Battery Cable Sizing

lazza · February 2012

HI Forum

We are about to install a system and I have a couple of question on the batteries. We are about to install 12 2V 900Ah C100 batteries for a 24V system.

The batteries come with 90mm2 (#3/0 AWG) cabling. However I calculate that we only need cables of 50mm2 (#1/0 AWG) for the system (inverter is 2kVA). Can I put a different size cabling i.e. 50mm2 (#1/0 AWG) from the battery negative and positive to the inverter, or must I use the same size cabling that has been used between each 2V individual battery, i.e. 90mm2 (#1/0 AWG)?
I prefer not to ground the negative battery terminal (if i understand what Bill has posted previously correctly: ... "if you have both AC and DC grounded circuits (grounded AC neutral and grounded DC return), the heavy DC currents could flow in the AC wiring returns. DC currents can be 3-10x the AC currents and may overheat the lighter AC wiring."). Does this mean i have to put in fuses in both the negative and positive sides of the circuit?

thanks for your assistance

Larry

vtmaps · February 2012

Re: Battery Cable Sizing

lazza wrote: »

The batteries come with 90mm2 (#3/0 AWG) cabling. However I calculate that we only need cables of 50mm2 (#1/0 AWG) for the system (inverter is 2kVA). Can I put a different size cabling i.e. 50mm2 (#1/0 AWG) from the battery negative and positive to the inverter, or must I use the same size cabling that has been used between each 2V individual battery, i.e. 90mm2 (#1/0 AWG)?

Since the batteries come with the heavier cables, why wouldn't you want to use them?

lazza wrote: »

I prefer not to ground the negative battery terminal (if i understand what Bill has posted previously correctly: ... "if you have both AC and DC grounded circuits (grounded AC neutral and grounded DC return), the heavy DC currents could flow in the AC wiring returns. DC currents can be 3-10x the AC currents and may overheat the lighter AC wiring."). Does this mean i have to put in fuses in both the negative and positive sides of the circuit?

If I recall, Bill's concern with DC currents in the AC wiring was because of the way ground fault protection is implemented in the NEC. The GFP is a small circuit breaker that interrupts the connection between Battery negative and ground. When the connection is interrupted, bad things can happen. The solution, I think, is to have the small breaker ganged to breakers which interrupt both PV+ and PV- cables.
--vtMaps

BB. · February 2012

Re: Battery Cable Sizing

Overall, I prefer to hard ground the battery negative (if negative ground). With a hard earth/safety ground, you do not need to fuse/breaker the return leads.

However, grounding is complex. Some examples:

We (in North America) are used to grounded neutrals (white wire, return wire, center tap of a 120/240 VAC split phase power transformer from pole). Most MSW Inverters do not have isolation between DC input and AC output. If you try and safety ground the DC negative and the "white wire" of the MSW inverter, it is usually a dead short across the inverter and will smoke it. Most TSW inverters use transformers for isolation, and therefore it is OK to bond the "white wire" to safety ground (and mimic utility power connection).

With DC powered devices that are designed (for example) to be used in a car or truck... The metal frame of the radio (example) is the only negative connection. What can happen is--You connect a radio to +/- from battery bank, then it gets grounded somehow to earth (metal table, antenna grounded to earth, etc.) or somebody/something makes a connection to the AC white/neutral/earth ground. Now you have several paths for the current to return back to the battery. The original battery black/return lead, and a more circular path through the green wire/neutral ground bond.

Because 12 volt current is 10x larger than 120 VAC current--it is fairly easy to have excessive current flow through the green wire (AC ground) that exceed its rated current.

If you do not bond a MSW inverter white wire, and use only one connection from your Battery Bank and another from your AC neutral panel to the ground rod (if TSW inverter), then you cannot get ground loops (shared/parallel current path trough both return and green wire ground wires).

The whole issue with NEC and the fuse in the safety ground to earth ground bonding is more complex. It makes the return a safety ground only up to ~1 amp. Above that level, the fuse pops and the safety ground (on the battery return) is no longer a safety ground but a hot lead instead (or else the fuse would not have popped). That has also sorts of safety related issues and system design problems (i.e., that metal case on your converted car stereo/antenna is now "hot" with respect to ground) that I won't go into here unless you want more details.

In the end, a hard bond, done correctly, between battery return (and AC main panel white wire bonding) is both safe, and add to the safety, of the overall power system.

-Bill

nsaspook · February 2012

Re: Battery Cable Sizing

My 2c.

1. Size does matter, in battery cables. That's 83+ amps for 2000 watts. #1/0 meets the NEC ampacity limits but does it meet your voltage drop limits (varies with cable length)?
http://en.wikipedia.org/wiki/American_wire_gauge
http://www.altronix.com/app_notes/calc.php

2. If your DC floats then yes, fuses on both sides are needed. With proper grounding (star-wired) to a single connection point and proper DC/AC isolation (no mixing inside panels, boards or equipment) of the main battery (more than 20A fused) feed wiring the chances of a high current DC fault into the AC circuit ground are low.

