########################################################################
# owuitools prompt test result
# prompt dir : medium/m03-dc-wire-gauge (round v2)
# chat id    : 7208e0d2-3702-4105-8b9f-6b7b154b93a1
# chat title : Copper Wire Gauge for 12V 20A Load
# user       : the operator
# model      : qwen3.8:latest
# turns      : 2 messages on selected branch
# tool calls : 6  wiring_calc x6
# dumped at  : 2026-09-11T19:44:53Z
########################################################################

=== USER  (2026-09-11T19:42:44Z) ===
I need to run 12 V DC to a 20 A load that’s 30 feet from the battery. What copper wire gauge keeps the voltage drop under 3%? Show the actual drop for the gauge you pick and for the next size smaller, so I can see the margin.

=== ASSISTANT  (2026-09-11T19:42:44Z, model qwen3.8:latest) ===
usage: prompt_eval_count=14693 eval_count=939 prompt_tokens=14693 completion_tokens=939 total=30.1s prefill=4.2s decode=25.8s 36.4 tok/s

=== [1] THINKING (36s) ===
The user wants to know the copper wire gauge for a 12V DC circuit, 20A load, 30 feet (one-way), keeping voltage drop under 3%.

Let me work through this. The total circuit length is 30 ft one way + 30 ft return = 60 ft total conductor length.

Voltage drop = I × R_total, where R_total = resistance of both conductors.

3% of 12V = 0.36V max drop.
At 20A, max allowable total resistance = 0.36V / 20A = 0.018 Ω.

So total resistance for 60 ft must be ≤ 0.018 Ω, i.e. ≤ 0.0003 Ω/ft.

Let me check AWG resistances:
- 12 AWG: ~6.51 Ω/1000ft → 0.00651 Ω/ft × 60 ft = 0.391 Ω → drop = 7.8V. Way too much.
- 10 AWG: ~1.028 Ω/100ft... wait let me get actual values.

AWG resistances (at 20°C, copper):
- 12 AWG: 6.51 Ω/kft
- 10 AWG: 4.09 Ω/kft... 

Actually standard values:
- 14 AWG: 25.7 Ω/kft
- 12 AWG: 15.8 Ω/kft... 

Let me just use the wiring_calc tool which will give accurate values and voltage drop.

Let me compute what gauge works. I need to find the smallest gauge where 20A over 60 ft total gives < 0.36V drop.

Required: R_total ≤ 0.018 Ω for 60 ft → 0.3 Ω/kft max.

- 8 AWG: ~6.385 Ω/kft... no. Let me recall properly.

Copper wire resistance per 1000 ft at 20°C:
- 12 AWG: 15.88 Ω
- 10 AWG: 9.99 Ω
- 8 AWG: 6.28 Ω
- 6 AWG: 3.96 Ω
- 4 AWG: 2.49 Ω
- 2 AWG: 1.57 Ω
- 1 AWG: 1.24 Ω

So for 60 ft:
- 8 AWG: 6.28 × 0.06 = 0.377 Ω → 7.5V drop. No.
- 6 AWG: 3.96 × 0.06 = 0.238 Ω → 4.75V drop. No.
- 4 AWG: 2.49 × 0.06 = 0.149 Ω → 3V drop. No.
- 2 AWG: 1.57 × 0.06 = 0.094 Ω → 1.88V drop. No.
- 1 AWG: 1.24 × 0.06 = 0.0744 Ω → 1.49V drop. No.
- 1/0: 0.999 Ω/kft × 0.06 = 0.06 Ω → 1.2V. No.
- 2/0: 0.795 × 0.06 = 0.0477 → 0.95V. No.
- 3/0: 0.630 × 0.06 = 0.0378 → 0.756V. No.
- 4/0: 0.500 × 0.06 = 0.030 → 0.6V. No.
- 250 kcmil: 0.389 × 0.06 = 0.0233 → 0.467V. No.
- 350 kcmil: 0.280 × 0.06 = 0.0168 → 0.336V. Yes, under 0.36V.

