What size battery disconnect switch

[JustinSchoeman]

If it has a valid CE/SANS/SABS approval, then it would be rated for 200A continuous. There might be some environmental derating in extreme cases - just read the data sheet.  If it has a valid safety rating, then there is already a safety margin built in.

If it is cheap junk with a fake CE sticker on, then it will never be safe, no matter how much you over spec it.

If it is incorrectly installed with improperly clamped cables, then it will never be safe, no matter how much you over spec it.

[Gnome]

On 2022/03/03 at 1:13 PM, JustinSchoeman said:

Who said 48V?  The manual has a Voltage range of 40-60V, so theoretical max output only attainable at the higher DC voltages. Go over 240A, and the overcurrent alarm will come on. Sustain it for 10s, and the inverter will turn off. No need to spec for higher currents than what the inverter will use.

So for 10 seconds it can go over 240A.  Either way, the principle is wrong.  You spec for the wire and the maximum current the supply can provide (both in terms of breaking capacity and amps).  If we followed your logic our circuit breakers in homes would be completely different.  Same for car fuse boxes and so on.

But obviously all those engineers are wrong, they don't know what they are doing.  DIYers have it right

[PowerUser]

18 minutes ago, Gnome said:

So for 10 seconds it can go over 240A.  Either way, the principle is wrong.  You spec for the wire and the maximum current the supply can provide (both in terms of breaking capacity and amps).  If we followed your logic our circuit breakers in homes would be completely different.  Same for car fuse boxes and so on.

But obviously all those engineers are wrong, they don't know what they are doing.  DIYers have it right

I totally agree with @Gnome!

@JustinSchoeman your advice is very wrong and completely irresponsible towards others. You can do whatever you want at your place but you take your own responsibility. 

[JustinSchoeman]

1 hour ago, Gnome said:

So for 10 seconds it can go over 240A.  Either way, the principle is wrong.  You spec for the wire and the maximum current the supply can provide (both in terms of breaking capacity and amps).  If we followed your logic our circuit breakers in homes would be completely different.  Same for car fuse boxes and so on.

No. You do NOT spec based on what the supply can provide. Well you do, but that is the interrupting capacity, which is something completely different. Most supplies can supply many thousands of Amps - you can't possibly hope to size your wiring to that.

You size the wire based on the normal maximum operating current of the device.  If the device is legally sold in South Africa, it will have a compliance sticker on the side saying what that current is (240A in this case).

This will set you minimum wire size as 120mm^2.  You then place a fuse on the source side of the wire to limit what the source can supply to what the wire can safely carry (so any fuse smaller than 260A).

There is no legal requirement to size anything for more than what is stated on the device's compliance sticker.

There is no safety advantage in sizing anything bigger, as the fuse will protect the wire in the event of a dangerous over current.

Some fuse manufacturers recommend (but not a legal requirement) that you up-rate the fuse 33% IF you will regularly be operating at close to the fuse's maximum rating for extended periods. And you can do this (as long as you up-size the wire too, so that the fuse is still less than its current carrying capacity). But this is not a legal or safety requirement - it is simply to prevent nuisance trips.

All electrical equipment and wiring has a short-time over current capacity.  This is specified in the SANS docs, and it is perfectly safe to use this over current capacity within the stated limits. So, even if you disagree with the manufacturer's legally specified max current, there is no reason to size for intermittent over currents.

And it is common industry practice to size cabling to continuous operating current rather than short term peak currents.  You will seldom size for inrush currents, and rarely make much concession for starting currents.

 

I have never seen an inverter installed according to what is being recommended here.  I mean let's look at a really simple 5kW inverter example with a short term peak of 10kW (i.e. most of the basic inverters around).

10kW/48V = 208A

which gives a minimum cable size of 95mm^2 and a 210A NH-1 fuse in a Keto-1 disconnect...

Yet, somehow, everybody gets their CoC with 35mm^2 wire and a 120A fuse. And I have never heard of such an installation catching fire, or even having nuisance trips?

 

1 hour ago, PowerUser said:

@JustinSchoeman your advice is very wrong and completely irresponsible towards others. You can do whatever you want at your place but you take your own responsibility. 

It is nice to know that our SANS wiring specs are irresponsible.