Floating DC panel two fuse/breaker example.
http://www.vt-inc.com/products/fdp-dc

lazza · February 2012

Re: Battery Cable Sizing

HI Forum

Thanks for your replies

I therefore take it that grounding the negative terminal of the battery is recommended, except in the case of MSW inverters.

The inverter we shall be using is the a Victron Phoenix Compact 24/2000- the manual is at the following link: http://www.victronenergy.com.es/upload/documents/Manual%20-%20Phoenix%20Inverter%20Compact%202000%20-%20rev%2002%20-%20EN%20NL%20FR%20DE%20ES%20SE.pdf .... I presume this has isolation between the DC and AC parts of the system.

Therefore I should ground:

the inverter chassis
the charge controller chassis
the AC PE wire
the AC nuetral wire (done within the inverter)
any other metal structure within the system

all to one single point, connected to the ground rod???

ps I dont think my cabling question was understood. I plan to use the 90mm2 cabling that comes with the batteries BETWEEN each battery, but 50mm2 cabling from the battery to the inverter.... will this be ok?

niel · February 2012

Re: Battery Cable Sizing

i'll make a few comments on the proposed wire sizes. now your 50mm2 is just a bit smaller than our #1/0, but it is close. from calculating with the v drop calculator, it shows for a 5ft run (about 1.6 meters) to be less than .45% and is acceptable. any chances of longer lengths of wire or larger current draws may be enough to go to the next higher up in thickness (i believe your 70mm2). this brings us up to the battery interconnects as this is all in series and can be considered an addition to the run length. you do not state the total length of wire used for the interconnects, but i would advise you to go with the 70mm2 for the inverter run and you can optionally use either 70mm2 or your proposed 95mm2 for the battery interconnects. it is primarily the parallel configurations that would need the thicker battery interconnects, but a series configuration can be the same as the run wires and just add the length of the interconnects to the run length and the same wire size can be used throughout. this may save you some $ by going 70mm2 on the interconnects, but it will cost you more as i advise going 70mm2 for the inverter run length as well.

if you wish you can see all this in my v drop calculator listed in my signature line. i don't have it setup for metric use on wire sizing, but you can roughly figure the 70mm2 as our #2/0.

westbranch · February 2012

Re: Battery Cable Sizing

Manual states:
In order to fully utilize the full capacity of the product, batteries with sufficient capacity and
battery cables with sufficient cross section should be used. See table.
24/2000
Recommended cross section (mm2)
6 m 50
24/2000
Recommended battery capacity (Ah) 200 – 500

so you have 2x battery capacity and 50 mm XC should work

niel · February 2012

Re: Battery Cable Sizing

i revisited my thinking on this due to westbranch pointing it out what the manual says. the 50mm2 is fine to use if up to about 6m or 20ft total wire length or 10ft run, but that would include battery interconnections. if all battery interconnect wires and the wire run is at or under those lengths then you can use the 50mm2 wire and still have less than .9% v drop loss.

i also missed the point of the 90mm2 coming with the batteries so definitely use them. wire v drops and their losses are additive and you can use the v drop calculator if you wish to view your options and make a decision on what you may want to go with. i should also point out that 83a will probably not be drawn that often either, but it can be.

lazza · February 2012

Re: Battery Cable Sizing

Thanks for your replies.

Yes my question really is only if it is ok to have 50mm2 cable from the inverter to the battery (from the +ve lug to the inverter and from the -ve lug to the inverter).... and then the 90mm2 between batteries as this comes with the batteries?

50mm2 would be fine for the whole run (less than 4 m) to achieve <1% voltage drop including the interconnect wires, but as the batteries come with the 90mm2 already, it makes sense to use them.

I just wanted to know if it's ok to have 90mm2 and 50mm2 cables in the same run.

niel · February 2012

Re: Battery Cable Sizing

if by the same run you mean 90mm2 interconnecting the batteries and then an appropriately sized length of 50mm2 going to the inverter, then yes, by all means.

lazza · February 2012

Re: Battery Cable Sizing

So if I go for a "floating" DC system, do I need fuses on both sides at all 3 points: ie

1. +ve & -ve Between the Panels and the Charge Controller
2. +ve & -ve Between the Charge Controller and the Batteries
3. +ve & -ve Between the Batteries and the Inverter

That's alot of fuses

... can i skip any?

nsaspook · February 2012

Re: Battery Cable Sizing

lazza wrote: »

So if I go for a "floating" DC system, do I need fuses on both sides at all 3 points: ie

1. +ve & -ve Between the Panels and the Charge Controller
2. +ve & -ve Between the Charge Controller and the Batteries
3. +ve & -ve Between the Batteries and the Inverter

That's alot of fuses ... can i skip any?

