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Archive for the ‘Utility disconnect’ Category


Utility disconnect

Monday, July 21st, 2008


I am installing a 1kw solar grid tied system with an Outback VFX 3524 inverter.

The local utility requires a system disconnect next to their meter. The local utility has determined the placement of the disconnect device outside the home but they will not indicate what size or model of DG disconnect should be used for this installation. My electrician is also uncertain of what is required here.

Can you recommend a suitable outdoor rated disconnect for this installation? Is the size of this disconnect determined by the the output of the solar (1kw max) or the 3.5 kw inverter?




First, make sure your VFX Inverter was ordered for “sell-back” grid inter-connect (Model #GVFX3524) as the basic VFX models do not have this optional feature and will not work in grid-tie mode.

Second, a 1 Kw solar array will have very little capacity to sell anything back to the utility grid after charging a battery bank, so this may not be worth the trouble until you enlarge the solar array.

Finally you have hit a raw nerve with all of us in the solar industry as these outside dis-connects are next to worthless and serve no purpose. All solar inverters labeled as meeting all requirements for grid inter-connection will shut down the micro-second the grid goes down and will not put out any power. This is because by design the inverter is constantly measuring the voltage, current, and sine wave form coming from the grid in order to match it for power sell back. If it can’t find the grid it cannot function and shuts off. This means the only purpose for this switch is if the Power Company “thinks” their linemen will shut this switch off if they are working on the lines in the area as a safety precaution.

However, I have not met any linemen that had any idea there was a solar power system anywhere near where they were working on a line, and they do not have record books they carry around that can cross reference the location where they are working with any solar homes around them. Also, many people blow up their portable generators each winter when they connect them into their dryer outlets to back feed their home during a power outage. This works great unless the forgot to turn off the main house circuit breaker and then as soon as the power comes back on their generator is toast. Because some people actually do this and back feed into the utility line during an outage, this 120/240 VAC power goes up to the house side of the utility pole transformer and comes out on the utility line side as 480 volt or higher which can kill a lineman not expecting a downed line to be energized. This is why linemen are taught to place grounding clamps on both sides of the line they are working on just in case the load side is being back-fed from something unknown.

Now back to your problem. Since this is a requirement of your local utility and not the National Electric Code, the type of dis-connect is determined by them. For example, some utilities allow just a rain-proof circuit breaker that is labeled as the “Solar Disconnect”. Some utilities require a dis-connect with a lever handle that can accept a padlock, and some really nasty utility companies require a very high cost lock-out switch with a window in the door that you can see when the switch contacts are open. Since the most power you can sell back with a 1 kW solar array is 8 amps @ 120 volts (1000 watts / 120 volts) plus 10% added efficiency losses, and since circuit breakers are usually sized for 125% of the load, a 10 amp circuit breaker or dis-connect switch is close enough for your application.

Keep in mind your inverter is 120 volts AC and your utility power is 120 / 240 volt split phase, so you will only be selling power back on one leg of their 240 volt line. Square D makes a nice outdoor un-fused dis-connect with an external handle that can be locked. Most utilities are allowing this switch, but the smallest it comes in is 3-pole with a 30 amp rating so its fairly large for your application. See if they will allow an air conditioner type 10 amp 120 VAC outdoor dis-connect. It does not have a lock type handle, but they will never use it anyway!

Good luck,

Jeff Yago



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