Inverter & Battery Cables

Make the Right Connections for Best System Performance
Intermediate

Inside this Article

Copper battery cable
Fine stranded, UL listed copper battery cable.
4/0 UL battery cable
This fine-stranded 4/0 AWG, UL-listed cable is designated as THW, making it appropriate for battery–inverter cabling.
Welding cable vs. battery cable
Welding cable (top) and UL cable (bottom) look very similar, but welding cable is not NEC-approved. Be sure to carefully inspect the markings before you make a purchase.
Copper cable lugn
A bare copper closed-end-type lug made for 4/0 cable.
Tin-plated battery cable
A tin-plated copper lug is crimped onto its cable, protected with heat-shrink tubing, and marked for polarity.
Tinned and crimped lug.
A crimped, open-ended lug.
Pulled out, poorly installed battery cable
A tug on this poorly crimped cable pulled it out of its lug. Good crimps are necessary for safety and low-resistance connections.
Heat shrinking
When heated, heat-shrink tubing with sealing glue inside will keep out corrosive fumes and support the fine wire strands.
Quality crimping tool
Using the right crimping tool for the cable size and lug type is crucial for making a good mechanical connection between the wire and lug.
Components & finished cable
The cable components before (top) and after (bottom) assembly, crimping, heating the shrink-tubing, and labeling with colored tape.
Copper battery cable
4/0 UL battery cable
Welding cable vs. battery cable
Copper cable lugn
Tin-plated battery cable
Tinned and crimped lug.
Pulled out, poorly installed battery cable
Heat shrinking
Quality crimping tool
Components & finished cable

Poor-quality and improperly installed battery and inverter cables can cause problems in the function and safety of a battery-based system. Here’s how to select the right cables and install them correctly, for optimal system performance.

There is a perception that battery and inverter cables are expensive—and it is a tempting place to cut costs—but buying cheap cables can result in significantly reduced performance of the battery bank and inverter(s). It’s a lot like putting cheap tires on a high-performance car—you save some money, but you don’t get the performance and safety you might need. The common problems seen with cabling in battery-based renewable energy (RE) systems are typically due to low-quality cables and hardware, in combination with poorly made crimps and connections.

You can purchase preassembled cables or have them made to order, but you can also build them yourself. The details are important—battery cables and their ring terminal connectors (also called “lugs”) carry high current and are used in harsh environments where they can be exposed to sulfuric acid, hydrogen gas, high temperatures, and dissimilar metals.

Cable Ampacity

For battery/inverter RE systems, the largest conductors in the system are usually the ones connecting all of the batteries together and then exiting the battery box to connect to the inverter. Since nearly all battery-based inverters operate at 48 VDC or lower, the cables need be large to handle high currents without significant losses. Sizing of these cables is based on the battery voltage, the inverter’s continuous amperage rating, and the length of the cable. Commonly, these cables are either 2/0 AWG (acceptable for use with a maximum of 175 A breaker or fuse) or 4/0 AWG (acceptable for use with a maximum of 250 amp breaker or fuse), but will need to be individually calculated. For example, the installation manual for OutBack Power Systems’ VFX3524 (3,500 watts; 24 VDC) inverter recommends 4/0 AWG for a battery-to-inverter cable length of 10 feet or less. This size cable would result in a voltage drop of less than 1% at full rated output of the inverter, resulting in 34 watts of losses in the 10-foot-long positive and negative conductors. Shorter cables would reduce the losses proportionally. 

Cable Types

High-quality battery/inverter cables are made of fine-strand copper conductors with a flexible insulation covering and are available from manufacturers such as Polar Wire Products or Cobra Wire & Cable. Although finely stranded cables are not required, they make installing and servicing the system easier and reduce stress on the battery and inverter terminals. All high-quality battery cables are made with UL-listed wire and include a National Electrical Code (NEC)-required designation, such as RHW, THW, or THHW.  

Lower-quality battery cables are often made from automotive or welding conductor cable. This type of cable is cheaper and easier to obtain—but is not acceptable by the NEC since it is not UL-listed or marked with the NEC wire type. While some types of welding cable have a UL listing, they have been approved using a different set of UL standards and tests, and are not marked with the required NEC wire-type designation.

