Put the same job into our calculator and into a free one and ours will usually tell you to buy more copper. That is not caution and it is not upselling. It is four corrections, three of them to the physics and one to the method, and this page shows all four with their sources so you can check us rather than take our word for it.
The short version
- Tinned copper is allowed a higher resistance than plain copper. BS EN 60228 tabulates them separately. About 2% at the sizes we sell.
- Flexible Class 5 copper is allowed a higher resistance than ordinary stranded Class 2. About 7% at 25mm², before any tinning.
- Resistance rises with temperature, and cable does not run at 20°C. A conductor at 60°C carries 15.7% more resistance than the figure on its datasheet.
- The cable has to carry the fuse, not just the load. Size it to the bare load and you get a lead nothing in any range will protect.
Stack the first three and a tinned, flexible 25mm² conductor in a warm engine bay has 1.29 times the resistance the standard American tables assume. The fourth correction is the one that most often changes which cable you actually buy.
1. Tinned copper has a higher permitted resistance than plain
Every strand of marine cable is plated with tin so it does not corrode under the lug where you cannot inspect it. That plating is not free electrically, and the conductor standard says so: BS EN 60228 gives plain and metal-coated conductors separate columns of maximum DC resistance.
Maximum DC resistance at 20°C, Class 5 flexible, in ohms per kilometre:
| Size | Plain | Metal-coated (tinned) | Difference |
|---|---|---|---|
| 6mm² | 3.30 | 3.39 | +2.7% |
| 10mm² | 1.91 | 1.95 | +2.1% |
| 16mm² | 1.21 | 1.24 | +2.5% |
| 25mm² | 0.780 | 0.795 | +1.9% |
| 35mm² | 0.554 | 0.565 | +2.0% |
| 50mm² | 0.386 | 0.393 | +1.8% |
| 70mm² | 0.272 | 0.277 | +1.8% |
| 95mm² | 0.206 | 0.210 | +1.9% |
A calculator that works from a single copper figure is using the plain number for a tinned cable. It is a small error on its own. It is not the only one.
2. Flexible cable is already above ordinary stranded, before any tinning
Battery cable is fine-stranded so it will bend round a bulkhead. Those strands are laid in a helix, so each one is slightly longer than the cable, and the standard allows for that with a higher permitted resistance.
At 25mm², BS EN 60228 permits 0.727 ohm/km for Class 2 ordinary stranded and 0.780 for Class 5 flexible plain. That is 7.3% more, and it applies before you tin anything.
This is the bigger of the two conductor effects, and it is the one most often missed, because a calculator built on the resistivity of copper is really modelling a solid bar.
3. Temperature, which nobody else in this market corrects for
Every resistance figure on every datasheet is quoted at 20°C. Cable in an engine bay, in a bilge or bundled under a floor does not run at 20°C. Copper gains resistance as it warms, by a well established relationship:
RT = R20 × (234.5 + T) / (234.5 + 20)
234.5 is the inferred zero-resistance temperature of annealed copper. You will also see this written as "add 0.393% per degree", which is the same equation and not an approximation of it, because 1 / 254.5 = 0.0039293.
| Conductor temperature | Multiply resistance by |
|---|---|
| 20°C | 1.0000 |
| 30°C | 1.0393 |
| 50°C | 1.1179 |
| 60°C | 1.1572 |
| 70°C | 1.1965 |
| 105°C | 1.3340 |
A 60°C conductor carries 15.7% more resistance than the same cable at 20°C. That is more than the tinning and the stranding put together, and it is the correction that free tools leave out entirely. It is also why our calculator asks where the cable runs, which no other UK tool does.
4. What the American tables actually assume
Most volt drop tables in this category are reimplementations of ABYC E-11, which is the US small craft standard. Its tables are built on a constant, K = 10.75 ohm-circular mil per foot.
Convert that and it is a resistivity of 0.017871 ohm·mm²/m. Annealed copper at 20°C is 0.017241. The ratio puts ABYC's basis at a conductor temperature of about 29°C, and there is no temperature note anywhere in the tables.
So the tables everyone copies are a room temperature, plain copper, solid conductor basis. They are not wrong for what they are. They are just not a description of a tinned flexible cable in a warm engine space.
What that adds up to
Take 25mm², which is the size we sell most of.
- ABYC's basis works out at 0.715 milliohm per metre.
- Tinned Class 5 to BS EN 60228 is 0.795, which is 1.11 times that, at 20°C.
