10AWG wire Vs. 14AWG wire for a Battery in a UPS

[NSP]

Hi,
UPS being used at home is 600VA(360 Watt) which means it can draw a load of 30Amp(360W/12v=30Amps) from the attached 7.5Ah Battery. This means that battery wires should be able to handle 30Amps, therefore 10AWG wire which is rated at 35Amps should be used as per charts on the internet. However, the battery wires in the UPS I have are only 14AWG which are rated for 20Amp as per charts on the internet! How can the UPS manufacturer get away with this? Is it because the length of the 14AWG wire from Battery to UPS PCB is only 8 Inches therefore it can handle up to 30Amp? Does this mean that 30A plus fuse should be used in the circuit between battery and the UPS PCB? Is there something I am missing in my understanding?

Thanks,
Navi

 

[Chalo]

My go-to wire gauge chart says 14 AWG is good for 32A when used as "chassis wiring" (as opposed to 6A for "power transmission"). I've used it for 35A controller-battery leads before with no issues whatsoever. The leads were in free (moving) air, though, and had adequate connectors at the ends. For a stationary wire in an enclosure, the limits would surely be lower. But for only an 8" wire run I doubt it would become a problem. Maybe if there are thermal bottlenecks at both ends.

 

[NSP]

My go-to wire gauge chart says 14 AWG is good for 32A when used as "chassis wiring" (as opposed to 6A for "power transmission"). I've used it for 35A controller-battery leads before with no issues whatsoever. The leads were in free (moving) air, though, and had adequate connectors at the ends. For a stationary wire in an enclosure, the limits would surely be lower. But for only an 8" wire run I doubt it would become a problem. Maybe if there are thermal bottlenecks at both ends.

Good to know that there are charts that show chassis wiring amps. my customization is adding another 5 inches (14AWG) from the battery to the Parallel connector and another 5 inches (14AWG) from parallel connector to battery case terminal and then another 9 inches using (12AWG wire) from battery case terminals to UPS "Custom External connector" and then another 5inch of 14AWG wire from external connector to internal motherboard manufacturer 14AWG wire which is about 5 inches. This totals to (5+5+9+5+5=29)Inches or approximate 2Feet5inches from BMS P terminals to UPS PCB connectors, which is 23 inches more than the original factory design . Am I playing with fire using 2'5' of 14AWG mixed with 12AWG wire in this approximate 2.5 feet run? Should I try to reduce the length further?

 

[amberwolf]

Does this mean that 30A plus fuse should be used in the circuit between battery and the UPS PCB?

What fuse does the UPS use originally?

 

[NSP]

What fuse does the UPS use originally?

See that is the thing, it doesn't, or at least I can't find one on the PCB or on the wire! That is why I want to place one or (Three) on the extended wires I am using. Two fuses (Lets say 20 or 30Amps) between Battery to Battery parallel Circuit (14AWG wire) and and then one fuse (lets say 30 or 40amps) on "Parallel Connector" to UPS case "External Custom Connector" wire which is (12AWG).

Edit: Strangely, the mains supply side (220-230v) also does not have any visible fuse, there is a Varistor (MOV-14D561K) on the PCB but no fuse, so I added a 6Amp-250v, fast blowing, glass fuse to the AC mains input wire as a precaution.

 

[Comrade]

There is no such thing as current ratings for gauges. It's a tradeoff between cost, weight, flexibility, durability, power loss and safety.

8 Inches therefore it can handle up to 30Amp?

That's roughly 0.7 watts dissipated through the red and 0.7 watts through the black wire. You would not even be able to detect any heating by touch. So safety is not an issue.

 

[offGridDownUnder]

See that is the thing, it doesn't, or at least I can't find one on the PCB or on the wire!

Perhaps the 14 gauge wires are the fuse? (Yes, that's a joke.)

 

[NSP]

My go-to wire gauge chart says 14 AWG is good for 32A when used as "chassis wiring" (as opposed to 6A for "power transmission"). I've used it for 35A controller-battery leads before with no issues whatsoever. The leads were in free (moving) air, though, and had adequate connectors at the ends. For a stationary wire in an enclosure, the limits would surely be lower. But for only an 8" wire run I doubt it would become a problem. Maybe if there are thermal bottlenecks at both ends.

I think I found the "Holy Grail" of wire charts (At Jasco Automotive website). This one is meant for automotive 12V DC system and according to it I will be OK even using a 16AWG wire as long as length does not exceed 1.9 feet, that is about 23 inches. So my custom wiring is not only OK but it is better than the minimum recommendation. Now, I am going to add a 35Amp fuse somewhere in the circuit. It feels so good to be safe

 

[Comrade]

I think I found the "Holy Grail" of wire charts

But my application requires a maximum of 1.9% voltage drop, so that table with 2% drop is completely useless.

 

[NSP]

But my application requires a maximum of 1.9% voltage drop, so that table with 2% drop is completely useless.

Haha, give or take +/- 1% margin! I am nearing perfection here