Comments (15)

ideas2014's picture

dear Michael
dont worry i am not doing any thing myself ,,i just learn for my knowldge and understanding tt all ,,,when it comes to action i will engauge the experts ....thawhy i am here happy from this productive discussion and ideas sharing from people in this field ..
if u know any friend or isntaller in this field pls send me email contact
rgds

Michael Welch's picture

Here is a great place to start. Anyone who is fully NABCEP-certified has good experience and knowledge. Just make sure they are experienced also in battery-based systems.
http://www.nabcep.org/certified-ins...

ideas2014's picture

dear MIchael
thanx for your sharing ,,by the way i tried the link u sent to check the voltage drop accoridng the length i want to use which 10 ,m and found there is option if i take will make the voltage dropp lesser , which is 2 condcutors per phase parrelle ,,,
is that mean i use 2 cables from same dia for same cross section for the copper 110 mm2
can any one elaborate on this and send me any helping iamges to imagin how this cable should looks like
should i use for + , - ? or enough for +
really i gained lots of knowdlge from this discussion and experinces sharing
thanx

Michael Welch's picture

I interpret it to mean the number of conductors in that box of the size in Wire Size box. Then you need to do the same thing for both the positive and negative conductors. In the case of this computation, you will need a total of four of the 0000 AWG cables, 33 feet long.

But I must say that the more I read your posts, the more concerned I am that you don't have the working knowledge necessary to successfully do this project. I think you would do yourself a BIG favor by involving an experienced, reputable installer of battery-based energy systems. If there is not one locally, I suggest a qualified electrical engineer.

Justine Sanchez's picture

Hello!
Thanks for your post. In this case we were showing the full cross sectional area (outer diameter OD value) for this particular Cobra flex cable and calculated the cross sectional area like so:
for example with the 1/0 AWG they list the O.D to be 0.575 inches, which is 14.6 mm and divide that by 2 to get a radius of 7.3, which we can plug in to the area formula:
3.14 x r2
3.14 x (7.3mm x 7.3mm) = 167 mm2

You are correct that the inner cross sectional area (of just the copper) would indeed be a smaller value.

Best,
Justine Sanchez
Home Power Magazine

BARRAU Lucas's picture

Hi everyone,
I think there is a mistake into the conversion in mm2

AWG 1/0 is equal to ~55mm2
AWG 2/0 is equal to ~70mm2
AWG 3/0 is equal to ~85mm2
AWG 4/0 is equal to ~110mm2

Regards !

bob tarzwell's picture

jim I have done hundreds of solar battery install and I don't think I have more then a few that have 10 ft of wire between batteries and inverter . So I personally think its a would like to have rule not a hard fast rule. yes you can double and triple up on wires and extend your length use a copper buss bar to tie then in together rather then the top of a battery . I go down to the scrap dealer and look at what he has and as long as my resistance I need is ok I will even use 1 4/0 wire and 2 2/0's if that's what I can find . As long as you keep your wire voltage drop to a minimum then you should be ok , note I do use ferrite core noise suppressors on each end of the longer wires. and I run them separate but close together . Ps im installing in the out islands in Bahamas so correct wire and other items is not always available we use what we have .

bob tarzwell's picture

voltage drop in cables is a well understood engineering calculation . knowing the cable size look up in a wire chart the resistance per foot , times that by the length and you have a total resistance ,if you have two same sized wires divide the resistance in half , next calculate your current draw , 30 kw with 48 volts is 30,000 /48 = 625 amps, at 625 amps to lose only 1volt you would need a resistance lower then .0015 ohms a meter of 4/0 is .00016 ohms 2/0 is .0002 and 1/0 is .0003 ohms per meter . a 10 meter run at 625 amps would need a single 2/0 wire at .002 ohms. now 1 volt is a fair loss you said it was for occasional 1 hr use so to have a loss of 650 watts/hr in the wire would be acceptable, in a in use all the time grid feed like I have at 24 kw I wanted less loss so I aimed at a lower voltage loss of .1 volts . I would suggest two runs of 4/0 or bigger wire will allow you to run for longer periods if needed with out to much wire loss, the other area to consider is how low a voltage your inverters can take , look up the spec if 48 volts is it, then the 1 volt your loosing may limit your lower battery cut of set point. This is not normal solar engineering but a special case.