[BritishRacingGreen]

Very Interesting but very important debate , this thread .

One cannot fault any argument provided , its up to you to rate the fuse as small as you possibly can , but in the defense of Justin Schoeman , I want to note the following :

1. A Fuse , even fast-blow is never designed to protect a silicon based subsystem from failure , because silicon (eg. Mosfet)  can and will will fail long before the Fuse will start disconnecting . In this regard the Inverter must look after itself , and its typically equipped with lightning fast current limiting circuits ,overvoltage crowbars and power limiting circuits. This is the primary line of defense .  In the same manner , the battery subsystem looks after itself , by means of the BMS . 

2. The Fuse is only there to prevent catastrophically thermal related failure in the 'plumbing' , when the electronic defense systems has now already failed. the plumbing  being the conductors , the connections , the PCB tracks etc. It will prevent thermal runaway (fire) , and it will prevent physical , mechanical damage to equipment .  This will typically allow a inverter PCB to be repaired , as opposed to throwing it away.

3. Therefore it is actually realistic to rate the fuse above the maximum margins of the function. Because we don't want the fuse to get ruptured during the windows where the inverter should protect itself. It leads to unnecessary spurious blow of fuse .

4. For these reasons , whenever a fuse blows , it must never be replaced without having a thorough investigation to the root of the problem. Never keep a fuse handy , do the investigation first. In case of doubt , consult  an expert.

Just my R0.02 

 

 

Edited March 4, 2022 by dropkick

[Deeos]

Per the product specifications I require the following 

60mm dc cable 

300 dc fuse

And set the inverter per the attached.

 

I will call Hubble on Monday to confirm this and to enquire if the Cloudlink will work with the 12kw inverter 

[Deeos]

Hubble has advised I use the following settings 

 

Max charge 160amps

Max discharge 400amps 

The cloudlink will work with the Deye 12kw inverter 

[Star Harvester]

On 2022/03/02 at 1:52 PM, Leshen said:

I personally have an 8kw Sunsynk with 2 x 6.4kwh BSL and 160A fuses and 50mm cable. Been operating like that for almost 2 years with not a single issue. Max I have seen is around 140A. Still yet to see an 8kw Sunsynk delivering more that 185A from the battery bank when it surges. 

@JustinSchoeman@Gnome@PowerUser  At the risk of raising an old debate from the grave and zombifying this discussion, I found this thread quite interesting and would like to drop my 2 cents worth.

While it is obvious that the bigger the cable the better the safety, the less the voltage drops, the lower the temperatures, the higher the efficiency etc. some guys take this a step too far and suffer high financial costs (seeing as copper is not cheap).

I saw some cable cross section numbers being dropped on this thread which made me feel like a vienna on a boerewors shelf in Shoprite Checkers....

Like @Leshen I am also running 50mm^2 cable which came pre-fitted to my 10/8 Freedom Won with a Multiplus II 8kVa and have not had any problems. This despite the Multiplus being able to supply short burst peak power of 15kW and the Freedom Won having a 200A continuous and 300A peak discharge current spec. So this lead me to browse around a bit and I came across this table in another thread. As you can see duty cycle plays a role in a cable's current rating.

 

Furthermore, most of you probably know the equation below or a similar equation which can be deduced by looking at electrical resistivity and making use of the old primary school equation V = IR. With copper's resistivity of 1.724 x10^-8 as per Google, and taking into account, Victron for instance recommends a voltage drop below 2.5% in Wiring unlimited, you can quite easily see that at 48V, a 50mm^2 cable with a 2m run between inverter and battery (x 2 for the return = 4m) and a target voltage drop of 1%, you can run up to 348 Amps. Please check my equations below as it was done in haste and I cannot guarantee there aren't any errors.

Point I'm trying to make is there are many factors such as length, voltage, material, acceptable voltage drop, duty cycle etc. which determine whether a specific cable size is adequate for a specific application.

Don't just throw a frankfurter at it if maybe a vienna would have been perfectly reasonable while hurting your pocket less.

A =    

EDIT: Just realized something, fuses only break the circuit well above rated current, sometimes multiples depending on the specific fuse's time current curve. This implies you do need quite a comfortable safety margin on what your cable can handle and it is probably taken into account when these (what may seem like conservative) cable current ratings are calculated. So ignore what I said above, stick to the regulations, and when in doubt throw an anaconda of a cable at it to be sure... if you can afford it.

