The charger is a 140W GaN brick. The laptop supports 140W input. The cable is the one that came in a drawer, and the laptop charges at well under half speed.
Nothing is broken. The cable is doing exactly what it was built to do, and the charging system knows it. That is the part people resist: a cable is not a passive hose. It takes part in the negotiation, and if it cannot carry the current, the charger and the laptop agree on something lower without telling anyone.
The Negotiation Has Three Parties, and the Cable Gets a Vote
Before power flows, a negotiation happens between the charger, the device, and the cable.
Total Phase describes the sequence plainly: if the source and sink device are asking for 100W but the cable is only capable of a maximum of 10W, an issue arises — and if they supply the full 100W anyway, the cable will malfunction and things can get dangerous. So a power delivery negotiation must occur that includes the source, sink, and the cable.
One step in that sequence is that the source declares its power capabilities as constrained by the cable. The charger does not offer everything it can do. It offers what the cable allows.
That explains almost every "my charger does not charge my laptop fast" complaint. The charger is fine and the device is fine; the negotiation concluded lower because the cable said so, quietly, with no indicator to tell you why. You get the minimum of three numbers, not the maximum. A 140W charger, a 100W laptop and a 60W cable produce 60W.
The E-Marker: The Chip That Tells the Charger What the Cable Can Take
The reason the cable can speak at all is a small chip embedded in the connector head, called an E-Marker.
The rule is specific. Total Phase states that an E-Marker is required on all USB Type-C cables that support 5 amps and/or exceed 60 watts of power carrying capability. Cables expected to run above 480 Mbps — High-speed USB 2.0 — also require one, which in practice means almost every USB 3.1 cable carries a chip.
A second threshold matters more here. The SOP’ negotiation, the part of the protocol that addresses the cable itself, is required for any power contracts above 3 Amps or 60 watts. Below that line the cable is not asked. Above it, the cable’s answer is binding.
So what happens when there is no chip to ask?
Acroname puts a number on it. Non-eMarked cables do not provide the same level of performance and devices will have to assume a conservative rating for the cable — a maximum of 20V at 3A, which is 60W, with a maximum data speed of USB 3.2 at 5Gbps.
A cable with no chip is not rejected and does not fail. It is treated as a 60W cable permanently, on every device it is ever plugged into. If your laptop wants 100W it will not get it, and no error message on earth says "your cable has no E-Marker." You get a slower charge and no explanation.
100W Was the Ceiling for Years. It Is Now 240W.
For a long time USB-C topped out at 100W. That ceiling has moved, and the naming changed with it.
Per usb.org, the USB PD Revision 3.1 specification enables delivering up to 240W of power over full featured USB Type-C cable and connector. Before that update, USB PD was limited to 100W using a solution based on 20V using USB Type-C cables rated at 5A. The mechanism is three new fixed voltages: 28V, 36V, and 48V, enabling up to 140W, 180W and 240W power levels, respectively.
Texas Instruments lays the two ranges side by side.
Power range | Available current and voltages | Power delivery profile range | Notes |
|---|
SPR | 3A: 5V, 9V, 15V, 20V; 5A: 20V | 15W–60W; >60W–100W | 5A tier requires a 5A cable |
|---|
EPR | 5A: 28V, 36V, 48V | >100W–240W | Requires entry into EPR mode, and an EPR cable |
|---|
The old range is now called Standard Power Range (SPR) and the new one Extended Power Range (EPR).
Two cable consequences follow from that table. The cable you need depends on which range you want: the SPR 5A tier requires a 5A cable, and EPR requires an EPR cable. A 20V/5A cable is not the same product as an EPR cable, even though both are "5A cables."
And the 20V/5A generation is on its way out. Plugable reports that the original 5A rated cables are being deprecated, and eventually original 20V/5A cables will no longer be certified by the USB-IF — the intent being to move the market to where all 5A cables are EPR-capable. If you are stocking inventory, that is a dated planning input, not a rumor.
Why EPR Is Also a Cable Specification
EPR is not a bigger number on the same handshake, and two of its rules make it a cable question rather than a charger question.
