wire gauge for dual driven appliance (solar inverter)

Question

I've been thinking about the #8 wire between a sub-panel, on a 50 amp breaker, and a 10kw solar inverter.

Between the 50 amp breaker and inside the inverter, in the event of a fault, the wires would be driven on two sides, potentially at upwards of 90 amps continuously. The size of the wire between the inverter and sub-panel was sized by the 50 amp breaker - #8, and is contained within PVC conduit. Inside the inverter the wire is both #8 and ~#16. The #16 wires goes to a WattNode power meter that consumes ~1 watt.

How is that #8 is allowed between the appliance and the breaker, and within the appliance, ~#16?

This is within the US and is for a SolarEdge 10k inveter. Sub-panel has 100 amp bus bar, w/utility at top and solar at bottom.

Answer 1

The NEC has no bearing on factory wiring within listed equipment

The gauge of factory wiring in listed equipment is governed by the applicable UL standards and by the testing UL conducts in accordance to those standards (usually various temperature rise tests) as part of the listing process, not by the NEC. This is reflected in NEC 310.1:

310.1 Scope. This article covers general requirements for conductors and their type designations, insulations, markings, mechanical strengths, ampacity ratings, and uses. These requirements do not apply to conductors that form an integral part of equipment, such as motors, motor controllers, and similar equipment, or to conductors specifically provided for elsewhere in this Code.

As a result of this, we must look outside the NEC for an explanation of this. While I have not seen explicit provisions for these sorts of potential or control taps in what I have been able to read of UL 1741, the UL 508A standard for industrial control panels contains a provision in 41.1 Exception 1 that can be used for an argument by analogy in this case:

41.1 The conductors of a control circuit tapped off the load side of the branch circuit protective device shall have overcurrent protection sized in accordance with the ampacity of the control circuit conductor as specified in Table 28.1 and Table 38.1.

Exception No. 1: When the control circuit is tapped off a motor branch circuit protective device and the control wires do not leave the industrial control panel enclosure (such as when a start-stop button is provided on the enclosure cover) the motor branch circuit protective device provides the required overcurrent protection when its rating does not exceed that specified in Table 41.1.

Given that Table 41.1 in UL 508A-2014 permits a 14AWG control tap on a motor branch circuit of up to 100A in size and a 16AWG tap on a circuit up to 40A when the conditions of Exception 1 are met, I would say that the safety case for the potential tap wiring you see in your inverter works analogously to the logic behind the above quoted exception.

Answer 2

Wiring inside the appliance is subject to the rules of UL, not NEC.

I'm kind-of surprised by the #8 wire on a 50A breaker, as #6 is required for a 50A breaker normally. (you are only allowed to use the 60C current rating on branch circuits and feeder <100A). They may just be saying that it's rather unlikely that the #8 wire will be loaded beyond 40A, because reaching the full 10kw would require perfect conditions and panel sizing. But if #8/50A breaker is in the labeling and instructions, then it was part of the UL listing, and UL tested the failure modes there.

I see what you mean about a wire fault mid-run causing 90A of flow, but that would only be at the fault - the wire from the inverter would only be 41.6A tops, and the wire from the panel is breaker-restricted at 50A.