Blue sea says I should use 0/4 AWG. Why?

[JanC]

Hello again,?

I ordered this product https://www.bluesea.com/products/7748/SafetyHub_150_Fuse_Block and I'm just trying to figure out what size cable should I use between this Safetyhub and my 24v battery. As you can see in the description, it says for maximum combined operating amperage 200A use 0/2 AWG, for 350A use 0/4 AWG. They don't differentiate between 12v and 24v systems and I think using 0/4 wire in this situation would be an overkill considering I plan to put a 300A MRBF fuse on the battery terminal which suits 0/2 cable perfecty. For the time being I don't want to put more than 200A through this safetyhub, but in a few months I might want go to about 230-250A maximum. Do you know if those stats from Blue Sea were meant for a 12v system? Would I be okay using 0/2 AWG?

Thanks

Jan

 

[Ampster]

The fuse is to protect the wire. Wire sizing should be done based on voltage drop you are comfortable with. When using 12 volt devices a one volt drop is more critical than on 24 or 48 systems. I always oversized the wires on my sailboat that ran on 12 volts. It is really a user choice about what risks you are comfortable with.

 

[Rocketman]

amps don’t care about the volts. - so if you are going to run at that high of amps - use that size of wire.
There is also a 3/0 wire(three-aught) that is between 2/0 and 4/0.

Also, a short part-second spike is much different that a sustained load. If it’s just a short spike of above 200a then back below 200a - you will probably be fine with a 2/0 wire.

 

[SeaGal]

Guess you meant 2/0, not 0/2?

Cable must have greater capacity than the fuse.

Voltage 12v or 24v doesn't affect current capacity of cable.

Temperature and length of cable should also be taken into consideration to determine voltage drop too.

Edited to add: What they said ^^^^... I was too slow!

 

[JanC]

The fuse is to protect the wire. Wire sizing should be done based on voltage drop you are comfortable with. When using 12 volt devices a one volt drop is more critical than on 24 or 48 systems. I always oversized the wires on my sailboat that ran on 12 volts. It is really a user choice about what risks you are comfortable with.

amps don’t care about the volts. - so if you are going to run at that high of amps - use that size of wire.
There is also a 3/0 wire(three-aught) that is between 2/0 and 4/0.

Also, a short part-second spike is much different that a sustained load. If it’s just a short spike of above 200a then back below 200a - you will probably be fine with a 2/0 wire.

Guess you meant 2/0, not 0/2?

Cable must have greater capacity than the fuse.

Voltage 12v or 24v doesn't affect current capacity of cable.

Temperature and length of cable should also be taken into consideration to determine voltage drop too.

Edited to add: What they said ^^^^... I was too slow!

I use this calculator - https://www.explorist.life/wire-sizing-calculator/ . When you put 300A, 24v and 2ft length, it says 2/0 AWG. I never plan to go above 300A and my main fuse is at 300A as well. So why do they say in the description that 2/0 is only good for 200A max.

Based on the wire sizing calculator I chose a 300A fuse that would protect a 2/0 AWG because 2/0 AWG can handle more than 300A. So I don't understand why does Blue Sea recommend going with 4/0 AWG if you want to go over 200A when 2/0 AWG could easily handle the load without much voltage drop for my intended distance.

I was mentioning the 12v vs 24v because I was told many times (I also saw it in wire sizing calculator) that if you have higher voltage, you save money on cables. That's actually one of the reasons I chose to go with 24v system. But perhaps my understanding of electricity is insufficient, because you are right that in this instance, the voltage does not make any difference on the recommended wire size.

I'm probably never going to go over 200A continuous, but the fuse for my 2000w inverter is at 150A and I will have some DC sockets, lights and perhaps a DC fridge/freezer in future, so I want to keep a headroom in case I change my mind.

 

[Ampster]

So why do they say in the description that 2/0 is only good for 200A max.

They have no control over how long a run someone will use that 2/0 cable .Also, they are protecting their liability because they sell a product and their product liability carrier probably reviewed their documents. That is the only context that makes sense to me.

 

[SeaGal]

I was mentioning the 12v vs 24v because I was told many times (I also saw it in wire sizing calculator) that if you have higher voltage, you save money on cables. That's actually one of the reasons I chose to go with 24v system. But perhaps my understanding of electricity is insufficient, because you are right that in this instance, the voltage does not make any difference on the recommended wire size.

You are correct for any given power, but not for any given current.. Just remember V=IR and P=VI.

So, a fuse or cable rated at 200A will handle 200A at either 12V or 24V. But at 12V it will provide power of 2400W (= 12 * 200), whereas at 24V it will provide power of 4800W (= 24 * 200).

So, on the grounds that most people need a system to deliver a fixed amount of power, then by running a higher voltage system you will pass less current through the cables (for that fixed amount of power) and hence can reduce the current carrying capacity of the cables. HTH. ?‍?

 

[featherlite]

I use this calculator - https://www.explorist.life/wire-sizing-calculator/ . When you put 300A, 24v and 2ft length, it says 2/0 AWG.

I thought the explorist calculator calculates a one way wire run, rather than for the double round trip. Am I wrong? I like to use that calculator but always assumed it was one way wire length. Thanks for your help!

 

[JanC]

I thought the explorist calculator calculates a one way wire run, rather than for the double round trip. Am I wrong? I like to use that calculator but always assumed it was one way wire length. Thanks for your help!

No it's the combined length. Both positive and negative cables. When you click next to the red asterisk it shows info about each input. I attached a screenshot.

 

[featherlite]

JanC,

Many, many thanks! Thank you for taking the time to show me that screen shot. Even after you showed me the pop up box, I had a hard time getting my cursor into the exact position to activate the pop up. Without your help, I never would have found it. I cannot THANK you enough!

 

[DrJoe]

Hello again,?

I ordered this product https://www.bluesea.com/products/7748/SafetyHub_150_Fuse_Block and I'm just trying to figure out what size cable should I use between this Safetyhub and my 24v battery. As you can see in the description, it says for maximum combined operating amperage 200A use 0/2 AWG, for 350A use 0/4 AWG. They don't differentiate between 12v and 24v systems and I think using 0/4 wire in this situation would be an overkill considering I plan to put a 300A MRBF fuse on the battery terminal which suits 0/2 cable perfecty. For the time being I don't want to put more than 200A through this safetyhub, but in a few months I might want go to about 230-250A maximum. Do you know if those stats from Blue Sea were meant for a 12v system? Would I be okay using 0/2 AWG?

Thanks

Jan

The reason why they dont specify a voltage with regards to the relation between wire size and maximum allowable current is because it doesn't matter.

The reason they specify wire size in regard to the maximum allowable current is because of temperature on the connection point. You need to "heatsink" that heat. And more copper means more heatsink, basically. and since the ampount of copper in the busbar stays the same, you need to increase the amount of copper in the connecting wires. you can calucalate the heatloss in that connection point:

P = I² x R // Power = Ampere² x Ohm --> The voltage doesn't matter

also permissible ampacity for the cable is a factor.

Edit: Regarding you question, unless the safety hub is in a very hot environment (say an engine bay), and given the fuse is sized correctly, you should be fine with a slightly smaller conductor. make sure you torque that connection to spec and maybe do a temp check later under full load.