How to back feed a 1200 amp service
- Thread starter sketchy
- Start date
- Status
You can back feed a service with a GFP main breaker as long as the breaker is not marked line and load. Some people are still confused by this but there was a really good article on back feeding GFP breakers in the IAEI magazine a few years ago that spelled it out.
You do need to consider the ground fault contribution from the PV system. If the main GFP is set for 500A and the PV system can supply 100A of ground fault current then there will need to be 600A of ground fault current before the main will trip and that might be a problem. It's hard to say since the NEC does not give a GFP setting requirement, only that there needs to be one with a maximum setting of 1,200A.
You do need to consider the ground fault contribution from the PV system. If the main GFP is set for 500A and the PV system can supply 100A of ground fault current then there will need to be 600A of ground fault current before the main will trip and that might be a problem. It's hard to say since the NEC does not give a GFP setting requirement, only that there needs to be one with a maximum setting of 1,200A.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
It may be worth noting that many/most (?) services over 1000 amps will have multiple service disconnects each less than 1000A so you might not run into GFP all that much.
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
This article?
In my opinion, 705.32 requires a separate GFP device on the solar system (invoking the exception), if you want to connect load side of a service and/or feeder GFP device.
That's not the article I was thinking of, and unfortunately I don't have the time to look it up right now. But it comes to the same conclusion. The key thing to keep in mind is that GFCI and GFP are two completely different things. I have not seen a GFCI that was not marked line and load.
shortcircuit2
Senior Member
- Location
- South of Bawstin
I'm trying to grasp a better understanding of the exception to 705.32 as it may or may not apply to a back-fed connection of a PV System output to a GF protected service or feeder.
Exception: Connection shall be permitted to be made on the load side of ground-fault protection, provided that ground-fault protection of equipment from all ground-fault current sources.
Wouldn't the GFP device on the service or feeder provide the needed GF protection of the interconnected PV System?
Why do you think a separate GFP device is needed for the solar system?
jaggedben
Senior Member
- Location
- Northern California
- Occupation
- Solar and Energy Storage Installer
Well, I have to confess my underqualification to definitively answer your first question. There are a lot of different current flow scenarios to consider, especially with three phase, and I'm not an engineer. Suffice to say the CMP identified an issue, and I can't assert that there are no scenarios where an upstream GFP device would be compromised
I can't remember exactly why I stated the opinion about a separate GFP device. I think it was just my impression that pv_noob's comment indicated the difficulty of meeting GFP requirements in another way.
The whole thing is a bit non-specific. I think the CMP wanted to recognize that there is supposed to be GFP under the PV system without adding any specific requirements.
Here's how I read it.
230.95 requires GFP basically on grounded 4 wire 480V 1000A+ services. It does not state that but that's what it was supposed to target. There is an exception for some industrial processes.
It does not say what the GFP setting has to be, as long as it is less than 1,200A. Kind of leaves a lot of leeway here. The load might be what defines the maximum GFP setting since the whole GFP concept was to protect equipment.
If you have one of these and you want to put in PV you basically don't want to to mess up the existing GFP settings without knowing why they are set where they are.
What is the existing GFP setting? Is it 500A, 1,000A? Do you know why it is set there and what an acceptable setting is?
How much ground fault current will the PV system add?
If it's 1,000A now and you are installing a PV system with a 100A ground fault contribution then you want to look at either increasing the ground fault amperage to 1,100A, still below the 1,200A limit, or reducing the main GFP trip to 900A so the total is 1,000A. It's not well defined in the NEC because it's really a limitation of the load equipment.
Here's how I read it.
230.95 requires GFP basically on grounded 4 wire 480V 1000A+ services. It does not state that but that's what it was supposed to target. There is an exception for some industrial processes.
It does not say what the GFP setting has to be, as long as it is less than 1,200A. Kind of leaves a lot of leeway here. The load might be what defines the maximum GFP setting since the whole GFP concept was to protect equipment.
If you have one of these and you want to put in PV you basically don't want to to mess up the existing GFP settings without knowing why they are set where they are.
What is the existing GFP setting? Is it 500A, 1,000A? Do you know why it is set there and what an acceptable setting is?
How much ground fault current will the PV system add?
If it's 1,000A now and you are installing a PV system with a 100A ground fault contribution then you want to look at either increasing the ground fault amperage to 1,100A, still below the 1,200A limit, or reducing the main GFP trip to 900A so the total is 1,000A. It's not well defined in the NEC because it's really a limitation of the load equipment.
- Status