ideas2014's picture

dear all ,,,
i have my own logic reasons to have the inverter away from the battery bank ,,i an not just asking question or raising problem for fun and waste our time all ...
my direct clear question ,,how much energy i could lose or voltage drop for 10 m length between battery and inverter for 30 kw ..
if i am talking about battery bank 48 VDC to keep my power steady in case grid faliure for one hour max ...
my renewable source of energy is neither solar or wind and my battery role is to absorb the fluccuation of my voltage ..

hop i didnt make it very diffecult
thanx again for every one sharing and enrich this discussion

Michael Welch's picture

Bob Tarzwell is correct and my response was wrong and I will delete it so that others are not misled. When I first plugged your figures into an online calculator, it gave me an 8% voltage drop at 24 ft. round trip, with 250 kcmil cables, which are even larger than 4/0. Such a large voltage drop is normally considered unacceptable, at least for home RE systems. I am not sure what went wrong with the calculation, but I should have suspected it was incorrect and rechecked the figures with another source. Mea culpa.

My coworker saw this comment thread and sent me this calculator, which does indicate that a 4/0 approximately 10 m battery-inverter cable distance will have a 1.4% voltage drop. Paralleling two 3/0 cables would keep the voltage drop well under 1%

http://www.calculator.net/voltage-d...

Multiple cables are acceptable, and I concur with the recommendation for using bus bars for all cable connections for multiple inverter cables. Use a convenient length of copper flat stock, and bolt the inverter(s) cables to it separately, without stacking the lug ends. If the battery has multiple strings, also bolt each string to the copper bus bar without stacking. This will create a situation where the potentials for all cables are equal, for practical purposes. The battery charging source(s) can also be landed there.

I think that most inverters can only land one cable, so you may be required to use a short stub of 4/0 if you are using multiple battery cables. Also, some inverters, like the SMA 4.5 kw 48 V can't attach cables larger than 3/0.

Though I have not done any calcs, I would think that a 3/16 or 1/4 inch by 1.5 inch piece of copper flat stock would be plenty big for a bus bar.

Ideas2014, most people would not attempt to satisfy the needs of the entire household in case of grid failure. It is common to add a critical loads distribution box which separates out the loads that are needed, often food refrigeration, some lighting, and medical equipment. Many folks are willing to get by without things like air conditioning and home entertainment centers for short periods, and save a bundle on battery/inverter costs by doing so.

Jim Hollander's picture

Very puzzled by the comment that you MUST have 10 feet or less between batteries and inverters even with 4/0 AWG cable.

Can you explain why using two(2) 4/0 AWG cables in parallel for each cable run would not allow you to extend that 10 ft. limit to more distance if you were simply unable to satisfy the 10 ft. rule?

Ampacity is something I always felt I understood, but to not be creative in solving a problem, is just...not being creative.

Yes, I have the Ugly's Spiral bound guidebook, and other electrician handy guidebooks with tables, and use the tables for voltage drop determinations.

I'd like to hear more about the absolute rule on 10 feet if you can?

Thanks in advance.

Jim in NH

bob tarzwell's picture

ideas2014 there is no reason you can not have more then 10 ft of wire between batteries and inverters , im running 24 kw 3 radians 8kw almost 40 ft from the batteries, yes I have multiple big wires ie 8 2/0 wires its all a mater of voltage loss ,you can measure between the inverter in and battery out by adding a wire to your voltmeter to extend it . Under full load you don't want more then a few volts loss or your loosing efficiency.

ideas2014's picture

nice article really but i wish to rasie question about the cable length from batter to inverter ,,this is any equation for that to calculate ,,i have project case that need longer cable more than 10 feet ,,,any suggestions pls share with me

Michael Welch's picture

Hi there. I can help you calculate cable size, but I need the following additional info:
1. The combined length of the positive and negative battery cables.
2. The DC system voltage
3. The size of the inverter in watts.

ideas2014's picture

thanx michael for your help
1- the combined length for + ,- is 20 meter
2- DC voltage 48
3- Inverter size is 10 kw
do u any recommendtion of reliabel inverter fro grid tie Battery based according yr experinces and practices
thanx

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