# How do you size a fuse for your system?



## ishiwgao (May 5, 2011)

MathisLaurant said:


> I have a 100.8 volt system and want to protect Cells. The Cells can safely do 250amp after that is pushing it so I don't want to past that.
> 
> i am guessing that i can just pick any 250 amp fuse but there are a few different options and voltage ranges at the 250 amp like slow blow, fast blow, 250a @ 1000v, 250a @ 80volts , etc.
> 
> ...


fuses should be sized for the wire, not for the system. this is a common misconception 

read up a bit from google on this and you should know how to.


----------



## MathisLaurant (Feb 21, 2017)

Ok, well i've over sized my wires that is for sure. I am using 2/0 for my 100.8v. so if i am thinking correctly, the fuse should be the "weakest link" in the wire so that the wire dosen't act as a fuse. and if i want to protect at 250 then i need a fuse that is at 250 amps


----------



## MathisLaurant (Feb 21, 2017)

also in sizing a fuse does the voltage matter? 
I assume that if one is to select a 32v fuse at 250 amp rating

and if you use that same that 32v- 250a fuse on a 100v and 250 amp it just makes it so that the fuse blows faster. 

right?


----------



## brian_ (Feb 7, 2017)

MathisLaurant said:


> also in sizing a fuse does the voltage matter?
> I assume that if one is to select a 32v fuse at 250 amp rating


Yes, the voltage rating of the fuse must be at least as high as the voltage of the application, like any other electrical equipment.



MathisLaurant said:


> ... and if you use that same that 32v- 250a fuse on a 100v and 250 amp it just makes it so that the fuse blows faster.
> 
> right?


Wrong. 
The current (and time) is what blows the fuse, regardless of the voltage. Remember that there is nearly zero voltage across the fuse, since it has little resistance. Whether both ends of the fuse are 32 volts above ground or both ends of the fuse are 100 volts above ground makes no difference to whether or not the heating due to current flow melts the fuse... only the amount of current and how long it flows.


----------



## Duncan (Dec 8, 2008)

Hi Mathis
Fuses are time and current devices -
If you have a 250 amp fuse it is expected that it will last forever at 250 amps - it would be nice if it would immediately blow at 260 amps 

Unfortunately it does not work that way - if you go past the "forever" current the fuse will heat up and eventually blow - but if you are only 10% high it may take an hour or longer to blow

I'm using the fuse built into a Chevy Volt service isolator as my main fuse - in the Chevy the most it sees is about 300 amps

In my Device the battery current peaks at 1200 amps - but because that is only for a second or so the fuse does not blow

So when do you use 250 amps? - is that just a peak current for a couple of seconds?
Or is that a continual current as you drive up a hill?

If 250 amps is your peak current and you normally use 150 amps then something like a 180 amp fuse may well be ideal

Google - fuse current time graph
And have a read of the articles


----------



## Duncan (Dec 8, 2008)

Hi Mathis
Fuses are time and current devices -
If you have a 250 amp fuse it is expected that it will last forever at 250 amps - it would be nice if it would immediately blow at 260 amps 

Unfortunately it does not work that way - if you go past the "forever" current the fuse will heat up and eventually blow - but if you are only 10% high it may take an hour or longer to blow

I'm using the fuse built into a Chevy Volt service isolator as my main fuse - in the Chevy the most it sees is about 300 amps

In my Device the battery current peaks at 1200 amps - but because that is only for a second or so the fuse does not blow

So when do you use 250 amps? - is that just a peak current for a couple of seconds?
Or is that a continual current as you drive up a hill?

If 250 amps is your peak current and you normally use 150 amps then something like a 180 amp fuse may well be ideal

Google - fuse current time graph
And have a read of the articles


----------



## MathisLaurant (Feb 21, 2017)

Thanks guys,

so my motor, the DLC-28 or ME1302, continuous will do 15kw. so, correct me if i am wrong... 
@100.8 volt that is about 148 amps lets say 150 amps to maintain that power, right

This same motor will do 38kw peak, so doing the same math that is 380 amps rounding up. 

since i don't plan to push it beyond 250 amps

i need a fuse that is like this: 

125v and 175 amps 

http://www.cooperindustries.com/con.../product-datasheets-a/Bus_Ele_DS_1037_ALS.pdf

something like this?


----------



## Tony Bogs (Apr 12, 2014)

On average the 175A fuse will blow out after about 5 minutes at 250A
and it can hold 380A for about 10 seconds.

380A for 10 seconds can push a poor quality battery with a dangerous chemistry into the thermal runaway region.


----------



## kennybobby (Aug 10, 2012)

Don't try to use a fuse to limit the current--use a current measurement circuit to monitor the level and pull back the current command if it exceeds your set point. This is a basic negative feedback control system.

Why would you want to end up on the side of the road with a blown fuse and not able to get back home?

Fuses are intended to protect the wiring, not used to limit the performance.


----------



## Tony Bogs (Apr 12, 2014)

Yes, a fuse is a safety device. Primary purpose is the prevention of overheating that can result in fire. 
It is at its best when protecting parts and circuits with a simple I2R profile, for instance wiring.


----------



## MathisLaurant (Feb 21, 2017)

Well if this helps to clear thing up

I am using Nissan leaf Cells 82Amphr Cell. 