Edited November 18, 2023 by Star Harvester

[Gnome]

On 2022/03/04 at 6:33 PM, JustinSchoeman said:

Yet, somehow, everybody gets their CoC with 35mm^2 wire and a 120A fuse.

CoC is self reported by people who often have no more than a high school diploma (if that).  You can get a CoC for any installation.  And a quick look at theforumsa where electrical installers post pictures of CoC'd installs shows just how bad it can get.

On 2023/11/18 at 8:54 PM, Star Harvester said:

Like @Leshen I am also running 50mm^2 cable which came pre-fitted to my 10/8 Freedom Won with a Multiplus II 8kVa and have not had any problems. This despite the Multiplus being able to supply short burst peak power of 15kW and the Freedom Won having a 200A continuous and 300A peak discharge current spec. So this lead me to browse around a bit and I came across this table in another thread. As you can see duty cycle plays a role in a cable's current rating.

Source

This is from the 5kVA installation manual for Victron Multiplus II.

Basically what you are saying is, I know more than the engineers who designed this product.

On 2022/03/04 at 6:33 PM, JustinSchoeman said:

I have never seen an inverter installed according to what is being recommended here.

You've never seen an installation done according to what the manufacturer recommends for Victron?

Arguing with you is like punching a brick wall, you've come to a forgone conclusion and you are quoting SANS standards intended for low voltage AC installations (ie. 220v) as if they are applicable to extra low voltage, high current, DC battery systems.

You debate based on personal anecdotes and "evidence" like "well it hasn't broken yet"/"works for me"/"everyone has <...>"/"nobody has <...>". At least use datasheets from reputable companies or research papers to substantiate your claims.  As far as I can tell your entire approach to advise on this forum is based on personal dogma rather than scientific method backed by qualified individuals.

Edited November 26, 2023 by Gnome

[JustinSchoeman]

On 2023/11/26 at 2:10 PM, Gnome said:

Arguing with you is like punching a brick wall, you've come to a forgone conclusion and you are quoting SANS standards intended for low voltage AC installations (ie. 220v) as if they are applicable to extra low voltage, high current, DC battery systems.

 

The SANS standards quite explicitly cover low voltage DC systems, and for good reasons:

"SANS 10142-1:2017
Edition 2
Introduction
In this edition an attempt has been made to move towards the IEC codes:
extra low voltage (below 50 V) and d.c. applications (up to 1,5 kV) have been
introduced as new requirements owing to the extensive usage of, and
increased fire risk that result from, high load currents."

 

On 2023/11/26 at 2:10 PM, Gnome said:

At least use datasheets from reputable companies or research papers to substantiate your claims

Sunsynk:

Luxpower:

(2AWG = 33mm²)

Growatt:

etc.

All 5kW inverters rated for 10kW surges (5 or 10 seconds).  All specified to use wire that can not be safely used at 10kW.

Victron is indeed the exception - but it is NOT related to safety.  Victron is a Low Frequency inverter (single conversion), and is extremely intolerant of ripple on the DC bus (which is directly connected to the batteries). The Victron wiring is instead specified in terms of maximum voltage drop, which must be very low to meet the minimum THD requirements.

High frequency inverters run their DC bus from a DC-DC converter which can regulate the DC bus (relatively) independently of the battery voltage. Their wiring is specified in terms of safety and regulatory compliance only.

The original question is in regards to a Deye inverter (high frequency).

[Gnome]

 

------

P = IV
P = 110A * 0.14v
  = 15.4 watts

P = IV
P = 50A * 0.062v
  = 3.1 watts

P = IV
  = 110A * 0.27v
  = 29.7 watts

Most of the "professional" installations I've seen run even more than 1 meter of wire between the battery and inverter so I put the 2 meter cable there too.

---

Lastly because @JustinSchoeman claims that the German inverter, namely Victron are "sensitive" to voltage drops (of 0.14v, doubt) and that is why they recommend thicker cable, here is ANOTHER German inverter company, except they don't use a "low frequency" design.  SMA Sunny Island inverter.

Apparently if you buy an inverter in China the physics are just different than if you buy an inverter in Germany