Does anyone have any idea why New York City, USA chose to use electronically ballasted 150w S56 HPS rather than 150w S55 HPS lanterns with 120v chokes?

Hmm that's interesting. Are you referring to the 1999 to 2012 period?

Hmm that's interesting. Are you referring to the 1999 to 2008 period?

I just dont know about that period of lighting history but I just want to know why New York City chose those lanterns over lanterns with 150w S55 ballasts in them.

Starting in 1999 NYCDOT replaced 60,000 of their 400 watt HPS cobra-heads with 250 watt heads. I don't believe these were electronically-ballasted. Between 2007 and 2012, New York converted most of the 250w HPS heads to electronically ballasted 150w, and their older 150w to electronically ballasted 100w. I assumed these were S55 and S54 type electronic ballasts, but you pose a good question which I wasn't aware of. Maybe Ugly1 will know why 100v lamps were used.

Is it possible that the city used the 100v S-56 so they could potentially convert to MH in the future? 

Is it possible that the city used the 100v S-56 so they could potentially convert to MH in the future? 

ANSI does not see them as compatible, so I don't think so.

Isn't the S56 more efficient lamp?
The electronic ballast does not suffer from the higher ballast losses as the CWA or HX has compare to the S55. And its losses would be more comparable to those of a series choke.
Normally the difference in ballast losses is cancelling out the efficacy benefit of having the arc voltage optimized for maximum efficacy without the voltage restriction, vs when targetting a plain series choke ballast.

Plus I would guess the S56 may offer wider selection of lamp types, like low Hg or Hg free, noncycling, higher CRI types (dunno what property NY is after) - as many of the design changes affect the requirements for ignition and OC- voltages, where tze S56 offers more margin...

From skimming over details in this thread and from pictures of the actual luminaires in question. I got a few points that could probably shed some light.

- Apparently the switchover starting in 2007 was to use electronically ballasted HPS and replace existing magnetically ballasted HPS luminaries in addition to any remaining MV luminaires that got missed out.

- The 150w S56 HPS apparently is a old NYC standard/requirement. NYC is rather known for having rather stringent standards that other cities don't have. For example they required glass lenses and reflectors in traffic signals long after other cities have switched to plastic lenses and metal reflectors. They also apparently require special approval for emergency lighting units and exit signs.

- Given that S56 150w lamps are rarely used compared to S55 150w, I doubt electronic ballasts were ever made for them specially. What Cooper (the manufacturer of the luminaries) opted to do was to install electronic Hatch 150w M102 PSMH ballasts in the luminaires and certify them on the nameplate as S56 HPS luminaires. They probably deemed it electrically similar enough to use them even though ANSI never said the two were interchangeable.

- The Hatch ballasts even though they didn't list S56 lamp compatibility, were specially made for this NYC order as the suffix -NYC appeared on the date codes of them.

- Ironically given the configuration of the luminaires, a conversion to 150w CMH would pretty much only require a lamp change.

I don't think the 150w electronic HPS lights in NYC were S56? They should say 15H if they were. The older magnetic 150w ones did have the 15H tags.

I don't think the 150w electronic HPS lights in NYC were S56? They should say 15H if they were. The older magnetic 150w ones did have the 15H tags.

They are S56, I know this because I own one and the info sticker inside it says 'Lamp Type: S56'.

They are S56, I know this because I own one and the info sticker inside it says 'Lamp Type: S56'.

I have one too. Actually I have several lol. And yes they are S56.

Wow, thanks for the information! In my hometown the city once had AEL 125s with 15H tags. The city didn’t know there were two different 150w HPS lamps. So they all glowed dim and yellow like LPS at night with the S55 lamps.

Wow, thanks for the information! In my hometown the city once had AEL 125s with 15H tags. The city didn’t know there were two different 150w HPS lamps. So they all glowed dim and yellow like LPS at night with the S55 lamps.

Yeah the electric company that services my hometown had some 150 watt S56 Thomas & Betts model 113's and ITT model 13. But they mostly had S55 113's and 13's. I can only guess they had a few S56 fixtures due to ordering mistakes. The ones I have ever seen glowed dimly like LPS as well.
Actually they also have some S56 Cooper OVH mixed in their system too.

Starting in 1999 NYCDOT replaced 60,000 of their 400 watt HPS cobra-heads with 250 watt heads. I don't believe these were electronically-ballasted. Between 2007 and 2012, New York converted most of the 250w HPS heads to electronically ballasted 150w, and their older 150w to electronically ballasted 100w. I assumed these were S55 and S54 type electronic ballasts, but you pose a good question which I wasn't aware of. Maybe Ugly1 will know why 100v lamps were used.

The amount of fixtures they wasted during all these conversions still horrifies me.