OCPD and Tap Rule for Multiple String Inverters on one Circuit

[Neal30]

We are going through some design scenarios. One of our electricians has brought up the method of interconnecting multiple 28KW string inverters on one circuit, utilizing taps and no individual OCPD's for inverters. The basic design that I feel more comfortable with, involves landing each inverter on a 3 pole breaker inside a 200A panel, then running back to a 200 A fused disconnect. There are costs associated with the panelboard and breakers, but I am just not 100% that they are interpreting this tap rule correctly.

A basic design layout below:

Option #1
28 KW String inverter Max AC Current 39 Amps, 480V wye output. We are installing about 24 of these inverters, my thought is to put 4 inverters, each on a 3 pole 50A breaker, into a 200 A panel board, and then to a 200 A fused disconnect, then connecting to the transformer.
(6 times)

Option #2
28 KW String inverter Max AC Current 39 Amps, 480V wye output. Proposed connecting 4 inverters via a tap connection using a polaris lug in a tap box, or some type of direct burial rated tap underground. They are thinking that we would not need any OCPD on each inverter, due to the tap rule. The entire circuit of 4 inverters would be protected by a 3 pole 200 A fused disconnect, then connected to the transformer.

I would like to get some insight from an industrial or commercial standpoint, my gut feeling says that if there was a short in the inverter, then the full load up to 200 A from the utility and up to 117 Amps from the other three inverters (39x3) would be available on those tap wires.

These inverters have integrated AC and DC disconnects, the DC strings are fused internally in the inverter. There is no internal AC OCPD in the inverters.

Thanks for your time, any insight is appreciated.

 

[GoldDigger]

The applicability of the tap rules is doubtful when you are dealing with sources rather than loads.
And your scenario of multiple inverters contributing current to a (non-bolted) fault is one of the concerns.
In the case of a bolted fault the inverters would shut down.
Consider the case of multiple panel strings into a combiner box on the DC side.
If there are more than two strings, each string will require an individual fuse or breaker meeting the panel manufacturer's max OCPD size, but the conductors do not need to be sized for greater than that current (larger than the panel Isc).
But in the DC case there may or may not be a source of DC fault current on the combined side of the combiner box.
I personally would not consider the individual inverter output circuits to be protected under the tap rule(s) even if I felt that the smaller wire size was justified by "real life" considerations.

Tapatalk!

 

[GoldDigger]

This does bring up the interesting question of whether the required string OCPD on the DC side has to be at the combiner or could be located at the panel end of the wiring to the combiner.
Either way the panels are protected, but the second way does not protect the wires as well.

Tapatalk!

 

[Smart $]

Just exactly how is tapping (actually, combining) of multiple inverter outputs without individual inverter OCPD permitted? I don't see it.

First, we have...

690.9 Overcurrent Protection.

(A) Circuits and Equipment. PV source circuit, PV output
circuit, inverter output circuit, and storage battery circuit
conductors and equipment shall be protected in accordance
with the requirements of Article 240. ...

The only section which may apply...

240.4 Protection of Conductors. Conductors, other than
flexible cords, flexible cables, and fixture wires, shall be
protected against overcurrent in accordance with their ampacities
specified in 310.15, unless otherwise permitted or
required in 240.4(A) through (G).

...

(E) Tap Conductors. Tap conductors shall be permitted to
be protected against overcurrent in accordance with the following:
(1) 210.19(A)(3) and (A)(4), Household Ranges and Cooking
Appliances and Other Loads
(2) 240.5(B)(2), Fixture Wire
(3) 240.21, Location in Circuit
(4) 368.17(B), Reduction in Ampacity Size of Busway
(5) 368.17(C), Feeder or Branch Circuits (busway taps)
(6) 430.53(D), Single Motor Taps

...

I see nothing in 240.21 that applies. Note 240.21 covers these types: branch-circuit taps, feeder taps, transformer secondary taps, service conductor taps, busway taps, motor circuit taps, and generator terminal taps. I don't see any mention of anything PV related... :blink:

FWIW, from an interpretation standpoint, you don't have any feeder conductors until you get to the load side of at least one ocpd.

 

[Smart $]

With that said, I don't believe there is anything preventing one from using a single ocpd and running a full size conductor. For example a 200A ocpd and conductor where combined inverter output rating does not exceed 160A. However, inverter terminals will likely not accommodate such a conductor without a reducing-type wire terminal.

 

[SolarPro]

Even if the Code didn't require it, you really want to have a breaker on the ac output of each inverter for commissioning and troubleshooting purposes. Otherwise, every time one of your 3-phase string inverters pops a cap, you got to shut the whole system down. That defeats one of the main benefits of using multiple inverters.

 

[jaggedben]

Option 2 is permitted, in my opinion, by 705.65(A) Exception 2.

But I agree with SolarPro's comment. And a disconnect (if not OCPD) may be required by 690.15.

If you go with Option 1, I don't see why you need both the panel board and the fused disco. The panelboard seems sufficient. It can even be an MLO panelboard. See 705.65(B), Exception.

 

[Smart $]

Option 2 is permitted, in my opinion, by 705.65(A) Exception 2.

...

I agree... but the short-cicuit current from all sources is the sum of all 4 inverter's short-circuit current.

 

[Smart $]

FWIW, 690.9(A) says the same thing.