240.8 and 408.36 EX #2

[sandsnow]

These two sections have been in the Code since 1975 or earlier in someway, shape, different number or slightly different text.
I've looked in NEC handbooks back 1996. No help. I've read 408.36 EX 2 over the years and have never known what it applied to.

What does two main circuit breakers having a combined ratingnot more than that of the panelboard mean? Two supplies to the panel? So wecould have two 100amp feeders supplying a 225 amp bus panelboard? Doesn?t seemright.
If I have a 200 amp C/B in the service and a 200 amp C/B aspanelboard main, I wouldn?t combine the rating of the two and say the panelboard is protectedby 400 amps.

And if I can have two 100 amp supplies to a 225 amppanelboard, how does 240.8 fit in where it says breakers cannot be connected inparallel?

This post relates to the thread going in PV forum
http://forums.mikeholt.com/showthread.php?t=152928

 

[jim dungar]

240.8 deals with the physical act of paralleling protective devices. In the old days, manufacturers actually made 800A fusible switches that used (2) 400A fuses mounted side by side (this is still done MV).


408.36 deals with split bus panelboards. This section allowed you to have two sets of busbars treated as a single panel, it prohibited a triple bus panel. it was most applicable when the split bus panel was the service entrance or the first item after a transformer.

 

[kwired]

240.8 deals with the physical act of paralleling protective devices. In the old days, manufacturers actually made 800A fusible switches that used (2) 400A fuses mounted side by side (this is still done MV).


408.36 deals with split bus panelboards. This section allowed you to have two sets of busbars treated as a single panel, it prohibited a triple bus panel. it was most applicable when the split bus panel was the service entrance or the first item after a transformer.

I can see exception 1 applying to split bus panel (408.36) but exception 2 (see thread title) doesn't really make much sense to me either. It doesn't mention multiple bus in the panel, so it would seem to mean overcurrent devices in parallel are protecting the panel:?

 

[sandsnow]

I can see exception 1 applying to split bus panel (408.36) but exception 2 (see thread title) doesn't really make much sense to me either. It doesn't mention multiple bus in the panel, so it would seem to mean overcurrent devices in parallel are protecting the panel:?

Bingo. Exactly what i was wondering.

 

[jim dungar]

I can see exception 1 applying to split bus panel (408.36) but exception 2 (see thread title) doesn't really make much sense to me either. It doesn't mention multiple bus in the panel, so it would seem to mean overcurrent devices in parallel are protecting the panel:?

Exception #1 is for services.
Exception #2 is for all other panels (it almost made more sense when we had Lighting and Appliance versus Power panels).

Also, the parallel connection of (2) "individual protective device + individual conductors" is not the same as paralleling just the protective device. Consider how we feed a Main-Main double ended switchboard with closed transition switching.

 

[sandsnow]

Exception #1 is for services.
Exception #2 is for all other panels (it almost made more sense when we had Lighting and Appliance versus Power panels).

Also, the parallel connection of (2) "individual protective device + individual conductors" is not the same as paralleling just the protective device. Consider how we feed a Main-Main double ended switchboard with closed transition switching.

Individual Protective Device + Individual Conductors is what we are being faced with. That is what is being proposed by the applicant and shown by the manufacturer. When you simplified it, it was proposed to have 4 SP 70A C/B not ganged together supply a panelboard. The second panelboard would have 4 SP 70A C/B not ganged together. In between it was proposed to run two conductors for each C/B, a hot and a neutral. Each set of conductors would run through a piece of equipment that would allow normal power to pass through (unmolested) and allow PV to backfeed (unmolested), but would transfer to batteries when utility goes down. The diagram is in the other thread.

Back to your example of the double ended switchboard. I am not faimiliar with closed transition. I have see main + tie breaker + main with Kirk Key interlock so one of those is always open.

 

[sandsnow]

I found the ROP from the 2008 NEC. Section 408.36 EX 2 refers to split bus panelboards. It was interesting in that some comments wanted a direct reference to split bus panelboards put in the Code text.