Having trouble with inverter/charger wire size.... (whats new!)

cdherman · February 2013

Just got a Xantrex Prosine 2.0 Inverter/Charger off eBay. (12v, 2000w sustained capacity) The unit is a replacement to an old APC 1400 battery backup that I had for years to power critical loads in my home during storms. Router, cordless phones, vonage box, internet modem, a light in the basement etc.. Old APC unit eats bats, since in overcharges them, and I could not fgure out how to reprogram the unit (made in 1999). Gave up....

The Prosine is massive overkill for the loads I anticipate subjecting it to, but well.... sometimes you have to indulge yourself.

So the manual is recomending 250MCM cable for runs under 6', 350mcm for up to 12' -- and a 300A type T fuse. Their rationale -- the inverter is capable of output to 4500w briefly to start motors etc, so the cables need to be properly sized for those sorts of surges.

Man, those sorts of cables are pricey, and moreover, the lugs for them are all 3/8" or 1/2", which don't fit the 5/16" lugs of the unit.

My plan is for a pair of parralel 100A/Hr deep cycle AGM bats (closed space install), so even if I did have the cableing to run something with a motor or other surge needs, I would be a fool to do so.... I'd burn through the bats in minutes.

I was playing with the voltage drop calculator -- it maintains that bury or conduit 4/0 copper can only handle 230A. I'd like to use 4/0 as its cheaper and easier to get lugs to fit etc. From my reading of the Prosine manual, if there is a large instantaneous demand on the inverter that drives a high A 12v demand which cannot be met, for whatever reason, the unit may behave erratically (read, shut down), but they did not suggest damage to the unit.

I also see many varied estimations of 4/0 amperage capacity out there --- some as high as 400A. Anyone know why the voltage drop calculator seems so conservative?

In the end, anyone care to tell why I should not go with 4/0? I think I can get the cable run down to about 3' (or even less, but I want the cables to be balanced length.). I might even be able to get by with 2/0 or even less, if I am willing to sacrifice my surge capacities. But the price/performance curve seems to really take a turn downward when you go from 4/0 to 250MCM....

Thanks....

vtmaps · February 2013

Re: Having trouble with inverter/charger wire size.... (whats new!)

cdherman wrote: »

In the end, anyone care to tell why I should not go with 4/0? I think I can get the cable run down to about 3' (or even less, but I want the cables to be balanced length.). I might even be able to get by with 2/0 or even less, if I am willing to sacrifice my surge capacities.

If you want to meet code, you must follow the recommendations in the manual. If you use thinner cables, you should reduce the fuse size accordingly. The inverter should protect itself from the (apparent) low battery voltage.

The two short cables that connect the batteries in parallel must be the same length. The cables that connect from the diagonal corners of the batteries to the inverter do NOT need to be of equal length, but of course should be as short as possible.

cdherman wrote:

Anyone know why the voltage drop calculator seems so conservative?

Which calculator are you using? The allowable current in a wire depends on temperature, insulation, location (within a chassis? within conduit?), and crowding (how many wires in the conduit).

--vtMaps

cdherman · February 2013

Re: Having trouble with inverter/charger wire size.... (whats new!)

vtmaps wrote: »

Which calculator are you using? The allowable current in a wire depends on temperature, insulation, location (within a chassis? within conduit?), and crowding (how many wires in the conduit).

--vtMaps

http://www.windsun.com/Hardware/Voltage_Calc.htm

It just gets confusing. Here: http://www.wireandcabletogo.com/Welding-Cable/ they describe Type K welding cable as having 30g strands, and being rated at 310A, Whereas Type M weldlign cable would have 34g strands and be rated at 405A

Whereas, our very own host here: http://www.solar-electric.com/hardware---wire.html says their welding cable is type M and has 30g strands.

I'd go for the 250MCM wire, but it seems like fittings are all much more rare and sized for larger lugs that I will ever have.

Even the crimper for do-it-yourself work says it only goes to 4/0. If the MCM wire is so rarely used, then I cannot figure out why I really need it. I mean, I follow the logic and math that if I want to start a motor that take 4000 watts 110V briefly, then for the inverter to meet that its got pull around 400A 12V for a moment. But if I am willing to forgo that capacity, then 4/0 should suffice.

Agree that after more reading, I should reduce the fuse to 250A. Seems in a 2 battery, 12v parallel string, I need fuse in each side, to keep things balanced. I had hoped that I can place the fuses between the batteries and then the inverter leads from oposite ends, such that at any time, an overload would pass through one or the other of the fuses. That placement however, looks funny on paper, so I suspect its wrong somehow (fuses in parrallel = problem I think, as it would double their A capacity)

So if I place the fuses in the leads from the inverter, one on each side, with equally long cables, then I would be balanced. But the throretical event that a batter shorts out across its terminals or internally and then the other battery dumps its energy into the first battery remains. Seems unlikely even since I am using AGMs...

Anyhow -- I digress. Thanks for the reply.....

Cariboocoot · February 2013

Re: Having trouble with inverter/charger wire size.... (whats new!)

If you're not using it at maximum capacity and it doesn't have to meet code (back-up unit; not hard-wired) you can shrink the DC wiring and as vtMaps pointed out the fuse.

2000 Watts @ 12 Volts is 167 Amps. Consider that to be your "high end". 4/0 will handle 300 Amps in transmission. A 200 Amp fuse would suffice. If you're really worried about popping fuses from time to time use a breaker instead.

Voltage drop on 6' of 4/0 at 167 Amps using 12 Volts is <1%. I wouldn't worry about it.

smatthew · February 2013

Re: Having trouble with inverter/charger wire size.... (whats new!)

I seem to recall that the ProSine inverters are more sensitive than most to the size of the battery cables. 4/0 should be fine, but keep the cables as short as reasonable.

niel · February 2013

Re: Having trouble with inverter/charger wire size.... (whats new!)

i think the problem would manifest out of the battery voltage being drawn down under a heavy load too. between the lowered battery voltage and any v drops on the wiring it could kick out the inverter for too low of voltage. even if the v drop was nearly non existent the battery voltage drop itself could be a deal killer. if you find it kicking out you can try to measure the voltage at the batteries and at the inverter to see if this is presenting a big problem. if what is measured at the batteries is quite low then it is a capacity problem. for example you could have a minimum voltage input requirement of 10.5v (typical of many inverters). if the loads pull the battery voltage down to 10.8v and the v drop through the wires is .3v (2.5% v drop) this represents the 10.5v kick out point.

also keep in mind that a 2000w draw at a pulled down battery voltage of even 11v is 181.8a plus the draw of the inverter to operate. this higher current draw presents larger v drops across the wires. btw, at 181.8a the v drop % is about 1.25% and the voltage dropped is .136v and is far lower than my example is so the 4/0 should be fine. any problem, as i said, would come from the battery voltage itself sagging too low, but the v drop would add to that sag as my example does indicate.

i feel the worst case is the need to get more battery capacity.

Having trouble with inverter/charger wire size.... (whats new!)

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