EPR requires an initial SPR explicit power contract before EPR can even be considered, and the source verifies that both the sink and the cable are EPR capable. The cable is checked in its own right, not inferred from the connectors.
The higher voltage also reaches the components inside the connector. Plugable points out that the bypass capacitor in a USB cable has a new minimum voltage rating, moving from 30V to 63V. A cable designed around a 20V ceiling does not become a 48V cable by relabeling it. For anyone specifying construction, that is the difference between a cable that will be certified and one that will not.
The 0.286 Ohm That Decides Everything
Now the part that explains why the cable that charges a phone fine will not charge a laptop.
Texas Instruments measured the numbers. A typical USB cable interface contact resistance is approximately 30mΩ, and since there are four connections — two on each cable end — that represents 0.12Ω. Assuming standard 24AWG wire per power line and a 1m length, total wire resistance is 0.166Ω, for a total expected cable and contact resistance of 0.286Ω.
Push current through it. At a maximum output current of 2.1A, the expected voltage drop across the cable is 0.6V. The voltage at the far end falls from 5.0V to 4.4V — which TI notes is at the lower voltage limit for USB devices.
A 12% loss, entirely inside the cable, on a one-meter cord. Two things then happen at once: the device receives less voltage than it wants, and the charger reduces current to a lower but still acceptable level, extending total charge time.
Resistance is proportional to length, so scale it and the arithmetic worsens. TI’s conclusion is the whole section in one sentence: heavier gauge wire may help, while extra-long cables with smaller gauge wire will charge below the maximum rate.
This is why thin, light cables are a charging risk. A 28AWG cord packs well and has more resistance per meter than 24AWG, so under high current it drops more voltage and the negotiation lands lower. For a phone at low wattage, no problem. For a laptop pulling 5A, it is the bottleneck.
The workaround inside chargers is to raise the output set voltage at no load — typically 5.0V, pushed toward 5.15V or 5.20V, with 5.25V the maximum for USB 3.0. It is why a good charger appears to over-deliver slightly at idle: it is pre-compensating for the cable.
Labeling, and What a Charge-Only Cable Really Is
The USB-IF closed a labeling gap here, and it is one of the few things you can check without a tester. Plugable reports that EPR cables must be visibly marked so that users can identify cable support of up to 240W in order to be certified — and that this marking was previously optional. If a cable claims 240W EPR and carries no marking, the certification claim deserves a question.
There is also a distinction invisible from outside: Acroname notes that USB-C connectors have 24 pins carrying anywhere from 7 to 16 different signals, so two cables with identical-looking ends can contain very different internal sets.
A charging-only cable is a deliberate product, not a defect, and it is often what ships in the box. Acroname is direct about the trade-off: a charging-only cable may simply have support for USB 2.0 at 480 Mbps, which is a significant limitation, against up to 80Gbps for a USB4 full-featured cable. That is the difference between a cable that moves electrons and one that moves a video signal. The failure mode is assuming a charge-only cable will also drive a monitor.
Our own USB charging cable range is built with the conductor gauge and E-Marker configuration matched to its stated rating, the USB 3.1 Type-C cables cover the data-and-power combination, and USB 4 cables are where the full 80Gbps class begins. Every assembly is verified through test and inspection, and the certifications we hold are listed by standard. For background on connector selection, our existing pieces on why Type-C matters and C-to-C versus A-to-C cover the shape of the connector family.
Frequently Asked Questions
My laptop charges slowly with one cable and fast with another. Which is at fault?
The cable. Above 3A or 60W the cable is asked what it can carry and the charger caps itself to that answer. Swap one variable at a time; the cable is the fastest to test.
Does a longer cable always charge slower?
Not always, but resistance rises with length, and TI’s example shows 1m of 24AWG losing 0.6V at 2.1A. Heavy gauge long can beat thin short.
Can I use an EPR charger with a non-EPR cable?
Yes, and you will get SPR performance. EPR requires an EPR cable, so a 240W charger on an ordinary 5A cable charges at 100W or below rather than failing.
