AC disconnect interrupt ratings

[solarix]

Can anyone explain to me what the interrupting rating needs to be (and why) for the AC disconnect on a typical grid-tied system?
NEC 690.17 says that it needs to be "sufficient for the current available at the line terminals of the equipment" What is that?
My local inspector is saying that the interrupting current can be 12,500 amps which your average disconnect can't handle.
I thought the service panel limited the current to the value of the OCP device. I understand that the breaker is not rated to be used to interrupt fault currents, but should it not limit the current so the disconnect AIC rating doesn't have to be so high?
Another buddy says that we need to use at least 2.5 feet of 10ga wire between the breaker and the disconnect (in this 5kW case) to satisfy the AIC rating. Inspector is saying we need to add fuses to the system which, of course, the utility won't allow inside the disconnect as they lock it for safety reasons.
On this issue, John Wiles says: " many utility-interactive PV systems make the utility connection through a back-fed circuit breaker in an existing load center and the existing load center is designed to handle the available short-circuit currents. No additional current limiting is required." Who is correct???

[BB.]

Really good questions.

I cannot give you exact answers--But as I understand, you are seeing some of the issues with multiple power/current sourced systems and the consequences.

A residential pole transformer is rated for 10,000 Amps into a dead short. So, any main breaker should be rated for 10,000 AIC.

And, in commercial installations that are installed very closely to the transformer (not sure of rating), the short amount of drop wire does not have enough resistance to limit fault current to the AIC rating of standard commercial breakers. So, instead, for those services, they have to use special fuses instead for the initial main panel fault protection.

I have not read the code on the requirements--But I would agree with you that the disconnect (assuming it is not the mains breaker) should only be rated to maximum Load interrupt current, and not the AIC rating (which, with a large Grid Tied solar array could, in theory, add to the maximum current of the transformer--I.e., the 10,000 Amps of the fault, and 100's more amps from the solar PV source too).

Then it gets more complex--Is there alternative paths for the sum of the fault currents or not (for example, with two parallel strings of solar panels, we don't need series protection fuses--But for three or more parallel paths we need a protection fuse for each parallel string).

If the inspector is somehow adding the theoretical dead short from a solar GT inverter to the AC AIC requirements--You may be able to argue that the GT inverter is not designed and not capable of driving any current if the voltage is below 212 VAC--By UL/NRTL Listing (or whatever your minimum system voltage is), so there should be no reason to increase AIC ratings or have a disconnect switch rated for AIC.

-Bill

[SolaRevolution]

This issue can be a real doozy.


In my limited grid-connection experience I have always had to defer AIC issues to the decisions of the utilitiy engineers. Quite handy really. The most contentious issues have been for systems with battery based grid-tie inverters. On two occasions I've had to wait for weeks while they settle it out among their own people, but so far it has not cost me or my clients anything out of pocket.

If you are using a rated panel board assembly there will be information that comes with it listing what types and ratings of fuses or breakers may be used in series with it. If you are doing a connection on the line side of a main service breaker the NEC may require that you get an engineers stamped approval.

"NEC 240.86(A) Selected Under Engineering Supervision in Existing Installations. The series rated combination devices shall be selected by a licensed professional engineer engaged primarily in the design or maintenance of electrical installations. The selection shall be documented and stamped by the professional engineer... "

If an AHJ really wants to be a bugger he can claim that the NEC requires an engineer's stamp to have a subpanel of a different brand than the main service disconnect unless the manufacturers recognize and list eachother in their documentation.




Also, not specifically solar but regarding the AIC ratings of services:

From the Pacific Gas & Electric (PG&E - a West coast utility) "Green Book":

"5.7.2. Main Service Disconnect Switch Rated for Amperes Interrupting Capacity (AIC) State and local codes require the service equipment’s main disconnect switch and fuse, or the circuit breaker, to be rated at the available short-circuit current value.

PG&E will design its facilities so that the short-circuit duty at the service termination will not exceed 10,000-amperes symmetrical for new, single-family, residential applicants that are supplied by an individual service drop or lateral that is rated at 225 amperes or less. This service includes mobile homes and duplexes.

Typically, it is not feasible for PG&E to design its facilities to limit the short-circuit duty to 10,000 amperes for other electrical services; for example, a 400-ampere, multimeter, residential or nonresidential installation. For these installations, on request, PG&E will provide the maximum available short-circuit current based on the service equipment’s capacity. If the applicant increases the service equipment’s capacity, the maximum-available short-circuit current may be higher."



For more NEC and electrical engineering oriented discussions you may want to check into Mike Holt's Forum:
http://forums.mikeholt.com/index.php


-Alex Aragon

[solarix]

In rereading NEC 690.17 I note that it says "sufficient for the nominal voltage and current" which I take to mean the breaker rating of 30 amps in this case. Since the main and branch breakers are A.I.C. rated, my take is that the disconnect only has to handle 30A to meet 690.17