Multiple Inverters in "Combiner" Panel 705.12(D)(2) Application Commercial/Industrial

[hcubed]

Multiple Inverters in "Combiner" Panel 705.12(D)(2) Application Commercial/Industrial

Hi,
I have a generic question regarding multiple inverters in a combiner panelboard in commercial/industrial 3-phase systems.

On a customer's existing main switchgear say we'll be installing a new circuit that goes to a new panelboard where we'll be interconnecting several new inverters (say four 20kW 3-phase inverters at 277/480V) - does 705.12(D)(2) apply to the panelboard? There will be no other loads in the new panelboard (and it will be labeled as such - "do not move or add circuits"). And now let's add one more complication - those inverters are actually bidirectional converters because they're battery systems, so they can technically act as a source or load. They would be controlled so that they're either all generating or all charging (you wouldn't ever have a situation where some were charging and some were generating simultaneously).

If 705.12(D)(2) doesn't apply then you don't have to worry about the 120% rule and you don't have to upsize the bus size on the panelboard, or install an MCB, helping to keep costs down.

Thanks.

 

[Carultch]

Hi,
I have a generic question regarding multiple inverters in a combiner panelboard in commercial/industrial 3-phase systems.

On a customer's existing main switchgear say we'll be installing a new circuit that goes to a new panelboard where we'll be interconnecting several new inverters (say four 20kW 3-phase inverters at 277/480V) - does 705.12(D)(2) apply to the panelboard? There will be no other loads in the new panelboard (and it will be labeled as such - "do not move or add circuits"). And now let's add one more complication - those inverters are actually bidirectional converters because they're battery systems, so they can technically act as a source or load. They would be controlled so that they're either all generating or all charging (you wouldn't ever have a situation where some were charging and some were generating simultaneously).

If 705.12(D)(2) doesn't apply then you don't have to worry about the 120% rule and you don't have to upsize the bus size on the panelboard, or install an MCB, helping to keep costs down.

Thanks.

You're best strategy is probably to go with the rule that reads along the lines of the sum of the breakers, excluding the main supply, shall not exceed busbar ampacity.

 

[ggunn]

You're best strategy is probably to go with the rule that reads along the lines of the sum of the breakers, excluding the main supply, shall not exceed busbar ampacity.

I agree; see 705.12(D)(2)(3)(c) in NEC2014. That's the language that legitimizes AC combiner panels for PV. 705.12(D)(2)(3) does apply, in any case. It gives you three (four really, but use (d) with caution) ways to qualify your panel bus.

If your AHJ is still in a previous code cycle, you may or may not have to count the OCPD of the sub and observe the 120% rule, and that would be a pain, but most AHJ's (at least all that I dealt with using previous codebooks) won't do that to you.

 

[hcubed]

Thank you. Huge help. And, yes, I'm working with the 2014 NEC, sorry for not clarifying.

Here's a follow up...now when we install into the existing switchgear the new circuit breaker that feeds the new panelboard, do we also have to follow one of the options in 705.12(D)(2)(3)? The reality of commercial and industrial switchgear is that the main switchgear breaker and the switchgear bus are typically the same rating (like 2500A) and it's almost impossible to install the new OCPD "at the end of the bus" because of the bus configuration of these large switchboards. Reducing the size of the main switchgear breaker is typically not economically feasible (unless it's one of those digital trip units). Perhaps 705.12(D)(2)(3)(c) is just what works best yet again...

Thanks.

 

[ggunn]

Thank you. Huge help. And, yes, I'm working with the 2014 NEC, sorry for not clarifying.

Here's a follow up...now when we install into the existing switchgear the new circuit breaker that feeds the new panelboard, do we also have to follow one of the options in 705.12(D)(2)(3)? The reality of commercial and industrial switchgear is that the main switchgear breaker and the switchgear bus are typically the same rating (like 2500A) and it's almost impossible to install the new OCPD "at the end of the bus" because of the bus configuration of these large switchboards. Reducing the size of the main switchgear breaker is typically not economically feasible (unless it's one of those digital trip units). Perhaps 705.12(D)(2)(3)(c) is just what works best yet again...

Thanks.

Yes, 125% of the inverter max output current (not the value of the backfed breaker) counts toward the 120% rule (subsection (b)) in every panel in series all the way back to the service, and if the sum of the panel OCPD and that 125% number is between 100% and 120% of the busbar rating, the backfed breaker must be at the opposite end of the bus from the feed from the service. If they sum to less than 100%, the backfed breaker can go anywhere on the bus under subsection (a).

You realize that to qualify under (c), you have to sum all the breakers excluding the panel OCPD, both load and source, right? Not many MDP's qualify.

 

[GoldDigger]

Not many MDPs qualify but a dedicated combiner or interconnect panel built using MDP components generally will.

 

[ggunn]

Not many MDPs qualify but a dedicated combiner or interconnect panel built using MDP components generally will.

Sure, but I was under the impression that the OP's followup Q was about the MDP.

Aside: one of my very first contributions to my present company (on my first day) was to point out that they had designed 420A of breakers into a 400A AC combiner panel, so it wouldn't fly; they thought they were supposed to sum 125% of the inverter currents. It wasn't what they wanted to hear, but they didn't fire me.

 

[jaggedben]

(Nevermind.)

 

[hcubed]

Thanks again for the help here. This helps clarify things. I realize Option 3 likely will not work with most MDP's, so it's either Option 1 or Option 2 (or Option 4, but I'm not sure how to do bus bar calcs).

 

[pv_n00b]

Thank you. Huge help. And, yes, I'm working with the 2014 NEC, sorry for not clarifying.

Here's a follow up...now when we install into the existing switchgear the new circuit breaker that feeds the new panelboard, do we also have to follow one of the options in 705.12(D)(2)(3)? The reality of commercial and industrial switchgear is that the main switchgear breaker and the switchgear bus are typically the same rating (like 2500A) and it's almost impossible to install the new OCPD "at the end of the bus" because of the bus configuration of these large switchboards. Reducing the size of the main switchgear breaker is typically not economically feasible (unless it's one of those digital trip units). Perhaps 705.12(D)(2)(3)(c) is just what works best yet again...

Thanks.

That's why you see a lot of supply side interconnections in large switchboards. Also a large number of switchboards are center fed making either a supply side interconnection or engineered interconnection the only options.