SVT vs SJT vs SPT Wire: How to Choose the Right Cord Jacket
Two power cords can carry the same gauge and the same plug and still be built for completely different jobs. The difference is the jacket — the outer cable type, printed as SVT, SJT, SPT, SJOW, or SJTW on the cord. This guide explains what each jacket type means, what it is built to handle, and how to pick the correct one for your application.
What the letters actually mean
Cord jacket designations are part of the UL / CSA naming system. The letters describe the service rating and the physical construction:
| Jacket | Full name | Construction | Duty rating |
|---|---|---|---|
| SPT-1 / SPT-2 | Service, Parallel, Thermoplastic | Conductors run side-by-side (parallel), no individual conductor insulation rating | Light, indoor, temporary |
| SVT | Small Wire, Thermoplastic | Round, thin overall jacket, individually insulated conductors | Light, indoor, consumer electronics |
| SJT | Jacketed, Thermoplastic | Round, heavier jacket than SVT | General purpose, indoor, most appliances |
| SJOW | Jacketed, Outdoor, Weatherproof | Round, weather-resistant jacket | Outdoor, temporary power, tools |
| SJTW | Jacketed, Outdoor, Thermoplastic, Weatherproof | Round, water-resistant jacket | Outdoor, damp locations, extension cords |
| STW / STO | Service, Thermoplastic / Oil-Resistant | Thick, heavy-duty jacket | Industrial, heavy tools, wet areas |
The practical split: SPT is the thin parallel cord you pull out of a small lamp. SVT is the round light cord on a monitor or printer. SJT is the all-rounder for general indoor appliances. SJOW/SJTW are the outdoor-rated extension cords. STW/STO are the heavy industrial duty cords.
Choosing by environment
Indoor, light load
For desktop electronics — monitors, PCs, printers, small appliances — SVT or SJT is correct. SVT is lighter and cheaper; SJT has a tougher jacket and is the safer default when the cord will be moved or flexed regularly. Most desktop power cords ship as 18 AWG SVT or SJT.
Indoor, general / heavier load
For appliances, tools, or anything drawn at sustained current, use SJT. It handles more flexing and abrasion than SVT and is the standard for “just needs to work reliably indoors.” If you are going above 10 A or running a longer indoor cord, SJT at 16 AWG is the sensible choice.
Outdoor or damp locations
SJOW (weatherproof) and SJTW (waterproof) are built for outdoor and wet use. A standard indoor SVT or SJT cord will degrade quickly outdoors — the jacket breaks down and the insulation fails. For garden, construction, and temporary outdoor power, always use an outdoor-rated jacket and a GFCI-protected circuit.
Industrial / heavy duty
STW and STO (oil-resistant) are the heavy-duty choices for factory equipment, compressors, and tools that see oil, water, and physical abuse. They are thicker, stiffer, and more expensive — use them only where the duty genuinely requires it.
Jacket and gauge must be matched
The jacket sets the duty rating; the gauge sets the current capacity. A cord is only as safe as its weakest dimension — a heavy outdoor jacket on a thin 18 AWG conductor will still overheat under a 15 A load. Match the jacket to the environment and the gauge to the current and length, and the two must both be correct.
What the jacket has to do with certification
The jacket type is part of the UL 817 listing. A “SJT 18 AWG 5-15P to C13” cord is a specific certified product — you cannot substitute an SVT jacket under the same file. When you verify a certificate, the jacket on the label must match the jacket you actually receive. This is one of the most common customs and inspection failures for power cords.
How HENG-WELL can help
HENG-WELL manufactures power cords across the full jacket range — SVT, SJT, SPT, SJOW, SJTW, and heavy-duty STO — with matching gauges and plugs for indoor, outdoor, and industrial use. Tell us the environment (indoor, outdoor, damp, industrial), the load, the length, and the destination market, and we will specify the correct jacket, gauge, and certification before you order.
HENG-WELL — power cords and connectors manufactured to global standards since 2011.
