Quick summary: Real charging power equals the smallest value among three links: the charger, the cable and the device. A 65 W charger plugged into an ordinary cable delivers only 60 W, because a cable without an E-Marker chip is assumed to carry just 3 A. Buying the charger and cable as a set does not raise the power, but it ensures that the ratings of the first two links are published together and match each other.
Key points
- Charging is a system of three links: source, cable, load. The weakest link decides the result.
- A cable both carries power and consumes it. A 1 m 28 AWG cable at 3 A drops about 1.28 V; a 20 AWG cable only about 0.20 V.
- Higher voltage makes the cable “lose less”. The same 1.52 W of loss is about 10% at 5 V but only about 2.5% at 20 V.
- Cables above 3 A must have an E-Marker. Otherwise the source quietly lowers its power, often without any notification.
1. The problem: a 65 W charger plugged in, and the laptop receives only 60 W
You buy a 65 W charger for your laptop, plug it in, and the machine reports 60 W. The charger is not faulty and neither is the laptop. The culprit is often the cable: a 65 W charger has to supply 3.25 A at 20 V, which exceeds the 3 A limit of an ordinary cable, so the source lowers itself to what the cable can take.
When a device charges slowly, the usual habit is to blame the charger. In reality the cause often lies in the cable, or in the three links not “talking” to each other at the desired power. This article answers a simple question: if charging is seen as a system of three links, each with its own rating, which rating decides the result, and what changes when you buy the charger and cable as a set?
2. Voltage drop on the cable: how does a thin wire waste power?
Treat the cable as a resistor in series between source and load. The voltage reaching the device is Vload = Vsource − I · Rloop, where Rloop is the total resistance of the supply path (VBUS), the return path (ground) and the contacts at the two connectors. The power turned into heat in the cable is Pcable = I2 · Rloop. The resistance of a copper wire rises as the conductor gets thinner and as the wire gets longer. Table 1 calculates these quantities for a 1 m cable, one copper conductor per path, at 3 A.
| Conductor size (AWG) | Resistance (Ω/m) | 1 m loop resistance (Ω) | Voltage drop at 3 A (V) | Loss at 3 A (W) |
|---|---|---|---|---|
| 20 | 0.033 | 0.067 | 0.20 | 0.60 |
| 24 | 0.084 | 0.168 | 0.51 | 1.52 |
| 28 | 0.213 | 0.426 | 1.28 | 3.83 |
Table 1. Voltage drop and loss on a 1 m copper cable (supply and return paths, one conductor each) at 3 A. Resistance from the AWG wire gauge table at 20 °C [3]; contact resistance at the two connector ends is not included.
The voltage drop limit in the USB standard
The USB standard sets a limit for this quantity. According to the USB Type-C functional test specification from USB-IF, the maximum allowed cable voltage drop is 250 mV on ground and 500 mV on VBUS, at the cable’s maximum rated current [1]. At 3 A, that corresponds to a maximum resistance of about 0.167 Ω for the VBUS path and 0.083 Ω for the ground path; at 5 A it is about 0.10 Ω and 0.05 Ω.
Comparing with Table 1: a single-conductor 28 AWG cable clearly exceeds the limit, and a single-conductor 24 AWG cable is close to it. That is why real cables must use larger-section conductors or several conductors in parallel, and why the longer the cable, the harder the limit is to meet.
Higher voltage makes the losses smaller
Operating voltage matters as much as current. With the same 3 A on a 24 AWG cable, the 1.52 W loss is 1.52 / 15 ≈ 10% of the power when transmitting at 5 V (15 W), but only 1.52 / 60 ≈ 2.5% at 20 V (60 W). That is why USB Power Delivery raises power by increasing the voltage (9, 15, 20 V) rather than only increasing the current: the cable bears the same absolute voltage drop, but the percentage lost is much smaller.