The quick answer is no. The fusing on the panel input is slightly redundant as they are a current source and won't blow due to a input short normally but provide protection from backfeed power sources ( 2 or more strings of panels) on that part of the circuit.

oil pan 4 · February 2012

Re: Battery Cable Sizing

http://www.powerstream.com/Wire_Size.htm

Cariboocoot · February 2012

Re: Battery Cable Sizing

lazza wrote: »

So if I go for a "floating" DC system, do I need fuses on both sides at all 3 points: ie

1. +ve & -ve Between the Panels and the Charge Controller
2. +ve & -ve Between the Charge Controller and the Batteries
3. +ve & -ve Between the Batteries and the Inverter

That's alot of fuses ... can i skip any?

You can skip at least half of those as you only need fuses (circuit protection) on "one side" of a circuit: i.e. the positive leads.
You only need panel fuses if there are more than two parallel panel connections (either single panels or series strings of panels).
You do need a fuse between charge controller(s) and battery bank. You do need a fuse between battery bank and inverter or any other DC loads. If you have parallel battery connections it is a good idea to fuse each separately (like you would with panels).

nsaspook · February 2012

Re: Battery Cable Sizing

Cariboocoot wrote: »

You can skip at least half of those as you only need fuses (circuit protection) on "one side" of a circuit: i.e. the positive leads.
You only need panel fuses if there are more than two parallel panel connections (either single panels or series strings of panels).
You do need a fuse between charge controller(s) and battery bank. You do need a fuse between battery bank and inverter or any other DC loads. If you have parallel battery connections it is a good idea to fuse each separately (like you would with panels).

He wants his DC feed to "float" so both + and - wires will be "hot" in respect to ground. Without a hard ground on one side of the DC feed there is the danger (small) of a unprotected dual fault with one leg fusing and you need to disconnect both lines to be 100% safe when disconnecting a floating power circuit.

BB. · February 2012

Re: Battery Cable Sizing

It even gets a bit more complex than that...

Basically, if you get a single Power to Earth/Metal/Chassis fault, your floating power system in effect becomes a positive or negative grounded system... Typically without any warning that that has happened.

If, for example you place your fuses only in the "+" leads, and you get a "+" to metal short (battery cable gets a cut on the metal battery box, RV frame, etc.), you now have a positive grounded system with a 100-300 amp fuse/breaker/no protection. And, if you get a second fault anywhere between "-" and metal, there are no fuses to protect against over current in the "-" leads.

A true floating power system should have two ganged breakers (just like ganged breakers for 240 VAC North American split phase 120/240 VAC power) so that if there is excessive fault current in one 1/2 the circuit, power is cut to both halves of the circuit.

Also floating power circuits may have either a test/monitoring system between power and ground (say three lamps, one Hot to Hot, and one from Hot A to ground and a second from Hot B to ground). Hot to Hot will be 100% brightness and the two Hot to grounds will be at 50% (or less) brightness. Or replace this with electronic monitoring.

The second is a standard test that once a day/once a week/etc. that checks for any shorts between "Hot" and "Ground".

There is an assumption in NEC code that you do not have to protect against "double faults"--However, I read that as being you do have to protect against the initial fault and subsequent faults... I.e., a Short and a Failing to Open breaker, you do not have to protect against. However, you do have to protect against the first short, and any further shorts down the road. Hence the reason for dual/ganged breakers on floating power supplies.

-Bill

Cariboocoot · February 2012

Re: Battery Cable Sizing

Hmm. Must have had another stroke as I fail to see any purpose to having a full-floating DC system unless you actually need both positive and negative Voltage in respect to a common DC neutral line. This sounds suspiciously like the ill-conceived DC ground fault protection regs recently enacted.

If you really do need "minus 'X' Volts DC" you get two parallel but inter-acting circuits (one positive and one negative) and each needs protection according to my previous post. Even so, I'd tie the neutral to ground due to the dangers Bill mentioned.

Frankly I'll stick to the old-fashioned method of tying (-) to GRND as I've yet to see any real evidence that such is particularly dangerous.

nsaspook · February 2012

Re: Battery Cable Sizing

I agree that simply grounding one leg is the correct thing to do on most installations.

For electrically noisy environments with large transients the floating power system can have better common-mode noise reduction because it acts like a balanced transmission line where common-mode chokes can be used and are more effective than single-ended filter units.

These are low energy units but the principle is the same for utility scale chokes.
http://dkc1.digikey.com/us/en/tod/Murata/CommonModeChokes_NoAudio/CommonModeChokes_NoAudio.swf

lazza · February 2012

Re: Battery Cable Sizing

Thanks for your interesting posts

The reason I opted for floating system is that as of yet I have not seen anyone here connecting the negative battery lug to ground. In my pv installer qualification course they always put fuses on both sides +ve and -ve. Basically I didnt want to try anything "new".

