Grounding question

DIYGuy

I am building a Grid-Tie system with a SMA SB9000-TL-US transformerless Grid-Tie inverter. In this configuration I understand I do tie either  the positive or negative side of the array to ground. I will ground the array itself by using a grounding rod driven into the ground close by the array. The array is mounted on top of a pole barn about 250 feet from the inverter.

The inverter will be mounted inside my home by the breaker panels and grounded to the ground within the breaker panels. Do I need to run a separate ground wire to tie the ground in breaker panels to the ground rod out by the array?

Thanks in advance for any insight you can give me into this.

Bob

Ethan Brush

Hi Bob,

You do NOT connect an array conductor to ground.  IF you want to know why, its because a transformerless inverter does not isolate the DC side from the AC side.  So in fact, the DC conductors retain their reference to the AC system which is grounded (nearly always),  Connecting one of these conductors to ground would cause a short circuit.  You will need a disconnect at the pole barn, and an equipment grounding conductor run with the supply conductors all the way from the inverter, to the array.  And then your pole barn electrode will connect to the array also.

DIYGuy

Thanks for the clarification. I mistyped my message. When I wrote "I do tie either", I meant to write "I do NOT tie either" conductor to ground. Any insight on what gauge the grounding wire running from the pole barn to the inverter should be?

Also, regarding the DC disconnect, I am running 3 strings of panels and am planning on running each of these strings separately back to the inverter. Each string will be about 400 volts @ 9 AMPS. I understand I have to disconnect both the positive and negative sides of each string which will require a 6 pole disconnect switch.

Would it work to use a 6 pole contactor mounted up by the array connected to a single pole disconnect down below? In addition, my utility requires me to mount an AC disconnect by the meter/entrance to my house. I was considering using a 3 pole AC disconnect. Two poles for the AC and the 3rd pole to the contactor. When the AC was disconnected it would also disconnect the DC out at the array.

Hope this makes sense.

Bob

Ethan Brush

DIYGuy said:

Thanks for the clarification. I mistyped my message. When I wrote "I do tie either", I meant to write "I do NOT tie either" conductor to ground. Any insight on what gauge the grounding wire running from the pole barn to the inverter should be?

Also, regarding the DC disconnect, I am running 3 strings of panels and am planning on running each of these strings separately back to the inverter. Each string will be about 400 volts @ 9 AMPS. I understand I have to disconnect both the positive and negative sides of each string which will require a 6 pole disconnect switch.

Would it work to use a 6 pole contactor mounted up by the array connected to a single pole disconnect down below? In addition, my utility requires me to mount an AC disconnect by the meter/entrance to my house. I was considering using a 3 pole AC disconnect. Two poles for the AC and the 3rd pole to the contactor. When the AC was disconnected it would also disconnect the DC out at the array.

Hope this makes sense.

Bob


The problem with running the strings back individually is that you need to disconnect 6 poles.  Your disconnect needs to comply with NEC 690.17.  That may limit the availability of what you can use.  Perhaps it would  be simpler to combine at the pole barn?   I dont recall if that inverter has an integral or optional combiner.

The EGC is sized per 690.45 which directs you to 250.122 so 14 AWG copper is all you should need (if you do not combine out there, if you do then you are looking at a 10 AWG).

What year NEC is in effect where you are?

DIYGuy

Thanks for the feedback Ethan. I think we have 2014 in effect here. Here is the article which discusses the use of a contactor: http://www.solarpowerworldonline.com/2014/09/new-nec-codes-will-affect-installations-2/

The combiner is optional but I am going to purchase it. I wanted to run all of the strings separately to the house which will give me more options in the long term.

Ethan Brush

DIYGuy said:

Thanks for the feedback Ethan. I think we have 2014 in effect here. Here is the article which discusses the use of a contactor: http://www.solarpowerworldonline.com/2014/09/new-nec-codes-will-affect-installations-2/

The combiner is optional but I am going to purchase it. I wanted to run all of the strings separately to the house which will give me more options in the long term.

Ok if you are on 2014, do you have to provide for the 690.12 "rapid shutdown" requirement?  That often involves a contactor.  Equipment that accomplishes that is kinda limited.  You might check out midnite solars option.  Note that you could still do something "homebrew" to meet the 690.12 requirements - the components must be "listed and identified" but do not have to be listed and identified for the use.