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.

Lugs

There are many different types of battery cable lugs to choose from if you’re making your own cables. The following are things to consider:

Material. Lugs can be made from many different materials, including copper, steel, or aluminum. To establish high-quality, long-lasting connections, only copper lugs are recommended. Steel or aluminum lugs will corrode over time from environmental conditions or from galvanic corrosion that occurs when dissimilar metals come into contact.

Bare or tin-plated. Copper lugs come in two varieties: bare or tin-plated. The tin-plated copper lugs are usually a dull gray color, and are preferred, especially for use at battery connections, since the plating reduces corrosion that can occur between the copper lug and the battery’s lead terminals, especially when there is battery acid involved.

Open- or closed-ended. To prevent corrosion from entering the conductor strands, use closed-ended lugs at the battery. Open-ended lugs are less expensive and more commonly available, and can be used when making inverter and breaker connections, but should not be used for the battery connections.

Listing. A lug needs to be tested and approved to UL standards, and rated for the system’s maximum voltage, current, temperature, and conditions of use. When using a fine-strand cable, you’ll also need to select lugs that are listed for the wire type (for example, fine strand wire is typically Class K). This can be difficult to determine or verify from the information available from the lug markings and manufacturer’s datasheets.

Sizing. All crimp-type lugs are rated to fit a specific conductor size, so matching the lug to the conductor is required by the NEC to ensure a good connection. 

Bolt-hole diameter. It is also important to choose the correct bolt-hole diameter for the lug and terminal hardware combination. Drilling out the lug’s hole to accommodate a larger bolt is not acceptable as it may reduce the lug’s current rating; it also can result in higher resistance and excessive heat buildup that could potentially result in melted battery terminals or a fire.

Ring terminal size & shape. The “flag” or “ring” of the lug that attaches to a terminal comes in a variety of shapes and sizes. Lugs that provide a large surface area reduce resistance and the possibility of digging into soft lead battery terminals. Some breakers and inverters may need a smaller-size ring to fit on their terminals.

Avoid using set-screw-type compression lugs with finely stranded cable. Under pressure, the fine strands can twist and break off. The high number of strands makes the flexible cable’s connection “soft,” resulting in a connection that will be difficult to get tight and could potentially become loose over time. 

Tight Lug & Cable Connections

Crimping. When lugs are not securely crimped on a cable, the loose connection causes higher resistance to the flow of electrons during charging and discharging. This can devastate the performance of a battery string or entire bank and could possibly result in melted battery terminals and even fire—but those aren’t the only problems. Additionally, this is a hazard to someone maintaining the battery bank. During a routine inspection—for example, when the electrolyte level is being checked—a cable could accidentally be bumped loose from a lug and touch something else, causing a short-circuit or shock. If there is hydrogen gas present, a resulting spark could be very dangerous. Safety concerns, as well as possible performance degradation, can be eliminated by well-made cable crimp connections.

A low-quality crimping tool used to compress the lug onto the cable can result in a loose connection, with only a portion of the cable’s strands of wire making electrical contact. These cheap crimpers often use a single “pin” to press on the lug’s barrel and press the other side into a V-shaped groove, leaving voids inside of the lug. Over time, this poorly crimped lug will become loose, often overheating and failing. The lug  may even come off the cable entirely if pulled.

High-quality crimpers use specific jaws to accommodate different-sized cables and compress the lug’s barrel from multiple angles, either into a square or even a hexagonal shape. This produces a much tighter connection without any voids—making sure all of the cable’s outer wire strands make contact with the lug. These types of tools are more expensive but necessary for making proper connections.

Soldering is an additional means of sealing the connection, so even if you’re using solder-type lugs they first must be crimped on properly and then soldered. Few installers have the equipment to properly solder a cable connection without damaging the insulation; therefore, it is rarely done. Soldered connections are acceptable under the NEC, but may require additional scrutiny by an inspector to verify what lugs and processes were used. 

Protecting the cable. To further protect the battery cable strands from corrosion, seal the crimped connection with adhesive-filled heat-shrink tubing. This is available in a variety of sizes and colors from battery suppliers. It usually is made from a thick-walled plastic material and gives additional support to the delicate, fine-stranded wire where it connects to the cable lug. Don’t be tempted to use electrical tape—it is not as effective.

Terminal Connections

Making a secure, low-resistance connection to the battery or the inverter terminal is just as important as properly securing the cable to the lug. Use the right type and size of stainless steel hardware when attaching to the lead post which is quite soft and can be easily damaged. The correct washer, lock washer, and nut placement is critical to the connection staying tight. Thoroughly clean the lead battery terminal with a wire brush before attaching the lug to achieve a good connection. Then tighten the terminal’s hardware to the battery manufacturer’s specifications and add an anticorrosion coating. Do not put any anticorrosion coating between the terminal and the cable lug.

Be diligent about placing the hardware in the correct order. A hazardous condition can be created if a washer is placed between a cable lug and the inverter or battery terminal. In this scenario, the high current that is drawn by the inverter would have to pass through the washer, causing the connection to overheat—which can damage the battery or inverter terminal and even cause a fire. 

Cable Protection

In most battery–inverter systems, the battery cables are routed from the battery enclosure into a DC disconnect enclosure, through a breaker, and then to the inverter. The NEC requires that exposed conductors be protected. A common solution is to route the cables through conduit, which comes in many different sizes and types, including flexible or rigid, and metallic or nonmetallic.

