Bigger wire efficiency

[kwest364]

When sizing wire between batteries, fuses, inverter, bus bar, breaker box/fuses, etc.

My set up:
Panels: 1100W panels (6 x 185W)
CC: 60A
Battery system: 24V, 4800Wh, 12V x 4 @ 100Ah each, LiFePo4, 2s2p
Inverter: 3000W 24V


When is the wire too big?
When does upsizing become pointless?
Efficiency increase? Ie. once you size up to a specific size, you will have as much efficiency as possible, and sizing up would NOT increase it. Where is that point?

Ex.
Panels: 10AWG, but does 8AWG make a difference? Chart says 10AWG at 60° can handle up to 30 amps, 4AWG 60° insulation can handle 70A, and since my CC can do 60A, isn't it prudent to use 4AWG? I realize there are other factors that change PV wire size (CC panel wire receptacle size, how panels are wired (series vs parallel), but is my logic and understanding sound?

Ex.
Batteries: Wire between batteries: 6AWG, but size up 4AWG? Unnecessary? Worth it? Obviously, bigger isn't gonna hurt, but will 4/0 provide greater efficiency vs 6AWG? What's minimum wire size for max gain?

Ex.
Inverter wires, what size? Since the current is higher and due to inverter efficiency losses (80% efficiency let's say for worst case scenario), I'd assume that sizing up here would be critical to reduce V drop, efficiency, proper heat dissipation, etc. but size up and stop at what size?


I want a safe, high quality, long lasting system. Calculate the minimum wire size for the load, but then how much bigger is good to give a safety buffer and maximal efficiency?

How is wire size between batteries determined?



Thanks in advance.

 

[SamDeleted]

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[DougfromdaUP]

Your wires are usually sized for the peak current you will draw. Most of the time you are running much less.

If your wire has a 3% drop at full current, you’ll only lose 0.75% at half that current. So you efficiencies are much greater than at the worst case(full) current.

So it’s an engineering question. How much money do you want to spend on wire to save a tiny amount of power? Once you have big enough wire to be safe, it’s up to you to decide about the loss.

 

[fafrd]

When sizing wire between batteries, fuses, inverter, bus bar, breaker box/fuses, etc.

My set up:
Panels: 1100W panels (6 x 185W)
CC: 60A
Battery system: 24V, 4800Wh, 12V x 4 @ 100Ah each, LiFePo4, 2s2p
Inverter: 3000W 24V


When is the wire too big?

Not until the wire is too big for the lugs of your inverter (and even then there are workarounds).

When does upsizing become pointless?

When the small amount of incremental efficiency gain is not worth the significant amount of incremental cost.

Efficiency increase? Ie. once you size up to a specific size, you will have as much efficiency as possible, and sizing up would NOT increase it. Where is that point?

Never. Larger wires will always deliver lower resistance (for the same wire length).

Lower resistance will always deliver higher efficiency (lower I^2R losses).

Ex.
Panels: 10AWG, but does 8AWG make a difference?

Yes.

Chart says 10AWG at 60° can handle up to 30 amps, 4AWG 60° insulation can handle 70A, and since my CC can do 60A, isn't it prudent to use 4AWG?

Ampacity / maximum current ratings are a matter of safety. The wires you use must always be rated for Isc-max x 1.25 x 1.25 (Isc_max being Isc at coldest possible temperatures in your location).

After you are certain the sites you are using delivery sufficient ampacity, going larger is a question of increased efficiency and what you can afford.

I realize there are other factors that change PV wire size (CC panel wire receptacle size, how panels are wired (series vs parallel), but is my logic and understanding sound?

Ex.
Batteries: Wire between batteries: 6AWG, but size up 4AWG? Unnecessary? Worth it? Obviously, bigger isn't gonna hurt, but will 4/0 provide greater efficiency vs 6AWG?

Yes.

4AWG has 63% the resistance of 6AWG and will lose only 63% as much power as 6AWG wires in I^2R losses…

What's minimum wire size for max gain?

You have to know the current you care most about, calculate the I^2R losses for that current with the wire sizes you are considering, then look at incremental costs involved and decide what makes the most sense for your budget…

Ex.
Inverter wires, what size? Since the current is higher and due to inverter efficiency losses (80% efficiency let's say for worst case scenario), I'd assume that sizing up here would be critical to reduce V drop,

Your inverter will have a low voltage cut-off that you want to safely stay above. Once you know the low voltage cut off you want to discharge your battery to, the gap between that voltage and the low voltage cutoff of the inverter tell you how much voltage drop you can afford at the maximum currents you expect. That maximum voltage drop gives you yet another input on minimum wire size requirements (you want to safely assure your inverter always hits low voltage shutdown will before your BMS shuts down DC input power abruptly).

efficiency,

that’s purely a question of what you can afford and what higher efficiency is worth to you.

proper heat dissipation,

that is purely a question of ampacity. As long as the wire is rated for at least maximum sustained current x 1.25 x 1.25 (or actually 1.25 x fuse/breaker rating), heat should not be a concern).

etc. but size up and stop at what size?

If you have the budget to afford it, sizing cables for the maximum accepted by the lugs on your inverter is never a bad idea…

And if the wire lengths are long, going even larger may make sense.

I want a safe, high quality, long lasting system. Calculate the minimum wire size for the load, but then how much bigger is good to give a safety buffer and maximal efficiency?

Size for the largest currents you ever plan to sustain and then go above that only because you inverter makes it easy and the added cost is not an issue.

How is wire size between batteries determined?

Maximum sustained currents that will be carried by those wires.

Thanks in advance.