Author Topic: 120% RULE FOR LOAD-SIDE GTI CONNECTION  (Read 1895 times)

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jack11

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120% RULE FOR LOAD-SIDE GTI CONNECTION
« on: August 13, 2016, 01:42:58 PM »
From some article I've read on electric code:

Section 705.12(D)(2) states that the sum of breakers supplying power to a busbar or conductor must not exceed 120% of the busbar or conductor's rating. Both the overcurrent protection device (OCPD) between the grid and the service panel (typically the main AC breaker) and the OCPD from the inverter(s) are considered power supplies.

I believe that applies only if the two sources feed the busbar from the opposite ends, otherwise some sections of the busbar may be in overcurrent under some conditions.

So, since we are feeding the busbar from the opposite ends, and any load will take the current from the middle of the busbar, why not a “200% rule”?

Seems like under the 200% rule, and feeding from the opposite ends, no section of the busbar would ever be subject to overcurrent condition. Or would it?

And the max load that could be connected could be quite a bit larger than under the 120% rule, without adverse effects.

Plus, there is another safety margin built into this in the form of the "x 1.25" rule between a grid-tie inverter max current output and its circuit breaker, which further reduces the current coming into the busbar. So, that "120% rule" could go somewhat higher than 200% and still not overcurrent the busbar.

Why did they set this rule to 120% and not 200% or so, can anyone comment?

This has impact on some custom AC circuits I am trying to build.

OperaHouse

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Re: 120% RULE FOR LOAD-SIDE GTI CONNECTION
« Reply #1 on: August 13, 2016, 02:08:48 PM »
Not sure I see the conflict.  If you have two 100A breakers feeding, buss bar needs to be rated for 240A.

jack11

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Re: 120% RULE FOR LOAD-SIDE GTI CONNECTION
« Reply #2 on: August 15, 2016, 01:59:55 PM »
As an example, RMS AC current budget into a 100A busbar.

Under the 120% Rule.
From the grid: 100A (at the top of the busbar),
From the GTI: 16A (at the bottom of the busbar),
Max load that can be connected: 116A (in the middle of the busbar).

Under the 200% Rule.
From the grid: 100A,
From the GTI: 80A,
Max load that can be connected: 180A (64A gain with respect to the 120% Rule, without busbar overcurrent).

Under the 225% Rule.
From the grid: 100A,
From the GTI: 100A,
Max load that can be connected: 200A (84A gain with respect to the 120% Rule, without busbar overcurrent).

Anything over the 225% Rule means potential overcurrent in some portions of the busbar.

Any comments on this?