For future installations therefore- do I simply take an extra ground lead from the battery -ve terminal lug and connect it to the single common ground bus?

It would be good to save on fuses

cheers
Larry

BB. · February 2012

Re: Battery Cable Sizing

Yep. If you have a battery monitor (current shunt), I would pull the ground from the shunt/common return.

Note, this only makes sense to ground if the system is in a metal box (like a car, RV) or permanent installation with grounded metal plumbing, etc. (home, cabin).

If this is a portable installation in a plywood container (example), there is nothing to really ground to. And running a stake into the ground really does nothing if there are no metal water pipes, permanently installed on concrete generator, etc. About the only thing to ground to is the metal chassis of the various electrical devices (PV array frames, inverter/charge controllers metal case, etc.).

-Bill

Cariboocoot · February 2012

Re: Battery Cable Sizing

lazza wrote: »

Thanks for your interesting posts

The reason I opted for floating system is that as of yet I have not seen anyone here connecting the negative battery lug to ground. In my pv installer qualification course they always put fuses on both sides +ve and -ve. Basically I didnt want to try anything "new".

For future installations therefore- do I simply take an extra ground lead from the battery -ve terminal lug and connect it to the single common ground bus?

It would be good to save on fuses

cheers
Larry

Well my system has the negative connected to ground. It is not the same ground as the AC due to the nature of the install, but the grounds are connected (underground). I guess that makes me a ground-hog.

I have also worked on systems that do not have negative connected to ground. There is no need to fuse the negative side. It's just like any mobile application: fuse on the positive only. Nothing is gained from having two fuses in-line on the same circuit. As Bill said, in mobile apps there's nothing to ground to.

lazza · February 2012

Re: Battery Cable Sizing

OK, thanks. But I'm still not sure exactly what the logic is for grounding the -ve terminal. Is it to prevent current flow in case of potential differences between the -ve battery terminal and metal chassis of either your car, RV or house?

Cariboocoot · February 2012

Re: Battery Cable Sizing

lazza wrote: »

OK, thanks. But I'm still not sure exactly what the logic is for grounding the -ve terminal. Is it to prevent current flow in case of potential differences between the -ve battery terminal and metal chassis of either your car, RV or house?

Basically it is to cause a shut down of the circuit by tripping a breaker or blowing a fuse should something go wrong and the "positive" (or portion of) become connected to the chassis (metal casing of major components). With negative tied to chassis and Earth ground, any such connection should "read" as a dead short and trigger the circuit protection, de-energizing the whole system.

This is where we get into the flaws and arguments regarding DC GFI design: positive shorts to ground show up as +/- current imbalance triggering the GFI. But the negative "floats" (not connected to ground) so there is the potential for ground (all the component metal casings) to be positive-energize in respect to any negative wiring.

That's not very clear, but it is a complicated issue. Perhaps Bill will elaborate as he really has this issue nailed in my opinion.

BB. · February 2012

Re: Battery Cable Sizing

Lazza,

What kind of system are you installing? I assume a home? Metal plumbing through earth (propane/water pipes)? Is there utility power?

Grounding is a complex subject--and sometimes the things that "work" for one type of grounding (shorting out your AC outlet to ground) are very different (such as for lightning protection).

And you end up with complexities (grounding with utility power, emergency backup power, natural gas pipes, routing electrical safety ground inside of the building vs routing lightning grounds outside the building, etc.).

Besides the issue of earth/safety bonding the "neutral"/"return"/"negative" battery post to prevent the negative leads from ever becoming "hot" (and requiring their own fuses)--There is an issue with "floating" power and metal objects (rv chassis, radio mast, solar PV panels with floating frames and PV+/- power)... That is the issue of static buildup. Besides a direct hit from lightning, an antenna mast or PV array on a roof can slowly build up static charge from the surrounding air. And just like shopping cart or car that can get a static discharge when you touch the metal--you can get a static discharge from the DC (or even AC) wiring. And because there is more "metal" in the power system, you can get a pretty healthy jolt as the stored capacitance may be larger than that of a shopping cart or car.

Lastly, there is grounding of TSW vs MSW inverters... TSW Inverters tend to be "isolate" transformers between the battery and AC output. MSW inverters tend to have direct connection between the battery and the AC output. If you ground the Battery lead and then try to ground the MSW AC output (i.e., create a grounded neutral)--Many MSW inverters will just blow a fuse/breaker and/or even let out their magic smoke.

So--Can you tell us more about your installation. Giving generic information about grounding becomes very complex and confusing to most people (can be a college EE class in itself).

-Bill

Battery Cable Sizing

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