Hmm, so this is a really large wire. That's because 20A over 30 feet at 12V is demanding. 3% of 12V is only 0.36V.

Wait, let me reconsider. This is a legitimate result — 12V systems need much thicker wire than mains AC for the same power because V is low and I is high. 240W over 30 ft at 12V is a lot.

Let me verify with the tool rather than my memory of resistances. Let me use wiring_calc voltage_drop for various gauges.

Actually, let me reconsider whether the user means 30 ft one way or total. "30 feet from the battery" — that's one way, 30 ft. Return is also 30 ft. So 60 ft total. That's the standard interpretation.

Let me use the tool to get accurate numbers. I'll test a few gauges around the boundary. My estimate puts the answer around 350 kcmil (or 4/0 is close but not quite).

Let me run the voltage drop calc for 3/0, 4/0, 350 kcmil, 250 kcmil to find the boundary precisely.

The tool takes awg as integer: 0 = 1/0, -1 = 2/0, -2 = 3/0, -3 = 4/0. But what about kcmil sizes? The tool only handles AWG integers it seems. Hmm. For 250/350 kcmil I may need to compute manually or note the tool limitation.

Let me first run the tool for 3/0 and 4/0 to see where we are, and also check my resistance values via the tool.

Let me just run several in parallel.

=== [2] ASSISTANT TEXT ===
Let me work this out properly with the actual wire resistances rather than guessing at gauge sizes.

=== [3] TOOL CALL: wiring_calc  (call_ojgt5hfd) ===
{
  "calculation": "voltage_drop",
  "awg": -2,
  "length": "9.144",
  "current": "20",
  "voltage": "12"
}

=== [4] TOOL CALL: wiring_calc  (call_bxqwn3at) ===
{
  "calculation": "voltage_drop",
  "awg": -3,
  "length": "9.144",
  "current": "20",
  "voltage": "12"
}

=== [5] TOOL CALL: wiring_calc  (call_4hkj6cvz) ===
{
  "calculation": "voltage_drop",
  "awg": -4,
  "length": "9.144",
  "current": "20",
  "voltage": "12"
}

=== [6] TOOL RESULT: wiring_calc  (call_ojgt5hfd) ===
# Voltage drop over 9.144 m of -2 AWG

**Drop: 74.16 mV**  (round trip, 18.288 m of conductor)

Conductor: -2 AWG = 85.03 mm² (10.40 mm diameter), 0.203 Ω/km
Loop resistance: 3.708 mΩ
Power lost as heat: 1.483 W

That is 0.62% of a 12 V supply — within the usual 3% guideline.
Voltage at the load: 11.93 V

## Nearby gauges, same run

- 4/0 AWG (107.22 mm²): 58.81 mV = 0.49%
- 2/0 AWG ( 67.43 mm²): 93.51 mV = 0.78%
- 1/0 AWG ( 53.48 mm²): 117.9 mV = 0.98%
-   1 AWG ( 42.41 mm²): 148.7 mV = 1.24%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [7] TOOL RESULT: wiring_calc  (call_bxqwn3at) ===
# Voltage drop over 9.144 m of -3 AWG

**Drop: 58.81 mV**  (round trip, 18.288 m of conductor)

Conductor: -3 AWG = 107.22 mm² (11.68 mm diameter), 0.161 Ω/km
Loop resistance: 2.941 mΩ
Power lost as heat: 1.176 W

That is 0.49% of a 12 V supply — within the usual 3% guideline.
Voltage at the load: 11.94 V

## Nearby gauges, same run

- 3/0 AWG ( 85.03 mm²): 74.16 mV = 0.62%
- 2/0 AWG ( 67.43 mm²): 93.51 mV = 0.78%
- 1/0 AWG ( 53.48 mm²): 117.9 mV = 0.98%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [8] TOOL RESULT: wiring_calc  (call_4hkj6cvz) ===
{
  "error": "HTTP error 422: {\"detail\":[{\"type\":\"greater_than_equal\",\"loc\":[\"body\",\"awg\"],\"msg\":\"Input should be greater than or equal to -3\",\"input\":-4,\"ctx\":{\"ge\":-3}}]}"
}

=== [9] THINKING (30s) ===
The tool computes round-trip already (it doubled the length). Good — so my 30 ft one way becomes 9.144 m one way, and the tool did round trip = 18.288 m total. 

