6/3 romex no ground wire sub panel connection?

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I am confused about hooking up my PVPowered 3500 Grid Tie Inverter to the sub panel in my garage. My home was built in 1975 and the sub panel is wired to the Main panel with 6/3 romex, no-ground wire, at 240vac with neutral wired to grounding block at main panel. My question is, will I be allowed to wire the GT Inverter to this sub panel for my ac connection to feed the solar power to the main panel? I will be grounding the solar panels to the inverter and then to a ground rod at the inverter location. The sub panel ONLY feeds a 240vac outlet in the garage, NO 120 vac connections from the sub panel and the 240vac outlet will be abandoned before the system is charged. The sub panel neutral is connected to an isolated neutral block in the sub panel, So my only ground will be to the solar panels connected to the DC ground on the inverter and inverter to the ground rod. I am hoping the the grounded neutral at the main panel can be disconnect from the neutral grounded bus bar and connected only to the equipment ground bar at the main panel and will be sufficient for grounding the AC side of the inverter. So the short of it all is: can the white neutral wire be used for the EGC wire that is only connected to the ground buss bar at both the sub panel and the main panel instead of using a solid ground wire?  Any info will be greatly appreciated, and thanks for looking.

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I found the answer I was looking for so I thought I'd share it do to the lack of replies:
NATIONAL ELECTRICAL CODE

ARTICLE 250

 GROUNDING AND BONDING
250.118 Types of Equipment Grounding
Conductors.  
The equipment grounding conductor
run with or enclosing the circuit conductors shall be
one or more or a combination of the following:
(1)
A copper, aluminum, or copper clad aluminum
conductor.  This  conductor  shall  be  solid  or  
stranded;  insulated,  covered,  or  bare;  and  in  
the form of a wire or a busbar of any shape

BB.

Sorry, I missed your post...

I am not a code guy--But, as I recall, you can use the conduit as safety ground for residential. But for commercial, you must pull a green wire ground (and perhaps, new residential requires a green wire ground too with new NEC?).

If you do not need the neutral in your sub panel, yes, you can lift the white wire at both ends, "phase tape" both ends with green electrical tape, and tie it to the safety ground in the main panel and to the local panel ground in your garage. Note that the subpanel may have a moveable "bonding" screw in the isolated neutral bus so you can ground the bus to the panel box.

-Bill

BB.

I forgot to add--Your green wire/safety ground for the solar array (and inverter?)--Lighting wise, the array ground should go down the outside of the building to a ground rod next to the foundation.

For electrical safety (code wise), you should have a ground wire from the remote ground rod/array ground tied back to your main ground rod (and/or cold water pipe ground). You want the two grounds tied together so you do not have a electrocution risk. You want a solid copper connection between the two electrical grounds (trip breaker if there is a short circuit). Ground Rods can have as much as 25 Ohms to earth ground resistance. That is not enough to trip even a 15 amp breaker at 120 VAC:

• V=I*R
• I = V/R = 120 VAC / 25 Ohms - 4.8 Amps AC

-Bill

Skip

 Hi Bill, I want to thank you for the info, I am having a licensed solar installer install the system, I just needed to know that the existing wiring would work for the sub panel, my city puts homeowner solar installers through hell