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somewhereinusa

Question for the electrical knowledgeable

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I'm upgrading my house bank from 4, 6V lead acid golf cart to 4, 12V lithium batteries in parallel . 
My inverter has a max charge rate of 150 amps and each battery has a max continuous discharge rate of 150A.
According to the various cable size charts I have looked at 4 gauge is good for 200A at 4 ft with 2% loss. My cables will be less that 2 ft.
I can get a really good deal on some 4 gauge wire. Does that wire size sound right with a safety margin?
 
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I use marine grade wire. My chart shows 4 awg safe to 160 amps in open air.  At two feet and 150 amps, there is less than 2% voltage drop. My heaviest load is 150 amps with the microwave. But I feel better with 2/0 cable, and that is what the inverter specifications list. I also like less than 1% voltage drop for charging and other sensitive devices, such as my fridge, and the inverter. Inverters are not chargers, they make 120 v ac from 12 v dc.

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Yes. I would check if you inverter/charger gives specification. My magnum called for 4/0.

 

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Good question, FWIW here's my response:

  First of all, when looking at all the wire gauge and ampacity charts out there, many don't realize there's a HUGE difference based on a) If the conductors are enclosed in raceway (and how many conductors in what size enclosure) or conduit or earth and b) The class and type of insulation       VERSUS       a single conductor in free air which has a much higher current rating due to the increased ability to dissipate heat.

That being said, one chart I looked at indicated 4 AWG THHN copper wire in free air at 30 Degrees C ambient temperature was rated for 135 Amps.    For that wire size the absolute max rating was 170 amps at 125 Degree rise or 180 amps with FEP (Teflon) and Silicone). Me being conservative, Id use the lower 135 amp rating and no more then 150 amps to be on the safe side. 

When I size wire,  I first calculate the "maximum continuous current" load,,,,,,,,,,,,Then, using the  most conservative ampacity chart for single conductors in free air  (in your case you indicate 150 amps) I choose a conductor which has a minimum ampacity of 125% of that (in your case 1.25 x 150 amps = 188 amps) ,,,,,,,,,,,,,,,,,,,,Then I calculate voltage drop based on wire size and current and length and upgrade the wire size as necessary THEN FINALLY AND MOST IMPORTANT  I (so long as the same or bigger then I just computed)  COMPLY WITH MANUFACTURE RECOMMENDATIONS  which I often find to be larger then I calculated and to insure warranty compliance plus minimize voltage drop...…...I recommend you never go below manufacturer and warranty specs and also never go below the calculations I suggested above AND USE WHICHEVER IS THE BIGGEST WIRE.

if you haven't guessed already ITS MY ADVICE THAT 4 GAUGE IS INSUFFICIENT for a maximum continuous current of 150 amps. Sure, it will "work" although it may, if there's a prolonged 150 amp current draw, and depending on temperature and class and type of insulation,  overheat above its rating even if not enough to degrade the insulation or become a serious fire or life safety hazard.  While for 150 amps I see 4/0 wire as possibly over engineering HOWEVER that will dramatically reduce voltage drop and IFFFFFFFFFFFF the manufacturer requires its use in order to safeguard any warranties and provided its higher then my calculation method above, best to consider doing as they say (but wow that's awful big cable for 150 amps !!!!!!!!!!!!! even to a conservative designer like myself)

PS The NEC used to permit multiple parallel conductors for added ampacity but ONLY in certain as I recall larger wire gauges and I'm unsure if 4 Gauge qualifies.  For no more lengths then you require Id much prefer a simply larger then 4 Gauge wire if necessary...…..less voltage drop, less temperature rise, safer if bigger wire is used !!! 

DISCLAIMER I'm a longgggggggggg retired n very rusty and NOT up with the latest codes having retired as a Power Distribution Design Engineer so NO WARRANTY....Consult the NEC and any applicable RVIA or RV codes if in doubt and especially where fire and life safety are concerned, don't take my word for it...………………. (Although 4 Gauge is less then what Id use and recommend, yes it can still "work")

 John T   BSEE, JD

      

Edited by oldjohnt

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6 hours ago, somewhereinusa said:
I'm upgrading my house bank from 4, 6V lead acid golf cart to 4, 12V lithium batteries in parallel . 
My inverter has a max charge rate of 150 amps and each battery has a max continuous discharge rate of 150A.
According to the various cable size charts I have looked at 4 gauge is good for 200A at 4 ft with 2% loss. My cables will be less that 2 ft.
I can get a really good deal on some 4 gauge wire. Does that wire size sound right with a safety margin?
 
__________________

Don't forget to include a 200 amp catastrophic fuse at the batteries ... you don't want to allow more current than that to flow through the wire in case of a short circuit or overload.

As long as your maximum load is less than 200 amps you'll be fine (with the fuse).

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1 hour ago, Lou Schneider said:

Don't forget to include a 200 amp catastrophic fuse at the batteries

Good point Lou, you wouldn't believe how many DIY installations I see or read about (even You Tube How to Videos, Yeah Right) where the so called or what the installer thought was for "catastrophic protection" fuses were placed say at the Inverter input or maybe the MPPT controllers output   NOT  at the battery YIKES  !!!!!!!!!!!!!