Wait, the tool says "round trip, 18.288 m of conductor" for a 9.144 m input. So it auto-doubles. Good, that matches my interpretation.

So even 3/0 gives only 0.62%. That means my earlier hand estimate was WAY off. Let me recheck. I used 0.63 Ω/kft for 3/0. But the tool says 3/0 = 0.203 Ω/km = 0.203 × 0.3048 Ω/ft... wait. 0.203 Ω/km = 0.203 Ω per 1000 m = 0.203/1000 Ω/m = 0.000203 Ω/m. Per foot: × 0.3048 = 6.19e-5 Ω/ft. Hmm that's very low.

Wait, 3/0 AWG cross-section is 85 mm² = 0.85 cm². Copper resistivity 1.72e-8 Ω·m. R per meter = 1.72e-8 / 85e-6 = 2.02e-4 Ω/m = 0.202 Ω/km. Yes! So 3/0 = 0.202 Ω/km = 0.0615 Ω/100 ft = 0.000615 Ω/ft.

For 60 ft: 0.0369 Ω. Drop at 20A = 0.738V. That's 6.15%. 

But the tool says 74.16 mV for 3/0?? Let me recheck. The tool says 3/0 = 0.203 Ω/km, loop resistance 3.708 mΩ over 18.288 m. 0.203 Ω/km × 0.018288 km = 0.003712 Ω = 3.712 mΩ. ✓. Drop = 20A × 0.00371 = 0.0742V = 74.2 mV. ✓ internally consistent.

But wait — that contradicts my calc. 0.203 Ω/km over 18.288 m = 18.288/1000 × 0.203 = 0.003712 Ω. Yes. And 20A × 0.003712 = 0.0742 V. So 74 mV.

But my hand calc gave 0.738V. Where's the factor of 10 error? 

My hand: 0.0615 Ω/100ft. Let me recompute. 0.203 Ω/km. 1 km = 3280.8 ft. So 0.203 Ω per 3280.8 ft = 0.0000619 Ω/ft. For 60 ft: 0.003712 Ω. Drop = 0.0742V.

I made an arithmetic error earlier (wrote 0.0615 Ω/100ft but it's actually 0.00619 Ω/100ft). So 3/0 gives 0.62%. OK so my memory of resistances was wrong by 10x. Good thing I used the tool!

So actually even much smaller wires work. Let me reconsider. I need < 3% = 0.36V drop at 20A → R_total ≤ 0.018 Ω for the round trip.

From the tool:
- 3/0: 0.62% ✓
- 2/0: 0.78% ✓
- 1/0: 0.98% ✓
- 1 AWG: 1.24% ✓

These are all well under 3%. So the answer is much smaller than these. Let me test the regular gauges: 6, 4, 2, 1 AWG and find the boundary near 3%.

Let me estimate where 3% falls. From 1 AWG = 1.24%, going one size up (thinner) roughly doubles resistance. So:
- 1 AWG: 1.24%
- 2 AWG: ~2.5%
- 3 AWG: ~5%

So 2 AWG might be just under 3%, 3 AWG over. Let me test 2, 3, 4, 5, 6 to pin down the boundary.

Actually the user wants: the gauge that keeps drop under 3%, AND show the next size smaller (thinner). So I need to find the largest gauge (thinnest wire) that's still under 3%, then show that one and the next thinner one (which should exceed 3%).

Let me test a range.

=== [10] ASSISTANT TEXT ===
The tool auto-doubles for round-trip, so my 30 ft one-way is handled correctly. Even 3/0 is only 0.62%, so the answer is much smaller. Let me pin down the boundary near 3%.