I've been told that the nissan pack is fused at 250amp, so I just wanted to size accordingly to my system size. my controller can handle up to 600 amps but i actually blew up my last one ( thank God for warranty) and the fuse i had did not do its job in protecting it... so I trying no to make the same mistake. 

i will make sure the BMS and Controller talk to each other to protect the pack.

I appreciate every one explaining this to me. 
my idea of fuses was completely wrong.


----------



## Duncan (Dec 8, 2008)

Hi Mathis

I am using the Volt main fuse in my system - similar to the Leaf in power levels

My controller is set to 1200 amps - and I do use full power at the track

So far the fuse has not blown - although I did blow my armature windings at the last event


----------



## Tony Bogs (Apr 12, 2014)

Failure of the isolation of the windings is going to happen eventually anyway when a motor is pushed to its amp limits on a regular basis.
Thank goodness that you didn't blow your Leaf cells (yet).


----------



## mons2b (Nov 17, 2015)

Do you fuse for your input current on your inverter or fuse for its AC output current?


----------



## remy_martian (Feb 4, 2019)

Neither. You fuse for the wiring. The inverter needs to guard its own butt.


----------



## mons2b (Nov 17, 2015)

remy_martian said:


> Neither. You fuse for the wiring. The inverter needs to guard its own butt.


my wiring is 180amp welding cable. my proposed fuse is 225 amp. I am looking at 225 Amp A2D225R type fuses rated for 250 v. My battery is 144v. The inverter is not high powered. 120 amps max input.


----------



## remy_martian (Feb 4, 2019)

"180 Amp welding cable" means nothing. Most consumer welders have a low duty cycle.

What GAUGE wire is it?


----------



## mons2b (Nov 17, 2015)

remy_martian said:


> "180 Amp welding cable" means nothing. Most consumer welders have a low duty cycle.
> 
> What GAUGE wire is it?


35mm2 > I think in the USA they call that gauge 2


----------



## remy_martian (Feb 4, 2019)

Dude...that's kinda puny (that's what she said, as well?) and is as I thought:






What is wire ampacity and how do you find ampacity rating?


When selecting a cable, one must know its wire ampacity rating. This rating has been determined by the NEC (National Electrical Code)..




www.wireandcabletips.com





Depending on the insulation max temp (it's continuously stamped on the cable), you only have about 115 amps you can put through it continuously vs a welding duty cycle.


----------



## Duncan (Dec 8, 2008)

remy_martian said:


> Dude...that's kinda puny (that's what she said, as well?) and is as I thought:
> 
> 
> 
> ...


True - but with 60 Ah capacity you could only put that through for 30 minutes!


----------



## mons2b (Nov 17, 2015)

Duncan said:


> True - but with 60 Ah capacity you could only put that through for 30 minutes!


I dont understand. I have sent you a PM. The cable says its rated 184 amp continuous 










35mm² Flexible Cable


NZ Electric Vehicles - Electric Vehicle parts - Electric Vehicle dismantling - Electric Vehicle Hardware




www.nzev.co.nz





*35mm² Flexible Cable

Basic Cable Specifications:*


Continuous current rating: 184 amps (Unenclosed Touching, 30°C Ambient in Air)
Short circuit rating 4620 amps for 1 second when operating at rated temp. of 110°C
Nominal Conductor Diameter: 7.8 mm
Nominal Cable Diameter: 12.8 mm

*Construction:*


Conductor: Fine stranded annealed copper to IEC 60228 and AS/NZS 1125.
Insulation: LSFLEX® R-70 (X-HF-110), Cross-linked, Thermoset, Elastomeric.
Jacket: Low-Friction E-Rubber® S-20 Thermoplastic, Elastomeric, Flame Retardant. Splash resistant to oil, skydrol, petrol, acid and sea water. Resists Ozone and UV. Excellent abrasion resistance. Submersible to 500m. Best practice PVC.

*Detailed Specifications:*


Operating Temp: -40°C to +110°C.
Voltage rating: 600/1000 Volts.
Sheath Colour: Orange.
Standards: AS/NZS 1125, AS/NZS1660.5.1, AS/NZS1660.5.6, AS/NZS 3008.1, AS/NZ3808, AS/NZS 5000.1 & IEC 60228, IEC 60332-1, IEC 60332-3-2.


----------



## remy_martian (Feb 4, 2019)

Looking up 7.8mm, it's not a 2 gauge, but halfway between 0 and 1 gauge. With a high temperature insulation, 180A may not go up in flames.

It'll drop about 0.11V per amp, though, so you'll drop close to 40V per foot in your cable (you have two cables, + and -) at your 180 amps.


----------



## Frank (Dec 6, 2008)

You might want to check that calculation.... 

Sent from my SM-T380 using Tapatalk


----------



## remy_martian (Feb 4, 2019)

Lol...read the table wrong, as "per foot". Only off by 1000 😬


----------



## Duncan (Dec 8, 2008)

remy_martian said:


> Lol...read the table wrong, as "per foot". Only off by 1000 😬
> 
> View attachment 125316


f you are gonna get it wrong - do it in style!!


----------



## mons2b (Nov 17, 2015)

Duncan said:


> f you are gonna get it wrong - do it in style!!


Thanks for your help Duncan.


----------