How do I know if a cable has an E-Marker?
Not by looking at it. Check the printed rating on the jacket, the specification sheet, and — buying in volume — a cable tester that reads the E-Marker directly.
The Short Version
USB-C charging is a three-party negotiation, and the cable is a voting member. Above 3A or 60W it must carry an E-Marker chip, and without one the device assigns a conservative 60W assumption no charger can override.
USB PD 3.1 raised the ceiling from 100W to 240W through 28V, 36V and 48V fixed voltages, which is why EPR cables are a distinct product from the 20V/5A generation now being deprecated. EPR also demands a bypass capacitor rated 63V rather than 30V.
The arithmetic you can feel is resistance. A one-meter 24AWG cord with four connector contacts totals 0.286Ω, costing 0.6V at 2.1A and leaving 4.4V at the far end — right at the floor. Make it longer or thinner and the charger answers with less current.
Buy the gauge and the E-Marker to match the power you need, not the connector shape. If a charging problem is not resolving and the construction is unclear, send us the specification and we will tell you what it should read.
Sources
- Total Phase — What is an E-Marker in a USB Type-C Cable and How Does It Work?, covering the E-Marker requirement for cables supporting 5A and/or exceeding 60W, the additional requirement above 480 Mbps, the SOP’ negotiation required for contracts above 3A or 60W, the E-Marker’s role and last word in negotiation, the source declaring power as constrained by the cable, and the fields the chip reports — https://www.totalphase.com/blog/2020/10/what-is-e-marker-how-does-it-work/
- USB-IF — USB Charger (USB Power Delivery), covering the USB PD Revision 3.1 update enabling up to 240W over full featured USB Type-C cable, the previous 100W limit based on 20V with 5A cables, the new 28V, 36V and 48V fixed voltages enabling 140W, 180W and 240W, the adjustable voltage supply mode, and the Type-C Release 2.1 definition of 240W cable requirements — https://www.usb.org/usb-charger-pd
- Texas Instruments — Power Tips: Don’t Let a Drooping USB Voltage Slow Down Your Charger, covering the 30mΩ typical contact resistance, the four connections totalling 0.12Ω, the 0.166Ω resistance of 1m of 24AWG wire, the 0.286Ω total, the 0.6V drop at 2.1A and the resulting 4.4V at the far end, the higher gauge guidance for long cables, and the practice of raising the no-load set voltage to 5.15V or 5.20V with 5.25V the USB 3.0 maximum — https://www.ti.com/lit/pdf/ssztbz8
- Texas Instruments — USB Type-C and USB Power Delivery: Designing for both Extended Power Range and battery-powered systems, covering the SPR and EPR fixed voltage ranges in tabular form, the 15W to 240W power tiers, the requirement for 5A and EPR cables, the 48V practical safety-margin limit, the AVS 100mV step range between 15V and 48V, and the distinction between AVS and PPS with its 20mV steps — https://www.ti.com/lit/SSZTD49
- Acroname — Types of USB Cables: Not All USB-C Cables are Created Equal, covering the 24 pins and 7 to 16 signals per cable, the conservative 20V at 3A 60W assumption applied to non-eMarked cables along with USB 3.2 at 5Gbps, the progression from 3A to 5A and 60W to 240W, the 480Mbps limitation of charging-only cables, the 80Gbps capability of USB4 full-featured cables, and the redriver and retimer distinction — https://acroname.com/blog/types-usb-cables-not-all-usb-c-cables-are-created-equal
- Plugable — What Is 240W USB Extended Power Range (EPR)?, covering the SPR and EPR naming, the 28V, 36V and 48V at 5A configuration, the 48V practical limit, AVS steps of 100mV, the requirement for an initial SPR contract before EPR, sink-driven entry, the half-second Keep Alive messages and the roughly one-second hard reset to SPR and 5V, the deprecation of original 5A cables, the mandatory visible marking of EPR cables, and the bypass capacitor minimum voltage rating change from 30V to 63V — https://plugable.com/blogs/news/what-is-240w-usb-extended-power-range-epr