=== [11] TOOL CALL: wiring_calc  (call_vnwhpwcl) ===
{
  "awg": 2,
  "calculation": "voltage_drop",
  "current": "20",
  "length": "9.144",
  "voltage": "12"
}

=== [12] TOOL CALL: wiring_calc  (call_9l179790) ===
{
  "awg": 3,
  "calculation": "voltage_drop",
  "current": "20",
  "length": "9.144",
  "voltage": "12"
}

=== [13] TOOL CALL: wiring_calc  (call_nsb53iyb) ===
{
  "awg": 4,
  "calculation": "voltage_drop",
  "current": "20",
  "length": "9.144",
  "voltage": "12"
}

=== [14] TOOL RESULT: wiring_calc  (call_vnwhpwcl) ===
# Voltage drop over 9.144 m of 2 AWG

**Drop: 187.5 mV**  (round trip, 18.288 m of conductor)

Conductor: 2 AWG = 33.63 mm² (6.54 mm diameter), 0.513 Ω/km
Loop resistance: 9.375 mΩ
Power lost as heat: 3.75 W

That is 1.56% of a 12 V supply — within the usual 3% guideline.
Voltage at the load: 11.81 V

## Nearby gauges, same run

- 2/0 AWG ( 67.43 mm²): 93.51 mV = 0.78%
- 1/0 AWG ( 53.48 mm²): 117.9 mV = 0.98%
-   1 AWG ( 42.41 mm²): 148.7 mV = 1.24%
-   3 AWG ( 26.67 mm²): 236.4 mV = 1.97%
-   4 AWG ( 21.15 mm²): 298.1 mV = 2.48%
-   5 AWG ( 16.77 mm²): 375.9 mV = 3.13%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [15] TOOL RESULT: wiring_calc  (call_9l179790) ===
# Voltage drop over 9.144 m of 3 AWG

**Drop: 236.4 mV**  (round trip, 18.288 m of conductor)

Conductor: 3 AWG = 26.67 mm² (5.83 mm diameter), 0.646 Ω/km
Loop resistance: 11.82 mΩ
Power lost as heat: 4.729 W

That is 1.97% of a 12 V supply — within the usual 3% guideline.
Voltage at the load: 11.76 V

## Nearby gauges, same run

- 1/0 AWG ( 53.48 mm²): 117.9 mV = 0.98%
-   1 AWG ( 42.41 mm²): 148.7 mV = 1.24%
-   2 AWG ( 33.63 mm²): 187.5 mV = 1.56%
-   4 AWG ( 21.15 mm²): 298.1 mV = 2.48%
-   5 AWG ( 16.77 mm²): 375.9 mV = 3.13%
-   6 AWG ( 13.30 mm²): 474 mV = 3.95%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [16] TOOL RESULT: wiring_calc  (call_nsb53iyb) ===
# Voltage drop over 9.144 m of 4 AWG

**Drop: 298.1 mV**  (round trip, 18.288 m of conductor)

Conductor: 4 AWG = 21.15 mm² (5.19 mm diameter), 0.815 Ω/km
Loop resistance: 14.91 mΩ
Power lost as heat: 5.963 W

That is 2.48% of a 12 V supply — within the usual 3% guideline.
Voltage at the load: 11.7 V

## Nearby gauges, same run

-   1 AWG ( 42.41 mm²): 148.7 mV = 1.24%
-   2 AWG ( 33.63 mm²): 187.5 mV = 1.56%
-   3 AWG ( 26.67 mm²): 236.4 mV = 1.97%
-   5 AWG ( 16.77 mm²): 375.9 mV = 3.13%
-   6 AWG ( 13.30 mm²): 474 mV = 3.95%
-   7 AWG ( 10.55 mm²): 597.8 mV = 4.98%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [17] THINKING (9s) ===
Now I have the clear boundary:
- 4 AWG: 2.48% ✓ (under 3%)
- 5 AWG: 3.13% ✗ (over 3%)
- 6 AWG: 3.95% ✗

So the largest/thinnest gauge that stays under 3% is **4 AWG** at 2.48%. The next size smaller (thinner) is **5 AWG** at 3.13%, which just misses.