 John T

 

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One thing I didn't seem to make clear. I'm not talking about the wire to the inverter, it is 4/0 I'm talking about the battery connections from the stud/buss bar, to the individual batteries wired in parallel. Inverter is 3000 W. Using method #3 recommended here http://www.smartgauge.co.uk/batt_con.html  

All of my battery wiring is open air and copper, not copper clad.  

Quote

Don't forget to include a 200 amp catastrophic fuse at the batteries ... you don't want to allow more current than that to flow through the wire in case of a short circuit or overload.

As long as your maximum load is less than 200 amps you'll be fine (with the fuse).

Are you talking about fusing each battery? I have a 300 A fuse between the battery bank and the inverter as recommended by Xantrex.

 

Edited by somewhereinusa

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Thanks for the update somewhere, FWIW here are my thoughts:

The use of individual copper conductors in free air yields the highest ampacity ratings in typical wire charts as it allows for more efficient heat dissipation versus if they were enclosed in a raceway.   

I do not fuse "each individual battery" in a multiple bank, only from the buss/combination of such up to loads BUT I CANT QUOTE OR GUARANTEE  ANY APPLICABLE CODE to justify how I do it ??????????? I don't recall seeing or reading about each individual battery being fuse protected nor am I aware of the presence or lack thereof of any code requirements to do so, maybe someone has that info and can enlighten us all ??? Just because I haven't seen it done doesn't count for much...…….. 

Regarding catastrophic protection as Lou discussed and the 300 A fuse you mentioned, of course as you likely already know, that needs to be located at the batteries NOT up at the Inverters input.

  Nice system you're building, best wishes

  John T

 

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5 hours ago, somewhereinusa said:

I'm talking about the battery connections from the stud/buss bar, to the individual batteries wired in parallel.

The battery interconnects carry a portion of the load, resulting in final load amps at the inverter. You can size those cables according to your expected largest load.  When I was a 12v guy I did that to save the hassle of 4/0 interrconnects.  Works fine.

Edited by hemsteadc

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5 hours ago, somewhereinusa said:

One thing I didn't seem to make clear. I'm not talking about the wire to the inverter, it is 4/0 I'm talking about the battery connections from the stud/buss bar, to the individual batteries wired in parallel. Inverter is 3000 W. Using method #3 recommended here http://www.smartgauge.co.uk/batt_con.html  

All of my battery wiring is open air and copper, not copper clad.  

Are you talking about fusing each battery? I have a 300 A fuse between the battery bank and the inverter as recommended by Xantrex.

Lithium batteries differ from lead acid batteries in that their voltage drops very little as they discharge.  This means it doesn't take much in the way of a wiring fault such as a corroded or loose connection to shift the load away from one or more parallel batteries to the remaining ones, versus lead acid batteries where the voltage on a heavily loaded battery degrades faster and keeps thing more balanced.

Your choices are to either individually fuse each battery to a value that protects the smaller wires, or make the catastrophic fuse small enough to protect the smallest wire.

Or make sure there's nothing the smaller wires can short against or catch on fire if they start glowing red hot from a potential overload before the 300 amp fuse blows.

Edited by Lou Schneider

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Somewhere, hey this is getting deep but fun for us sparkies at least lol. Here is a further analysis based on your new information:

 

If your Inverter is 3000 Watt and if you feed it with your battery banks at lets just use 12 Volts (even if not exact only an average approximation remember) for these purposes, 3000/12 = 250 Amps

You say you are using Method 3 as suggested by Smartgauge:  http://www.smartgauge.co.uk/batt_con.html

You indicate you are fusing the Inverter input feed at 300 Amps:

If Method 3 causes each of your four batteries to supply approximately the same current to the Inverter that would mean each is producing 250/4 = 62.5  Amps . NOTE that's absolute worst case full loaded scenario which you may well never meet or for very long.

I would side the individual battery cables (as in Method 3) to have an ampacity of no less then 125% of the 62.5 or 78 amps.

THEREFORE in the event you choose for whatever reason (that's your call) to fuse each individual battery and using cables (each battery as shown in Smartgauge Method 3) rated at no less then 78 Amps, I would likely chose an overcurrent protection device of 80 amps.

There ya go, if you use Method 3 and you choose to fuse each battery individually, I would use 80 amp rated battery cables and 80 Amp overcurrent protection. In the unlikely event you run at full power for extended time and an 80 amp thermal magnetic breaker were to nuisance trip its thermal element , sure you might have to bump things up a size.........….

NOTE I have not addressed your other battery loads (you may well need more then 80 amp wire and fusing at each battery) just the Inverter for now (subject of your question), this is NOT the whole story but we can talk about other wiring, distribution and overcurrent protection when you need it. FWIW I didn't fuse each of my four batteries...???

Let us know how you end up

John T

 

Edited by oldjohnt

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Do not use a 300 amp fuse on 4 awg cables. At 300 amps the insulation will soften and start melting.

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