The Overhead Streetlights of the Local Streets Part 2: Clamshells and Cobraheads

This is Part 2 in an overview of overhead street lighting. Earlier we talked about NEMA lights, which are still commonly used in St. Paul. Today we cover some of the newer form factors.

 “Clamshell” Street Lights

Because of their large circular filaments, incandescent street light bulbs needed to be mounted vertically. But no such limitation applied to mercury  lamps, with their very small arc-tubes. About 1950, manufacturers started designing fixtures specifically for these lamps which were smaller and had an oval shape. Equipped with 400 watt mercury lamps they produced a relatively massive amount of light compared to what came before. Although new mercury installations are now banned due to inefficiency they were much more efficient than incandescents and were good at spreading light out to either side down the street, not into neighbors windows in front of and behind the light. Line Materials was first with their Ovalite, and in what has become the eternal problem for companies putting out a new product with a distinctive name,  all similar fixtures that the competition came out were informally called that. Today “clamshell” is collector slang for them.

The first “clamshells” had odd proportions, as they were marketed as being able to hold an economical incandescent lamp vertically, and then be updated to the expensive but efficient and high-tech mercury lamps later.  The original Line Materials Ovalite and GE Form 109 were two examples of these. Later they revised the design for mercury lamps only, as in the Westinghouse OV-20 and it’s smaller and larger siblings, the OV-10 and OV-35. The medium-sized ones with 400 watt lamps were by far the most common, since expensive mercury lamps weren’t used for local streets and these were the ideal wattage for business districts.

Form 109, Bar in Hudson, WI

GE Form 109, Bar in Hudson, WI

Here’s my Westinghouse OV-20. To keep the light somewhat sane in my garage, this uses a 250 watt coated mercury lamp (the original clear tubular lamps it was designed for, now long out of production, produced light somewhat between that and today’s 400 watt mercury lamps).

Westinghouse Clamshell

Westinghouse “Clamshell”

OV-20 Mercury Clamshell

OV-20 Mercury Clamshell.  The blue box is the external ballast. To keep it somewhat sane in my garage this is only a 250 watt lamp.


Some of these still survive in the field, particularly in small towns and private installations but they are rapidly disappearing with the LED onslaught. Here is one in Lake City, MN in 2014:

Westinghouse OV20 "Clamshell" Streetlight.

Westinghouse OV20 “Clamshell” Streetlight. Not the “paint can” ballast on the top of the pole.

With advancing technology shrinking the size of ballasts, it became possible to integrate the smallest of the lamp and ballast combinations into the fixture, But this form didn’t last. Instead it morphed into what we all know but may not love, the cobrahead.

The Cobraheads

The first Cobraheads were the Westinghouse Silverliners.

OV 25 Silverliner, the first Cobrahead model, Great Bear Center Parking Lot

OV 25 Silverliner, the first Cobrahead model, Great Bear Shopping Center, Bloomington, MN Parking Lot


Like “Clamshells”, Cobraheads generally came in three sizes, small (generally equipped with 100-250 watt lamps but a few are as small as 50), medium (250 and 400 watt lamps), and large (700 and 1000 watt lamps). The large sizes are essentially extinct; I don’t know of any in the area and the fixtures have been out of production for decades. A 400 watt sodium lamp produces as much light as a 700 watt mercury lamp and can be accommodated in a medium fixtures, and generally you want a different form-factor, like a high-mast tower, for 1000 watt sodium lights. This area tended to be conservative and use medium-sized fixtures for 250 watt lamps, the largest size found on local streets and the smallest on the freeways.

Here’s my 1960s GE M250R Cobrahead, showing what they look like on the inside:

Inside of my 1960s M250R

Inside of my 1960s M250R


Although the idea of a cobrahead was to integrate the control gear, they were available without. Perhaps this was for direct replacement of “clamshells”, or utilities had surplus ballasts lying around or they just wanted to be able to procure them separately. Here’s a remote ballasted OV-25 on Stinson Blvd., Minneapolis, where the ballasts are in the bases. Such an arrangement isn’t possible with high pressure sodium and pulse start metal halide lights, because they require a high voltage pulse to start so the control gear must be near the lamp.

Remove Ballasted OV-25

Remote Ballasted OV-25. Probably the oldest Cobraheads still on the streets of the area.


A Spotter’s Guide to Cobraheads

Recognizing cobraheads is challenging even for experts since the differences are subtle and the same model (especially true for GE) might have different molds.  This is not an exhaustive list, but includes some of the most common models in the area.  A few city owned lights on metal poles are older, but none of the wood pole lights are older than the mid-1980s Northern States Power Company conversion from mercury to sodium.  In all cases, looking at the lens is not useful for identification; most models could be ordered with several kinds . Although small fixtures could be equipped with 250 watt lamps, our area was very conservative and used medium luminaires for that wattage, they are almost gone from the freeways but can still be found at larger intersections.

General Electric Luminaires usually had their logo on the bottom and sometimes have a two piece door. The control gear is mounted to the smaller door facing the pole. This “Powr/Door” was a marketing tool, advertised that if you wanted to buy cheap mercury lights you could upgrade to the (at the time) more expensive sodium lights (or even some wonderful technology that hadn’t been invented yet), you would just have to replace the door rather than the whole fixture. In practice, this was rarely done since the cost and work with replacing the entire fixture wasn’t much more.  Small models were known as the M250 series, medium models were the M400 (designating the largest wattage lamp they could use); “A” denoted a two piece door; no suffix and later “R” denoted a one piece door.

There are a few pre-1986 M250As in city owned installations like this one:

GE M250A, Minneapolis

GE M250A, Minneapolis, MN


M250A2s, produced from 1986 on, have a very boxy look:

GE M250A2

GE M250A2


The single door version, the M250R2 was produced from 1985 on:

GE M250R2, one of the most popular models in the area.