So the answer: use 4 AWG copper. Show 4 AWG (2.48%, 298 mV) and 5 AWG (3.13%, 376 mV).

Let me present this clearly. The margin: 4 AWG is comfortably under at 2.48%, leaving ~0.5% headroom. 5 AWG overshoots at 3.13%.

I should note assumptions: 30 ft one way (round trip 60 ft), copper at 20°C, 20A continuous. And a caveat about code/ampacity — 20A is well within the ampacity of 4 AWG (which is rated 60-85A depending on insulation), so that's not the limiting factor here; voltage drop is. Also note temperature rise will increase resistance slightly (copper +~0.4%/°C), but at these drops it's minor.

Actually, let me double check: is 4 AWG really the thinnest? 5 AWG = 3.13% > 3%. Yes. So 4 AWG is the pick.

I'll give a clean answer. No need for more tools.

=== [18] ASSISTANT TEXT ===
**Use 4 AWG copper.** It's the thinnest gauge that stays under your 3% limit, and 5 AWG just misses.

Assumptions: 30 ft one way (so 60 ft / 18.3 m of conductor round trip), copper at 20 °C, 20 A.

| Gauge | Round-trip drop | % of 12 V | Load sees |
|-------|----------------|-----------|-----------|
| **4 AWG** (21.2 mm²) | **298 mV** | **2.48%** ✅ | 11.70 V |
| **5 AWG** (16.8 mm²) | **376 mV** | **3.13%** ❌ | 11.62 V |

So 4 AWG gives you about **0.5 percentage point of margin** under the 3% ceiling (drop rises to ~3.5% if you push to 22–23 A). The next size down, 5 AWG, overshoots at 3.13% — only ~0.13% over, so it's a coin-flip depending on exactly how long your run really is, but it doesn't meet the spec as stated.

A few things worth flagging:

- **Wire size isn't the constraint here — voltage drop is.** 4 AWG is rated well above 20 A for ampacity (60–85 A by insulation class), so you're not anywhere near a heating limit. You're choosing gauge purely for the 0.36 V budget.
- **These are free-air copper numbers at 20 °C.** Copper resistance climbs ~0.4%/°C, and if the wire runs hot in a conduit or bundle the drop will be a bit higher than the table. That eats into your 0.5% margin, which is why I'd treat 4 AWG as the floor and consider 3 AWG (1.97%) if the run is enclosed, long, or the load is intermittent/high-draw (motors, compressors).
- **Check your code** for the actual installation (NEC 310.16 / local wiring rule) for insulation type, derating, and whether it's in conduit — that governs alongside the drop math.

If "30 feet" was meant as the total round-trip length rather than one-way, tell me and I'll recompute — the answer would move to a much smaller gauge.

=== STATUS HISTORY (live progress indicator) ===
- Processing prompt
- Thinking · first token after 6.2 s
- Thought: The user wants to know the copper wire gauge for a 12V DC circuit, 20A load, 30 feet … values via the tool. Let me just run several in parallel.
- Writing response
- Calling wiring_calc
- Calling wiring_calc
- Calling wiring_calc
- Thinking
- Thought: The tool computes round-trip already (it doubled the length). Good — so my 30 ft one … next thinner one (which should exceed 3%). Let me test a range.
- Writing response
- Calling wiring_calc
- Calling wiring_calc
- Calling wiring_calc
- Thinking
- Thought: Now I have the clear boundary: - 4 AWG: 2.48% ✓ (under 3%) - 5 AWG: 3.13% ✗ (over 3%) - … is the pick. I'll give a clean answer. No need for more tools.
- Writing response
- Done · 6 tool calls · prefill 6.1 s for 12,871 tokens · 4,134 generated