The cables leaving the battery bank are usually not protected from overcurrent until they are connected to the DC disconnect enclosure, making them a substantial hazard if they are not well-protected. It is recommended to use nonmetallic conduit for these circuits to eliminate the potential for ground faults. The conduit should be attached to the battery and DC disconnect enclosures using a threaded male adapter; use plastic bushings on these sharp threads to protect the cable insulation. The conduit also needs to be well-supported and attached to walls or supports to prevent it from breaking and pulling on the conductors.

Comments (23)

ricks707's picture

The comment on soldering battery cable makes me cringe. Soldering a crimped connection that is not properly crimped increases resistance at the lug which will cause the lug to expand, working loose, and if you are not lucky, molten solder all over the place. And no you can't resolder it reliably. The only satifactory connection is a crimp, of the correct size, done with the proper tooling. I prefer the open ended type lugs, tin plated, done with hydraulic crimping tools. I flex the connection after making the connection, looking for ANY movement of the strands in the lug. After it passes this test, I coat the entire lug except for where it connects electrically with varnish. Please note that shrink tubing that is not adhesive lined is NOT waterproof! It actually makes the connection less reliable as it will cause water to be trapped in the connection area. Basic chemistry comes into play,two dissimilar metals, a little electrolyte? The only question is whether you built a good battery or a bad one. The result is the same, a bad connection. I spend may a year maintaining wiring in electroplating factories where high amp DC is the norm.

Edward-Dijeau's picture

I use "SET SCRWEW" single and double conductor lugs and every time I check the battery fluids, I try to re-tighten the lugs to the wires and the nut holding down the lug to the battery. Double lugs allow the parralel battery conections to be lifted and battery removal and replacment without beaking the "Chain" of current flow to the other batterys from the charge controler and PV array. I Also use multiple 2000/4000 and 1200/2400 watt inveters so I do not use wires larger than #1 AWG in parralllel.

Brady Sheridan's picture

Thanks for the information, now I have it figured out from the battery to the inverter. Can you point me in the right direction for the AC wiring to the panel? Off Grid, Magnum 4400 watt inverter, 16x 230w panels wired for 48v to batteries. Panel is 75' from power shed/inverter and will be run as split phase 120/240 (very deep well pump requirement). I know an electrician should do this (and they might) but I'm a numbers guy and like to understand how it is calculated.

Michael Welch's picture
Again, your inverter manual is going to have the best info (except for the potentially complex details of the National Electrical Code.) Do you have the manual handy? If not, here is a link: http://magnumenergy.com/wp-content/uploads/2012/08/64-0032-Rev-A-MS-PAE-...
Brady Sheridan's picture

Thanks Michael. The manual says "AC wiring must be no less than #10 AWG gauge copper wire and be approved for residential wiring per NEC(THHN as an example)." So that gives me a start but I suspect there is a recommendation/calculation based on the distance or does AC not loose power like DC?

Michael Welch's picture
No, both DC and AC suffer from voltage drop. The smaller the wire and the greater the distance, the larger the voltage drop. Typical design recommendations are for no more than 2% voltage drop in both the AC inverter output and PV DC input circuits.
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

Edward-Dijeau's picture

Look at parallel conductors and the "25 foot Tap rule" in the NEC. Low voltage, under 50 Volts to ground or betwean conductors, needs only the proper fuzing to protect the wire but you may find, because of the very large conductors used, you may want to decrease the fuze size to protect the equipment. They make "Double lugs" for 1-0 wires for connecting to inverters and you can parrallel Low Voltage wires from batteries smaller than 2-0 as long as the total length of each conductor in the parralel run is identical in length and identical in size. The inverters internal fuze protects the conductors as long as they are sized smaller than the full load capacity of the wire and the conductores are 25 feet in length or less. On distances over 25 feet, the conductors must be fuzed at the batteries. 2 - 100 watt slow blow fuzes would protect the 2 1-0 conductors for inverters where your total used load would not exceed 200 Amps or 2400 Watts at 12 Volts 4800 watts at 24 volts. Slow blow fuzes would allow for motor starting and peaks that some loads may experiance as long as this is within the rated "Peak" if your inverter. Voltage drop is based on load current and on low current inverter loads, many inverters will still run with a voltage drop down to 11.2 volts for a short period of time on a 12 volt system. I have found that by connecting the inverter as close as possible to the solar panel charge controler input, the higher PV voltage will help mitigate the voltage drop on load start up for motor leads like Air Conditioning or Refrigerators durring daylight hours. I also recomend you use separate inverters for each moter load otherwise muliple loads can triger a simutanious start when the voltage drop trigers the thermostate of the other unit. You only need one inverter for all your LED lighting loads in your home and small elecronics.

Edward-Dijeau's picture

One more thing. On long distance runs, it would be less expensive to use the stiffer 1 - 0 large strand copper wire in conduit rather than the high strand copper. They make "in line" spaces to convert from low strand to high strand 1 -0 wire at the termination points on the battery bank or inverter. Adding a second battery bank of even just 2 batteries, at the inverter location, will allow for cleaner start ups and peaks in loads. Remember to Fuse the second battery bank.

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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