3. How do the charger, cable and device “negotiate” power?
With USB Power Delivery, the source publishes a list of the voltage and current levels it can supply, the device picks the level it needs, and the source only enables that level once all conditions are met. One condition is the capability of the cable. A USB-C cable is assumed by default to carry 3 A (60 W at 20 V). To use currents up to 5 A (100 W at 20 V), the cable must have an electronic identification chip, the E-Marker, so the source can read the cable’s rating before raising the current [2]; the USB-IF specification also requires the cable to operate correctly at the maximum voltage drop on the ground and VCONN lines [1].
As a result, the real power of the whole chain equals the smallest of the three links: Preal = min(Psource, Pcable, Pload). Table 2 illustrates this with four scenarios.
| Source | Cable | Device | Power achieved | Limiting link |
|---|---|---|---|---|
| 30 W (20 V / 1.5 A) | 3 A | needs 30 W | 30 W | none |
| 65 W (20 V / 3.25 A) | 3 A, no E-Marker | laptop needs 65 W | 60 W | cable |
| 65 W (20 V / 3.25 A) | 5 A, with E-Marker | laptop needs 65 W | 65 W | none |
| 45 W (20 V / 2.25 A) | 5 A | supports only 5 V | 15 W | device |
Table 2. Power equals the smallest value among the three links. The 3.25 A at 20 V of a 65 W charger follows the Texas Instruments reference design [4]. A 3 A cable carries 60 W at 20 V.
The second row of the table is exactly the situation at the start of the article: a 65 W charger needs to supply 3.25 A at 20 V, above the 3 A limit of an ordinary cable, so only a cable with an E-Marker receives the full 65 W. With an ordinary cable, the source drops itself to 60 W, and the user usually gets no notification.
4. How does a matched charger–cable set differ from buying them separately?
When the charger and cable are sold separately, each link has its own manufacturer, rating and quality, and the user has to make sure they match. A charger–cable set changes that in that the ratings of both are published at the same time, so the basic principle “the cable’s rating is not lower than the source’s maximum current” can be checked. Table 3 compares several OH-Tech sets according to their published specifications.
| Set | Charger | Cable | Remark |
|---|---|---|---|
| OH-B30C | GaN 30 W; 20 V / 1.5 A; PD 3.0, PPS, QC 3.0 | USB-C – USB-C, 1 m; rated 60 W (20 V / 3 A) | The cable’s rated current is twice the charger’s maximum current |
| OH-B21C | 22.5 W; maximum current 3 A (at 5 V) | USB-C – USB-C, 1 m; rated 100 W (20 V / 5 A) | The cable has rating to spare and can be used with a stronger charger later |
| OH-B30L | GaN 30 W; PD 3.0, PPS, QC 3.0 | USB-C – Lightning, 1 m; 5 V / 3 A and 9 V / 3 A | The cable is rated up to 9 V × 3 A = 27 W, the voltage level typically used for fast charging Lightning devices |
| OH-B31L | 18 W; QC 3.0 / AFC / FCP; USB-A port | USB-A – Lightning, 1 m; up to 3 A | The cable’s rated current is not lower than the charger’s current |
Table 3. Selected OH-Tech charger–cable sets. According to the manufacturer’s published specifications.
Three benefits of buying a set
- It removes the biggest unknown: whether the cable can handle the charger’s current.
- You can cross-check on paper right away. The specifications of both links are in the same document; for example the OH-B30C publishes a 60 W cable for a 30 W charger, and the OH-B30L publishes a cable at 9 V × 3 A for Lightning devices.
- One point of contact for warranty, instead of tracking down three different manufacturers.
Limits of a matched set
The cable’s rating in a set is chosen to match the charger that comes with it. If you later buy a stronger charger, for example 65 W, the 60 W cable of the OH-B30C becomes the limiting link, as in the second row of Table 2. So when upgrading the source, check the cable’s rating again.
