

Re: Tap Rule Interpretation

Don is correct in his response. Remember that the code is the minimum requiremet necessary. A properly engineered design does not always call for the minimum. My preference would be to use the 100 amp main breaker for the panel.

 

[iwire]

Re: Tap Rule Interpretation

I am still confused or you could say doubtful

Why did they put this FPN under 240.21(B)(1) if not to remind us that panels need to be protected at their rating?

FPN:For overcurrent protection requirements for lighting and appliance branch-circuit panelboards and certain power panelboards, see 408.16(A), (B), and (E).

 

[don_resqcapt19]

Re: Tap Rule Interpretation

If it is a lighting and appliance branch circuit panel or a power panel having neutral and more than 10% of its OCPDs rated 30 amps or less, then the panel itself requires overcurrent protection. The only way to provide this protection is to use a main breaker. If this is a power panel where less than 10% of the breakers are rated at 30 amps or less, of if the 100 amp panel does not have a neutral, then the MLO panel is ok. My earlier post just keyed on protecting the tap conductors and not providing protection for the panel itself. Sorry for the confusion.
Don

 

[pierre]

Re: Tap Rule Interpretation

Don's last post, combined with his first post is all that is necessary to say about this type of installation. Some may not like it, but it is a minimum requirement and is safe as far as the NEC is concerned.

Pierre

 

[iwire]

Re: Tap Rule Interpretation

Thanks Don.

Bob

 

[semir]

Re: Tap Rule Interpretation

100 Amps panel (480/277V) has only 2 C.Breakers (20A/1P) with the total load of 10Amps.
This panel is dedicated only to Emergency lighting (277 Volts).
In my opinion can be MLO.