- At a realistic 60°C engine bay conductor temperature it is 0.920, which is 1.29 times.
Nearly a third more volt drop than the table predicts, on the same cable, on the same run.
5. The cable has to carry the fuse, not just the load
This one is not physics, it is method, and in practice it changes the answer more often than the other three.
A fuse cannot sit at the load current or it will nuisance blow, so it is sized above the continuous load. And the cable cannot be smaller than the fuse that protects it, or the fuse will happily pass a current the cable cannot carry. The chain runs:
load → fuse (above the load, with headroom) → cable (at or above the fuse)
Work it the other way and you get a cable that carries the load and nothing that will protect it. A worked example, on a 2000W inverter at 12V:
| Step | Sizing to the load | Sizing to the fuse |
|---|---|---|
| Continuous current | 185A | 185A |
| Cable | 35mm², 240A | 50mm², 345A |
| Fuse needed | 250A | 250A |
| Does the fuse protect the cable? | No. 250A on a 240A cable. | Yes. |
Both answers "pass" a volt drop check. Only one of them is a lead you can actually fit and fuse. Ours returns 50mm² with a 250A fuse, which is what the trade fits.
What BS EN ISO 13297 actually says
You will read all over this market that "ISO 13297 requires a 3% volt drop". It does not, and getting this right matters if you are specifying to a standard.
The standard sets its numeric limit in clause 5.5:
The length and cross sectional area of conductors in each circuit shall be such that the calculated voltage drop shall not exceed 10% of the nominal voltage.
Clause 5.6 then deals with equipment vital to safety, and its requirement is functional rather than numeric: such equipment shall be supplied with the proper voltage to achieve its rated performance. The 3% appears as a NOTE against that clause, saying a 3% drop is acceptable for such equipment.
In ISO drafting convention a NOTE is informative, not normative. So the position is: 10% is the limit the standard sets, and 3% is offered as one acceptable way of satisfying the separate functional requirement for safety-critical equipment. Our calculator offers both and describes them that way.
ABYC E-11 is the opposite, and worth knowing if you work to it: it makes both figures mandatory, 3% for panelboard feeders, bilge blowers, electronic equipment and navigation lights, and 10% for other circuits.
One more thing the standard does that most tools conflate: clause 5.4 requires DC equipment to function between 75% and 133% of nominal at the battery terminals, with an exception where equipment needs a higher minimum. That is a separate check from the percentage, which is why our calculator reports two numbers: the drop as a percentage of nominal, and the actual voltage arriving at the load.
What we do not claim
The point of this page is that you can check us, so here is what we cannot show you.
- BS 7671 does not apply to boats. Regulation 110.2 excludes vessels covered by BS 8450. It is the right reference for a caravan or motor caravan, through Section 721. We will not quote it at a marine circuit, and one UK calculator does.
- We say nothing numeric about BS EN 1648-2, the 12V standard for motor caravan habitation. A 3% figure circulates online attached to it with no clause number. We have not bought the standard, so we will not repeat the claim.
- We have not read ISO 13297 Annex A. Its conductor sizing method and its temperature derating range are behind the paywall. Our derating factors follow ABYC E-11 practice and are our own engineering judgement, and we label them that way rather than dressing them as ISO figures.
- Our cable's current ratings are the manufacturer's nominal figures, published with no conditions. AMC state plainly that they are "provided as a guide only, and can vary depending on the application, condition and environmental factors". No ambient, no installation method, no bundling basis. The derating we apply on top is ours. We would rather tell you that than present a guide figure as a rated ampacity, which is what the number you see quoted everywhere else actually is.
Sources
- BS EN 60228, conductors of insulated cables. Class 2 and Class 5 maximum DC resistance, plain and metal-coated columns.
- BS EN ISO 13297:2021, Small craft. Electrical systems. Alternating and direct current installations. Clauses 5.4, 5.5 and 5.6.
- ABYC E-11, AC and DC electrical systems on boats. Tables IX and X, and the constant K = 10.75.
- ISO 6722-1:2011, road vehicle cables, class B. The construction standard for the cores we sell.
- BS 7671, Requirements for Electrical Installations, Regulation 110.2 and Section 721.
- Temperature coefficient of copper: the inferred zero-resistance temperature of 234.5°C for annealed copper.
- AMC OceanFlex and AMC thin wall datasheets, hosted on our own site and linked from every cable product page.
This page is a summary of the standards as we read them. It is not legal or regulatory advice, and the installer remains responsible for the installation.