5. How to test for the weak link in your own charging system
The principle of “replace one link at a time” lets you find the weak link without expensive equipment:
- Plug a USB meter (showing voltage, current and power) between the charger and the cable, and note the power while charging.
- Swap the cable and measure again. If the power rises, the weak link is the cable.
- Swap the charger and measure again. Change only one link each time.
Check a few more points: the charger’s power label read at 20 V (not at 5 V), the cable’s current rating printed on the packaging (for example 20 V / 5 A), and the cable length, because at the same conductor size a cable twice as long has twice the resistance and loss.
6. How to read this article
- Table 1 is the “ideal” picture. To keep it simple, we treat each path as a single copper core at 20 °C. Real cables use several cores in parallel and warm up in use, so real-world numbers shift, but the trend holds: thinner and longer means more loss.
- Need more than 3 A? Ask about the E-Marker. Not every set states whether its cable has this chip. If you plan to charge a 65 W laptop or more, one question before buying can save you months of slow charging.
- Table 2 is an example, not a measurement. The four scenarios show the “weakest link decides” rule, which applies to every charger you already own.
- Full transparency. This article was written by OH-Tech and mentions our own products; every technical figure is cited so you can check it yourself.
7. Conclusion
Charging is a problem of the whole system, and a system is only as strong as its weakest link. A cable does not just carry power, it also consumes it, in proportion to its resistance and to the square of the current. The high voltages of USB PD both raise power and reduce the percentage lost in the cable, but for currents above 3 A the cable must have an E-Marker. Buying a set does not increase power, but it ensures that the first two links of the chain are rated to match from the start.
Frequently asked questions
Why does my device still charge slowly with a powerful charger?
Charging power equals the smallest value among three links: the charger, the cable and the device. Three common causes are a cable without enough current rating (an ordinary cable carries only 3 A), a device that supports only a low voltage level such as 5 V, or a charger and device that do not share a fast-charging protocol. Try swapping one link at a time to find the limiting one.
How does a charger–cable set differ from buying them separately?
In that the ratings of the charger and the cable are published together, so the chance that the cable cannot handle the charger’s current is very low. A set does not raise power compared with buying separately at the same ratings, but it reduces the risk of a mismatch.
Does a longer cable charge more slowly?
It can. At the same copper conductor size, a cable twice as long has twice the resistance, so the voltage drop and heat loss in the cable also double. At the higher voltage levels of PD, the effect is smaller than when charging at 5 V.
What is an E-Marker chip for?
An E-Marker is a small chip in the connector of a USB-C cable that tells the source how much current the cable can carry. A cable without an E-Marker is assumed to carry 3 A (60 W at 20 V). To use 5 A (100 W at 20 V), the cable must have an E-Marker.
Can I use the cable from this set with another charger?
Yes, if the connectors are compatible, but the power will be limited by the cable’s rating. For example, the 60 W (20 V / 3 A) cable of the OH-B30C used with a 65 W charger will only reach 60 W.
Which charger–cable sets does OH-Tech offer?
There are GaN sets such as the OH-B30C, OH-B30L and OH-B28C; 22.5 W sets such as the OH-B21C; 18 W sets such as the OH-B31L; and 20 W PD and 12 W sets. Prices are by quotation; contact OH-Tech or a dealer for advice.
References
- USB Implementers Forum. (2024, March 3). Universal Serial Bus Type-C (USB Type-C) Functional Test Specification, Chapters 4 and 5 (Compliance Rev 0.91), section 4.4.1 “IR Drop”. usb.org.
- USB Implementers Forum. USB Power Delivery Specification, Revision 3.1 (2021). USB-IF presentation “USB Power Delivery”.
- HyperPhysics, Georgia State University. Electrical wire gauges (AWG, resistance at 20 °C). hyperphysics.phy-astr.gsu.edu.
- Texas Instruments. (2023, November). 65-W, GaN-based USB PD 3.0 USB Type-C adapter reference design (TIDA-050072, TIDUF57). ti.com.