GE M250R2


Another M250R2 with a flat lens, these are the ones I see Xcel putting in now if a new fixture is needed:



Here’s a old  M400A, produced 1967-1985:

GE M400 Luminaire

GE M400A.


In 1986 the M400 models were replaced by the M400A2 and M400R2. Here’s an R2; simplified but similar looking:


GE M400R2


In  1997 by the M400R2 was replaced by the very different looking M400R3, also known as the MSRL.  There is also the two door A3 version but it there are any around they are rare. In 2008 production of the M400R2 started again and then was discontinued very recently:

"Shallow Glass" luminaire. Notice the glass sags down slightly below the fixture to spread light out better than a full cutoff fixture (albeit less than a semi-cutoff). The yellow sticker means it's a 250 watt high pressure sodium vapor.

GE M400R3


Cooper Industries luminaires are very “fat” for lack of a better term. They all are to some degree, but the model OVX was chunky to an extreme.

Cooper OVS

Cooper OVS. The circle on the bottom  is for a bubble level.


Cooper OVX

Cooper OVX


The model OVZ, which looks virtually identical, replaced the OVS. In 1992 the model OVX was introduced, which this area seems to have switched to rather than the OVZ.  The Cooper OVX was somewhat notorious because it was a small fixture that could be equipped with medium-sized 400 watt lamps, which tended to overheat and prematurely fail.

Also of note was the L-250, as used by a few non-NSP/Xcel utilities in the area.  This was designed as a Westinghouse model, When Cooper bought Westinghouse’s streetlight business in 1982 they marketed it for a while under the Crouse-Hinds (another company they owned) name:

Crouse-Hinds L-250

Crouse-Hinds L-250, Stillwater, MN


American Electric luminaires, are very generic looking. The  model 13 was generally a very light grey, appearing almost white in the sun, and had a blunt end:

American Electric 13

American Electric 13


In 1988 the model 113 replaced it. These were much more rounded and as ordered in this area had polycarbonate refractors that tended to yellow. One identifying feature of both is the metal extends slightly down encircling the refractor. In 2003 the very similar model 115 replaced the 113. I’ve not noticed any 115s in the area since Xcel tended to use GE fixtures by that time, the difference is the 115 had a bulge where the “10” sticker is in this photo.

American Electric 113 Luminaire

American Electric 113 Luminaire. This one has a polycarbonate refractor, which were lighter and cheaper but tend to yellow.


The medium-sized American Electric lights were the model 25 and it’s replacement the 125. A quick way to distinguish an AE 125 from a very similar looking GE M400R3 is the door of a 125 is tapered to meet the back of the housing:

More typical semi-cutoff luminaire

American Electric 25


American Electric 125


Also by American Electric is the small DuraStar 2000 and its medium-sized sibling, the Durastar 3000, made of polycarbonate rather than aluminum. These saw quite a bit of use in the southern part of the state as well as northern Iowa. Although anything but “Dura”ble after baking in the hot sun for decades, these were extremely light and easy to mount, the neck would mount to the pole and then the rest of the luminaire would twist on. Also odd is the external ignitor, the black box on the near side. This was a sold as a feature so you could replace it without even opening the door, but in practice it took all of  10 seconds to open the main door which you would do to try a new lamp first, and if a new lamp and photocell doesn’t fix it, industry practice is just to junk the fixture at that point anyway.

DuraStar 2000 LIght

DuraStar 2000 in my collection


Near as I can tell 90% of the cobraheads on surface streets in the area are GE M250R2s, American Electric 13s, or Cooper OVSs, these being apparently what NSP used for the mass conversion; the American Electric 113s and OVXs not existing at the time but used for later replacements.

 A Couple of Non-Cobrahead luminaires

The first High Pressure Sodium Lamps on metro freeways used an open cone shape refractor to accommodate the larger 250 watt lamps. These were found on I-35W south of downtown and I-35E north of downtown. The other mercury lights on the freeways were not converted until around 1990.

I-35W construction showing cone shaped NEMA lights in the background, early 1980s

I-35W construction showing cone-shaped NEMA lights in the background, early 1980s


There's still a couple of cone shaped lights left. Being mounted in the center, the light thrown backwards wasn't wasted.

There’s still a couple of cone shaped lights left in Hudson, WI. Being mounted in the center, the light thrown backwards wasn’t wasted.


Holophane, now owned by the parent company of American Electric has made glass for streetlights forever, and they also started making streetlights in order to sell more glass.:

Holophane Light

Holophane SRL, New Ulm, MN. I saw this listed in Holophane’s 1978 catalog but their utilitarian products were in production for decades.


St Paul used lights with the same style refractor but a much large head:



Here’s an unusual, unknown light from Rochester, MN:



Holophane also makes “fake history” “teardrop” models; the Minneapolis Parkway Models are Holophane MPU “Memphis” luminaires combined with some of the optional attachments:


Holophane “Memphis” Luminaire.


And the rarest of the rare, a “Turtle”. I saw this on a side street in Aurora, MN and posted a photo to a lighting message board, asking “what the heck is this?” Turns out it’s a UK model, GEC Z8422 , and everyone was amazed at one turning up in the United States. A small number of low pressure sodium vapor models were exported to Canada, but this was the first mercury vapor model seen in North America, and the first of any model in the US. One can only wonder how on earth it ended up there.

GEC Z8422 "Turtle"

GEC Z8422 “Turtle”


About Monte Castleman

Monte is a long time "roadgeek" who lives in Bloomington. He's interested in all aspects of roads and design, but particularly traffic signals, major bridges, and lighting. He works as an insurance adjuster, and likes to collect maps and traffic signals, travel, recreational bicycling, and visiting amusement parks.