TAPS

[EEDude]

Hi folks...
This is my first post so please forgive if this has been covered. I did look but came up short...
Rather than try to explain here's a pic of what I was looking at. I believe the 10' tap rule covers both of these applications? Just looking for a little affirmation.

For Fig A, switch gear has no main and is close coupled to the transformer.
Fig B is a tap directly off of an MCC bus (10' or less in conduit) to a MLO panelboard.
Hopefully, between my brief explaination and the figures it will be clear.
Looking for code references and interpretations....
What confuses me, is the "can't tap a tap" rule. Technically these branch breakers are not taps. Correct?
Thanks so much in advance.

Fig A

Fig B

 

[augie47]

Welcome !

I had a bit of a difficult time viewing but all seems to be in order for the 10 ft tap rule.
If the transformer in Fig A is a utilty transformer you techbically would have service conductors and not a tap. If it's part of a private distribution system you are fine under 240.21(C) rules,.;

Fig B looks to be fine under 240.21(B)(1).

In that your conductors terminate on a device (panelboard) and there are OCP devices ahead of all conductors leaving the panelboard you are not "tapping a tap". 240.21 prohibits a tap conductor from feeding another conductor EXCEPT thru an overcurrent device.

 

[charlie b]

Figure A has a problem. I agree with Gus that it meets the transformer tap rule of 240.21(C)(2). But in addition to the tap rules, you need to comply with the rules concerning overcurrent protection of the transformer itself. Table 450.3(A) {NOTE: I am on 2008 here in WA} requires us to limit the secondary overcurrent protection to 125% of the transformer's rated current. That limit is 2,256 amps. You can achieve this with up to six circuit breakers, and you only have five. But the sum total of the five overcurrent devices is 3,000 amps. It is not allowed to exceed the 2,256.

Welcome to the forum.

 

[augie47]

Good catch. I didn't even think to check that.

 

[charlie b]

And I think Figure B has a similar problem. Once again, the tap rules are satisfied. But the rule for protecting the panel from overcurrent, 408.36, is not. I don't see where we are allowed to protect the panel using up to six breakers. I think you need to either have a feeder breaker on the MCC or a main breaker on the downstream panel.

 

[augie47]

And I think Figure B has a similar problem. Once again, the tap rules are satisfied. But the rule for protecting the panel from overcurrent, 408.36, is not. I don't see where we are allowed to protect the panel using up to six breakers. I think you need to either have a feeder breaker on the MCC or a main breaker on the downstream panel.

I would agree. 240.21(C)(2) seems to be satisfied, but 408.36 does not

 

[petersonra]

Good catch. I didn't even think to check that.

i did but before I could reply I had to go take of a crisis and I just now got back.

 

[jim dungar]

Don't forget, Supervised industrial installations can have slightly different 'tap rules' per 240.92.

 

[EEDude]

Don't forget, Supervised industrial installations can have slightly different 'tap rules' per 240.92.

Thanks for the replies guys. I'm really liking this place...
Yes. I should have mentioned that Fig A is a supervised industrial location. Also, I don't believe this would be considered a panelboard. I threw the online together real quick to pose the question so, my bad for not being clear on that :ashamed1:.
Fig A is actually a unit substation with drawout power circuit breakers on the LV side.
It Looks like 240.92(1)(1) is happy by the ratio calcs (2000A bus on secondary)?

I can't seem to justify Fig B at all. Just based on 408.36. BTW this is an existing installation that I am performing a short circuit study on. My questions are outside my scope of work, but when I see something that doesn't feel right I like to do some research (I feel ethically compelled)... This installation is pretty old too. So maybe this was acceptable say, in the '60s?

Thanks again!!!

 

[EEDude]

Oh yeah....
Let me add another twist.
What if the panelboard in Fig B was integrated into the MCC? Or any other switchgear for that matter... Does it still need a main? Or at that point would you just consider these as branch circuits off of the main bus?
Man, these topics can grow quick...