# Low Voltage lighting / transformer question

**URL:** <https://boards.straightdope.com/t/low-voltage-lighting-transformer-question/372081>\
**Category:** Factual Questions\
**Created:** [September 9, 2006, 6:35pm UTC](https://boards.straightdope.com/t/low-voltage-lighting-transformer-question/372081 "2006-09-09T18:35:14Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![jdc](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@jdc](https://boards.straightdope.com/u/jdc)\
**Post date:** [September 9, 2006, 6:35pm UTC](https://boards.straightdope.com/t/low-voltage-lighting-transformer-question/372081/1 "2006-09-09T18:35:14Z")

</div>

6 fixture, containing 1 18W (12v) bulb each, no plan to add more.

Do I just add the wattage of all 6 bulbs (108 total) and get transformer rated that high, or higher?

For example, is this enough capacity?

Intermatic 121W Timer/Transformer for $31.49

Features include:  
Powers all 12V Lighting systems up to 121W  
Fully Programmable timer w/ two sets of on/off trippers  
Heavy Duty Polymer Construction  
Not Suitable for Use with Submersible fixtures or products

Technical Specifications:  
121 Watt Timer  
2 Sets of Trippers

All labeling of existing Intermatic transformer missing. It buzzes like crazy, always have and I want to replace it.

---

<div class="post-metadata">

**Author:** ![Harmonious\_Discord](https://avatars.discourse-cdn.com/v4/letter/h/74df32/32.png) [@Harmonious\_Discord](https://boards.straightdope.com/u/Harmonious_Discord)\
**Post date:** [September 9, 2006, 7:31pm UTC](https://boards.straightdope.com/t/low-voltage-lighting-transformer-question/372081/2 "2006-09-09T19:31:23Z")

</div>

121 watt unit would be about right. To large and you waste power. To small and the transformer gets damaged.

You add all the bulb wattage together for 108 watts. Add about 10% for a good buffer amount. You get 118 watts for target size of the transformmer.

---

<div class="post-metadata">

**Author:** ![jdc](https://avatars.discourse-cdn.com/v4/letter/j/919ad9/32.png) [@jdc](https://boards.straightdope.com/u/jdc)\
**Post date:** [September 9, 2006, 7:46pm UTC](https://boards.straightdope.com/t/low-voltage-lighting-transformer-question/372081/3 "2006-09-09T19:46:12Z")

</div>

Thanks for the advice.

---

<div class="post-metadata">

**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\
**Post date:** [September 9, 2006, 9:01pm UTC](https://boards.straightdope.com/t/low-voltage-lighting-transformer-question/372081/4 "2006-09-09T21:01:51Z")

</div>

Yeah, your proposed transformer will do the job. If I were doing it for myself I would go the next larger size though. You say you don’t intend to add to the system. I’ll lay odds you will want to at some time.

One thing you might check on the nameplate data. Do they give a temperature rise? That’s the rise at full load and it shouldn’t exceed about 40 C.