Max Inverter cable size causing problems

[DavidH]

Ok, in a "nutshell" I'm finding that the max AWG 4 wire the inverter will accept in the screw terminals is falling short of the recommended NEC wire size for wires in conduit when amps from the surge is taken into account. So I'm wondering what the solution is.

More details of the system which will not be grid-tied and a standalone system:

Enclosure and Mppt.JPG

This is exactly what I was thinking of attaching directly to the cement wall indoors (I live in PR and home is cinder block with concrete ceiling/roof. I have both items in this image, the MidNite Classic and the MNDC enclosure. Wires exiting at the bottom left go to the battery bank (x2 12V 100Ah to make 24V battery bank), wires exiting bottom right are PV wires and ground, wires exiting on the left side go to the inverter. (There are numerous knockouts that can be used though).

My inverter is a Victron Phoenix 24/1200. I do not plan to use it anywhere near full capacity, however, I have to size the wiring for the maximum surge (as I understand it). The surge is 2400W which would work out at 100A on the DC side (2400/24) but I was advised to factor in an inverter efficiency of 80% for a safety margin = 125A max on the DC side.

In the image above the wire leaving the left side of the MNDC enclosure should immediately enter conduit going to the inverter if it is indoors? This is what I've understood but this immediately would mean the max Ampacity allowed for the 4 AWG wire would be 85A @ 75C (and only 95A @ 90C) which is below the 125A calculated for a surge putting me in non-compliance with Table 310.15(B)(16). Now at the 1200W it's totally fine as I'll have a max 62.5A (and I'm in no way planning to use 1200W continuous) but I'm under the impression still have to size for double this due to surge potential which has me banging my head against the wall. The max wire the inverter can accept in the screw terminals is 4 AWG - why in the world would they design it like this or have I misunderstood something?

The two ampacity tables for conduit and free air. https://www.lapptannehill.com/resour...ampacity-chart

Any suggestions?
Can I somehow have the inverter 4 AWG wires not entering a conduit so that they comply with Table 310.15(B)(17)'s "free air" values? Maybe another enclosure to house the inverter that joins right up alongside the MNDC enclosure like the MidNite Classic 200 is attached in the image?
Or is there some adapter that would fit into the screw terminals but allow me to attach a larger wire size to be in compliance?

Or what if I use an 80A breaker between the inverter and the battery to protect the 4AWG wire (rated 85A @75C in conduit) from the surge?

Any thoughts appreciated!
Thanks

[DavidH]

Here's an image of the Phoenix Inverter's screw terminals (apparently rated at 101A) and this is the link to the datasheet. http://www.euroclamp.it/MPT2515

screw terminals.png

[Ampster]

Tell us more about the surge? How long does it last? You have control over the surge depending on what loads you intend to put on your inverter. Is there a reason you are sizing for the surge?
I would just use an 80 Amp breaker and 4 AWG wire.

[DavidH]

Hi Ampster,

I would love to not have to size for the surge because I'll not be putting anything on the system that would surge to that amount. I was under the impression (hopefully mistaken) that the wiring would need to account for the potential surge conditions but if like you say, the 80Amp breaker and 4 AWG will be fine, then that's what I'll go for.

I can't find anything in the manual about the length of time the inverter can cope with the surge of 2400W. I'm just reading a "very short time" on various websites.

[Salts]

Hi Ampster,

I would love to not have to size for the surge because I'll not be putting anything on the system that would surge to that amount. I was under the impression (hopefully mistaken) that the wiring would need to account for the potential surge conditions but if like you say, the 80Amp breaker and 4 AWG will be fine, then that's what I'll go for.

I can't find anything in the manual about the length of time the inverter can cope with the surge of 2400W. I'm just reading a "very short time" on various websites.

You can put any size cable on there that you want to so long as you have a fuse or breaker protecting it. I would suggest you just use the biggest wires that the terminals will accept and then put a DC circuit breaker on them rated for the wire specs. You need the breaker anyhow. Its not there to protect the charger, its there to protect the wire.

Most of the inverters that I see have surge ratings that are designed for start-up currents measured in milliseconds. My Sunny Island inverters are rated for 6000 watts continuous but can surge to 9000 watts for 30 minutes or 11000 watts for 60ms. 9000 watts at 30 minutes is a concern, but 11000 watts for 60 milliseconds isn't enough to heat up much of anything. In fact, the 11,000 watt rating is past what the breaker is set for, and this is why fuses and breakers have time delays. They're designed to pass the surge without blowing because its only momentary.

[Mike90250]

You can size the wire for YOUR max surge, not the full capacity of the inverter. You can also use a splicer to convert from 0 to #4 and only use #4 for the last 6", which greatly reduces the voltage drop. Here's specs for water pump motors, which would apply to many capacitor start motors


Well Pump Motor Specs.jpg

[DavidH]

Thanks Mike, Salts and Ampster for clearing up that misconception I had. That makes things much easier!!
Thanks for the chart and info on the splicer Mike.