E26 vs E27 Bulbs - Interchangeable? Not Necessarily!
A Convergence of Two Voltage Standards
Prior to the development of new, energy-saving bulbs such as CFL and LED bulbs, incandescent bulbs operated on a particular voltage level only - 120V AC for North America, and 240V AC for Europe (and other 240V AC countries).
There was no such thing as a "wide voltage" incandescent bulb that worked both with 120V AC and 240 V AC.
The 120V AC versions used E26 lamp bases, while the 240V AC versions used E27 lamp bases - similar but slightly different screws.
Because the voltage ratings differed, there was almost never a case where the incandescent E26 lamps would or could be used in an E27 socket at 240V AC, and vice versa, since there would be an inherent voltage incompatibility.
With the development of CFL and LED bulbs, however, lamps became compatible with a wide range of voltages. Specifically, some LED lamps can operate at any voltage level between 90V AC and 240V AC.
The same LED lamp can therefore be used on a 120V AC circuit in North America, as well as a 240V AC circuit in Europe.
But then, the question arises, should the LED lamp use the E26 or E27 Edison Screw standard?
And, furthermore, if an E26 LED bulb is rated for use at 240V AC, can it be used in an E27 socket, and vice versa?
Beware of Incorrect Advice!
Many manufacturers and light bulb sellers will simply state that E26 and E27 bulbs and sockets are interchangeable. They will claim you can safely install an E26 bulb in an E27 socket, and vice versa.
Furthermore, the common reasoning they give is that "well, E27 is 27 mm and E26 is 26 mm, and we know they fit, so close enough!"
This is inaccurate advice based on incorrect information!
When manufactured to the universally accepted IEC standards, E26 and E27 lamp bases actually have the same diameter specifications of 26.05 mm (min) to 26.45 mm (max).
What is different between E26 and E27 are the minimum distances between the bottom contact (live contact) and the screw threads. The motivation for this difference is due to the different line voltage levels the E26 and E27 standards are designed to - 120V AC and 240V AC, respectively.
You will indeed almost always find that E26 and E27 lamp bases are mechanically interchangeable - i.e. you will be able to physically fit an E26 bulb into an E27 socket, and vice versa.
However, just because a bulb fits and operates, does not necessarily mean that your lamp installation meets the electrical safety standards that the lamp and lampholders were designed for. In fact, a mismatched installation can lead to an increased risk for short circuits and electrical fires.
In general, bulbs with an E27 designation are safe to use in E26 lampholders, but E26 bulbs may pose an electrical safety hazard when used in E27 lampholders and 240V AC.
This is because E26 bulb bases are assumed to only receive voltages up to 120V AC, and not 240V AC. Higher voltages require a wider separation of live and neutral contacts, and therefore E27 has stricter requirements on the separation distance.
Below, we go in depth into the difference between E26 and E27.
Mechanical differences between E26 and E27
Manufacturing of Edison Screw lamp bases are governed by the IEC standards, which lay out the required dimensions under IEC 7004-21A-2, and IEC 7004-21-9, for E26 and E27, respectively.
Despite what is commonly described as a 1 mm difference between E26 and E27 screw dimensions, the screw dimensions requirements are virtually the same.
The most significant differences are seen in:
1) Overall screw length,
2) Screw thread diameter at trough, and
3) Insulator cap height.
Below are diagrams and specifications comparing E26 to E27:
Length of screw (min):
E26 - 19.56 mm
E27 - 22.0 mm
Screw diameter at thread peak:
E26: 26.05 mm - 26.41 mm
E27: 26.05 mm - 26.45 mm
Screw diameter at thread trough (max):
E26: 24.72 mm
E27: 24.26 mm
Distance between threads (OC):
E26: 3.629 mm
E27: 3.629 mm
Live contact pad width:
E26: 9.14 mm - 11.56 mm
E27: 4.8 mm - 11.5 mm
Insulator height (min):
E26: 3.25 mm
E27: 5 mm
Electrical Safety Differences Between E26 and E27
The mechanical differences we discussed above may seem inconsequential, but exist due to the corresponding voltage level that each lamp base is designed for.
E26 lamps are designed for the North American market, which primarily uses 120V AC, while E27 lamps are designed for other markets where 240V AC is the norm.
Because E27 bulbs are designed for use with higher voltage circuits, additional precautions must be taken to ensure that there is sufficient clearance between the live and neutral contacts.
Specifically, the mechanical differences we note above exist because:
1) E27 uses a longer length screw, because the longer length provides additional alignment precision of the screw in the socket. If the screw is improperly aligned, the "live" contact point may come closer to the neutral screw threads, increasing the risk of an electric arc or short circuit.
2) E27 uses a wider thread trough diameter, to provide a more secure and tight contact. This provides additional protection against moisture and debris from entering the socket. At higher voltage levels, the short circuiting and electrical fires risks are elevated by moisture and contamination.
3) E27 requires a taller insulator height to increase the distance between the live contact and the neutral screw base. The difference is relatively significant and necessary to avoid short circuit risks.
When purchasing LED bulbs, do not blindly assume that E26 and E27 are fully interchangeable. When possible, use E26 bulbs for E26 sockets at 120V AC, and E27 bulbs for E27 sockets at 240V AC.
In some cases, E27 bulbs may be safely used in E26 sockets, but E26 bulbs should not be used in E27 sockets.
Even though they may be mechanically similar and interchangeable, from an electrical safety standpoint, E26 offers lessened insulation and protection against short circuits and electrical fires when used in an E27 socket at 240V AC.
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