Hi ,me again,after some advice from here iv installed a fused isolator switch between inverter and battery,now battery terminals about 3m away from inverter,is it ok to wire battery to isolator with 40mm2 cable then 25mm2 from isolator to inverter.Only 1 5kw battery installation just now,am i right in saying if install another battery i would need to replace the 25mm2 from isolator to inverter with the 40mm??
Battery cable
- Thread starter joe9000
- Start date
stienman
Mostly Harmless
- Joined
- Jan 6, 2021
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- 478
The point of the wire is to carry current (not voltage!) from one point to another. The more current, the thicker the wire required.
There are two aspects to this relationship: safety and efficiency.
If you push too much current through a small wire, it will get hot and could start a fire. Further, this heating, even if it doesn't start a fire, is energy lost (lower efficiency).
So the wire gauge is determined almost exclusively by the current it will carry.
If the maximum continuous current the wire will see is 62.5A, then 25mm2 wire, which is equivalent to 3awg, will be safe - it can handle up to 75 amps - regardless of how many batteries you have.
There are two aspects to this relationship: safety and efficiency.
If you push too much current through a small wire, it will get hot and could start a fire. Further, this heating, even if it doesn't start a fire, is energy lost (lower efficiency).
So the wire gauge is determined almost exclusively by the current it will carry.
If the maximum continuous current the wire will see is 62.5A, then 25mm2 wire, which is equivalent to 3awg, will be safe - it can handle up to 75 amps - regardless of how many batteries you have.
As a side note, one thing not directly touched on is the insulation rating. You see this in the AWG charts for ampacity since the lower the insulation temperature rating the lower the number of amps it can safely handle for same gauge cable. There are silicon insulated cables for instance that can be rated for 200C which can safely handle larger amount of amps then cables larger than ones that have lower insulation ratings because it safe to allow it to heat up a lot more without damaging the insulation . This still does mean however you are wasting power if your cable is getting hot so still always better to use larger gauge cable. But this is why you may see same gauge cables having different ampacity ratings because the insulation of the cable is different.
The wire size determines multiple things:
- heating and heat dissipation during normal use: depends on current and installation environment (e.g. cable in free air, in conduits, in plaster, etc.).
- short circuit current: wire needs to be large enough to not overheat and to enable fast enough trip of circuit breaker.
- voltage drop: wire needs to have low enough voltage drop for your inverter to see the actual voltage on the battery with sufficient precision. Depends on wire length, current, size.
- heating and heat dissipation during normal use: depends on current and installation environment (e.g. cable in free air, in conduits, in plaster, etc.).
- short circuit current: wire needs to be large enough to not overheat and to enable fast enough trip of circuit breaker.
- voltage drop: wire needs to have low enough voltage drop for your inverter to see the actual voltage on the battery with sufficient precision. Depends on wire length, current, size.
as said before battery terminal is not quite 3mtr away from inverter terminal in loft the cable will come through ceiling into cupboard where battery is which isnt built yet(cupboard) but will be and connected to fused dc isolator then onto battery,its the coldest room in house during winter,as to what steinman said about wire heating up thats why i thought thicker wire but if 62.5 amps going through it,wont need to.?
