Xantrex XW60 CC MPPT any feedback Yet ?

Re: Xantrex XW60 CC MPPT any feedback Yet ?

This reminds me of the NEC/NRTL arguments I used to have when designing computer systems...

From my humble point of view, there is a pretty large disconnect going on here.

The Solar Charger/Inverter is rated to manage up to 60 amps of input power (and XX amps or XXXX watts of output power). This has no relationship to the about of input current/power available as both major brands are "self regulating" in terms of current/power throughput and neither will fail if "exposed" to current in excess of their input ratings (Xantrex for sure, I remember them stating that they will simply limit power to safe rated levels--I believe that Outback also self regulates too).

To take a side trip here, when we look for appliances, I don't plug my laptop into a 40 watt maximum outlet, or plug my 200 watt TV into a 200 watt max outlet, or my 1,000 watt heater, etc... They all plug into a US standard 15/20 amp 120 VAC service with a maximum rating of 10,000 amps of short circuit current available (limited by magnetic flux/saturation by the transformer on the pole).

So, back to the Inverter/charge controllers--they are (or at lest should be) rated for power use and should be rated as being safely connected to a circuit with XXX amps of available short circuit current available.

Going back to AC house wiring, the main panel circuit breakers are rated to interrupt 10,000 Amperes. The safety ground wires (as I recall from decades ago--I could be wrong) are rated for a minimum of 6 awg bare (or 8 awg insulated--and some deratings for length if long runs--IIRC this was for a 125 amp service) and are rated to safely pass enough current to trip the upstream breaker--125 amps in this case.

These breakers and ground wires (and by extension house wiring and appliances) all rely on a cascade of protections--and do not force (IMHO unreasonable) operational/protection requirements on any one particular piece of equipment.

The Transformer is protected by design to 10,000 amps maximum, the drop wiring is sized for maximum continuous load and to handle a 10,000 amp short term surge, the house main breaker and branch breakers are sized to both interrupt 10,000 amps, pass enough current to trip the local breaker, and pass 100% (+/-) of rated current. The house wiring is sized to pass 100% rated current to the outlets and enough current to trip the branch breaker. The device cord is sized to pass 100% of rated device load and enough current to trip the upstream protective device (notice that you can use 16 awg cords on a branch circuit that is wired for 15-20 amps with 14-12 awg house wiring). By the time you get to the device, it is built by design to not draw more than its designed current and to "safely fail" (for now, assume no fire or shock hazards).

And the safety ground from the outlet to the main panel ground is sized to pass enough current to trip the branch breaker without failing, and the main panel to earth ground is sized large enough to ensure that the main service breaker is safely tripped.

So, back to the solar arrays... The wiring should be sized to handle 100% of the rated power plus safely manage the maximum over current as defined by either the source's ability to supply current and/or by the protective device feeding the downstream device. So, in the case of solar arrays, the wiring should be sized by the Isc+125% and/or the protective device should be sized to insure that it can break the maximum current available from the solar panel (with batteries, this is a problem as they have very high currents available--virtually never a problem for the standard home solar array system).

And, I would even go farther that the array wiring only needs to be sized as the same size as a safety ground wire (don't have an NEC book handy, but IIRC for a 125 amp service that insulated ground wire would only need to be 8 awg)--which is much smaller than the normal NEC rating requirements for standard AC branch circuit runs (again, if I remember correctly).

So, back to the device itself. I have a little 40 watt power brick on my laptop right here. There is no way in heck that it can sustain a 15 amp internal short circuit as would appear to be required by the 60/125%=48 amp interpretation of NEC for the Xantrex requirement.

Instead, the little power brick is designed to use 94V2 or 94V0 flame rated printed circuit boards, may (or may not) have an input fuse (not required). And the plastic case probably has some metal for heat sinking and protection. And the case of the brick is (or should be) made with a flame retardant plastic.

