Central Inverter DC Fusing

[pcanning87]

I've seen that central inverters, say a SMA 500CP-US or SMA 2200-US, can be ordered without DC fusing. Is this configuration code compliant if you have external fuses or breakers? Don't you need the fusing inside the inverter to prevent backfeeding and comply with 690.9?

Thanks in advance.

 

[Carultch]

I've seen that central inverters, say a SMA 500CP-US or SMA 2200-US, can be ordered without DC fusing. Is this configuration code compliant if you have external fuses or breakers? Don't you need the fusing inside the inverter to prevent backfeeding and comply with 690.9?

Thanks in advance.

It is compliant to externalize the DC fusing or breakers to an adjacent subcombiner or an adjacent "non-combiner". Essentially, the local feeder between your external unit and the inverter becomes a 240.21(B) tap, if you are using an external "non-combiner". The tap conductors are eventually protected against backfeeding by the time the current meets the corresponding OCPD.

 

[pcanning87]

Essentially, the local feeder between your external unit and the inverter becomes a 240.21(B) tap, if you are using an external "non-combiner". The tap conductors are eventually protected against backfeeding by the time the current meets the corresponding OCPD.

In this case would 240.21(B)(5) allow an unlimited length such that the "adjacent non-combiner" doesn't actually need to be adjacent? Either way I certainly wouldn't design that way as a best practice.

 

[jaggedben]

I'd say backfeed from the AC side can be protected against by the AC overcurrent device. Unless there is a serious worry that the DC conductors would not be protected by the AC overcurrent device rating in the case of catastrophic inverter failure and AC current flowing through to the DC conductors, then I don't think you need to worry that you're not complying with 690.9. It's something to ask the manufacturer: how much AC current could backfeed through the inverter?

DC conductors are all sized for maximum possible current and only require overcurrent protection where they are parallel connected to circuits of higher ampacity. (Language in the 2017 NEC will make this clearer, which is a good thing.) There is no general reason that you need DC fusing on inverter input circuits, unless the manufacturer has specified so, perhaps to protect their product engineering.

I don't think it is proper to apply 240.21(B) to DC conductors on a PV system. Just my opinion.

Admittedly I don't have any experience designing systems with this size inverter.

 

[pcanning87]

My concern is that the combiner box outputs terminate at a common bus on the DC side of the inverter. This would allow a fault to be backfed through the DC bus from the other combiner box circuits. I currently have an external fuse unit immediately adjacent with OCPD for each combiner box circuit. I've definitely seen this done before, I'm just having trouble justifying it to myself with the Code.

 

[jaggedben]

I see. I'd agree that you either have to design the system so that circuits are otherwise protected, or have fusing in the inverter. It pretty much comes down the same rules as strings for small systems. If you have three or more combiner output circuits landing in the inverter then you should have fusing there. (*Technically you're also allowed to severely oversize the wiring.) Perhaps SMA offers the fuseless option for designers who prefer to use their own combiners with certain features to combine the entire output.

 

[pv_n00b]

I've seen that central inverters, say a SMA 500CP-US or SMA 2200-US, can be ordered without DC fusing. Is this configuration code compliant if you have external fuses or breakers? Don't you need the fusing inside the inverter to prevent backfeeding and comply with 690.9?

Thanks in advance.

I've done a bunch of these over the years. The no fuse option is if you are supplying your own external recombiner. In that case you can have up to two external recombiners that then connect to the DC input of the inverter. Any more than two and you are combining in the inverter and you are required to have OCPD, or really big conductors.