North American Cord Types Decoded: SPT, SVT, SJT, HPN, ST — and What AWG Really Gives You
The cable name tells you three things
A North American cord designation such as SJT or SPT-2 is not an arbitrary code. It describes the jacket construction, which in turn tells you where the cable may and may not be used.
| Family | Construction | Shape | Typical use |
|---|---|---|---|
| SPT-1 | Parallel, thin insulation | Flat | Small appliances, adaptor leads |
| SPT-2 | Parallel, thicker insulation | Flat | The general-purpose flat cord |
| SPT-3 | Parallel, thickest — for 3-conductor earthed work | Flat | 3-core earthed flat cord |
| NISPT-1 / NISPT-2 | Parallel, layered (not one-piece) insulation | Flat | Non-integral flat construction |
| SVT | Light round jacket, PVC | Round | Small appliances, internal wiring |
| SJT | General-purpose round flexible jacket, PVC | Round | The workhorse portable cord |
| HPN | Neoprene, parallel | Flat/parallel | Heat-resistant, higher current |
| HSJ | Heavy rubber, oil resistant | Round | Higher-power portable equipment |
| ST | Industrial heavy round jacket | Round | Generators, industrial leads |
| TPT | Tinsel, 27 AWG | Flat | Lamp wiring only |
One structural rule follows from the shape column: in a three-conductor earthed plug, SPT-1 and SPT-2 are accepted only in the integrally moulded (one-piece co-extruded) construction. Non-integral flat cable is not accepted in that position.
The suffix is a service condition, not decoration
North American cord designations carry suffixes that describe the environment the jacket is intended for:
| Suffix | Meaning |
|---|---|
| W | Weather resistant |
| O | Oil resistant |
| OO | Double oil resistant |
| T | Tinsel (in TPT) |
So SJTW is an SJT jacket additionally rated weather resistant; SJTOO is double oil resistant. Read the suffix before specifying a cable for a workshop, wash-down area or outdoor installation.
AWG: smaller number, thicker conductor
AWG works backwards from intuition — a smaller number is a thicker conductor, and a thicker conductor carries more current. The step that matters most commercially is between 18 and 16 AWG.
| Conductor | Ordinary SPT / SJT | HPN (neoprene) | Notes |
|---|---|---|---|
| 27 AWG | — | — | TPT tinsel only — 0.5 A, lamp wiring |
| 20 AWG | 2–5 A | — | Low-draw appliances |
| 18 AWG | 10 A | 10 A | Standard on most adaptor leads |
| 17 AWG | 12 A | 12 A | Round jacket only — no flat SPT |
| 16 AWG | 13 A | 15 A | The dividing line — see below |
| 14 AWG | 15–18 A | 15–18 A | 18 A only where plug and connector allow |
| 12 AWG | 15–20 A | — | Heavy extension leads |
| 10 AWG | 20–30 A | — | Industrial locking plugs |
The 16 AWG dividing line
This single row accounts for more mis-labelled cords than any other:
Ordinary SPT and SJT at 16 AWG reach 13 A. Only HPN at 16 AWG reaches 15 A.
So if a supplier quotes 15 A on a 16 AWG cord, the cord designation must read HPN — a neoprene parallel cord. If it reads SJT or SPT-2, the correct figure is 13 A.
This is not a rounding difference. Two cords of the same conductor size, sold into the same application, are legitimately rated differently because their jacket compounds perform differently under heat.
Three cables that get misused
17 AWG is round-only. 17 AWG exists, rated 12 A, but only as round-jacketed SVT or SJT. There is no 17 AWG flat SPT cable. A 17 AWG specification paired with a flat cable is an impossible combination.
TPT is not a power cord. TPT is 27 AWG tinsel — a very fine, highly flexible conductor intended for internal lamp wiring. It is rated 0.5 A. It must never be sold or specified as a supply lead, whatever its flexibility suggests.
Flat cable has prohibited positions. Beyond the integral-moulding requirement above, SPT flat cable is excluded entirely from:
– locking plugs (L5 / L6 series)
– IEC C19 connectors, the PDU and UPS end
– C15 high-temperature connectors
Those positions require round heavy-duty cord.
What to confirm before specifying
- Flat or round — and if flat, whether the earthed position permits it (integral moulding or not).
- Conductor size matched to the current and to the connector, not chosen independently.
- HPN specifically, if 16 AWG is being used at 15 A.
- Suffix matching the environment — oil, weather, or neither.
- Cable listed on the certificate that covers the finished assembly. Changing the cable changes the product.
The short version
| Need | Cable |
|---|---|
| General portable lead | SJT |
| Flat cord, low current | SPT-2 |
| Flat cord, 3-conductor, earthed | SPT-3 or integral SPT-2 |
| 15 A on a 16 AWG cord | HPN |
| Oil or weather exposure | add O / W suffix |
| Light internal wiring | SVT |
| Never a supply lead | TPT |
本篇是「北美电源线」系列第 2 篇。下一篇：北美体系下的 IEC 60320 连接器。
HENG-WELL supplies moulded power cords in SPT, SVT, SJT, HPN, HSJ and ST constructions with supporting documentation. Tell us the current, the connector and the environment and we will confirm the cable.