I don't care if the device is a solar charge controller, a Grid Tie inverter, an electric motor, a computer power brick, or anything else. The devices are safe to run at their ratings and will not exceed safe operating conditions as long as the input conditions (voltage, frequency, wave forms) remain within specified limits.

The only requirement is that they operate and fail safely with specified limits and that if the input source exceeds any of those limits, then it is the responsibility of the "facility" to provide upstream protection for the wiring and device.

So, getting back to "brass" tacks--what is the maximum ratings of the input box to the Xantrex Grid Tied inverter. That would include the size of the knockouts, the volume of the wiring/junction box and the wiring attachments. Notice (as far as I recall) there is no condition on the operational requirement of the device (other than that the terminal/j-box ratings must support that current/voltage). How the device under load fails (pop internal fuse, pop a trace, pop a transformer winding, etc.) does not matter--The only thing that matters is that the device under load fails "safely".

There can be some hand waving on whether or not the input current rating of the branch wiring for the Xantrex GT Inverter needs to be Iinverter-rating (not Isc or Isc) Irating*125% (as I recall, there were some exemptions where you could run at 100% of rated circuit current for some specialized installations)--but in actual design and operation, pretty much everyone well oversizes their input wiring to keep voltage drop/I^2*R losses to a minimum and using 125% (or even 125%*125%=Isc over-rating * Branch over-rating) ratings are going to easily met (in all but the shortest solar panel wiring runs).

So--the end results of my argument is that if the Xantrex unit (for example) is rated for 60 amp input maximum and the unit self regulates to 60 amps maximum, then I do not see a reason to run 48amp*125%=60 amp wiring from the solar panels to the inverter and limiting the solar panels to Isc/Imp of 48 amps.

Instead, I would suggest the system be designed for 60 amp input, the wiring should then be sized to 60amps*125% NEC branch circuit rating. And check the Isc*125% (fine with me) against safety wire requirements (if I am correct that safety ground requirements allow smaller awg wiring)--and use whichever requires the larger awg. It gets a bit more complex if there are combiner boxes and series protection required for parallel strings--but that is beyond my basic argument here.

For the solar panels, I would design them for Imp (or even higher--as 99% of the time most installations very rarely see Imp and 10-20% larger arrays will rarely be limited in harvest capacity in temperate climates--and in any case the GT inverter will self limit its total power conversion).

And in the case of failure (shorted input on the Xantrex inverter, or failed wiring insulation), then normal NEC design/installation requirements should easily and safely handle those abnormal conditions.

As always, this is my own opinion and is not a recommendation for anyone to violate NEC/NRTL/Building/Fire codes.

-Bill

Re: Xantrex XW60 CC MPPT any feedback Yet ?

By the way, fixed my post above where I was trying to type about branch wiring ratings based on inverter input current and not Isc ratings per say. (fixes in bold).

Well, that 1991 statement by Mr. Wiles was only sort of operative at that time and did not really apply to NEC anyway...

NRTL's (like UL) are an outgrowth of the insurance industry. The basic UL documents (as I see them) are 1) design guidelines, 2) a design validation framework and service, and lastly, 3) source inspections to insure that manufacturers along the whole component/product pipeline are using the materials that they said they would use are actually being used...

For example, I spec some wire (per NEC and UL requirements). UL inspects a prototype of my design, makes sure that I used the correctly rated insulation, awg, termination, do a power on test (measure a few temps and voltages), and (depending on where it is used) maybe a high voltage and leakage current test.

UL writes up a report for the product (or component like circuit breaker, plastic, PC board) and then comes every 6-12 months to the factory and makes sure that every "controlled" item I called out in my design is still being used and that the UL approved tags/markings are present on the raw components.

So... Writing up a strange belts and suspenders NEC interpretation to somehow account for none-of-the-above happening (not UL/NRTL'ing the device itself) is transparently useless... It does not make the non-UL device one bit safer.

-Bill