Take your pick. The 24V side is the opposite of the common side.

There have been some posts here about phase(?) on the transformer and relationship to neutral on the line voltage side but I have not encountered a situation that required switching them. Perhaps someone will comment who has experienced it.

 

Thats a possibility with systems where there are two transformers wired in parallel. Two 24v xfmrs with the secondaries wired in parallel will give 24 volts at the secondary. If they are wired out of phase, the secondary voltages will sum to 48 volts. Similar to measuring from hot to neutral at the panel vs hot to hot.

 

What do you mean by take your pick...any of the two wires coming of the the secondary side i could ground?

Thanks

 

yes. Whichever you pick to ground, the other side will be 24V. The 24V side is the one that gets switched, common should be uninterrupted.

 

Thanks ..

 

You're welcome. I hope it solves the issue for you.

 

polarity= L1 to ground, 120V. neutral to ground, 0 volts
ground/bonding= ground (EGC) to chassis, 0 volts
phasing a transformer= L1 to R, 96 volts. Out of phase= L1 to R= 144 volts.
correct phase= primary voltage minus secondary voltage, out of phase, voltages add together.

for what is worth.......

 

The only transformers that were grounded on the secondary side I had to replace because some electrician thought everything needs to be grounded! Don't ground low voltage secondaries! Why would you want to ground L1 or L2?? You do not ground live wires. if the secondary goes to ground it is screwed down to grounded metal. You are confused about the American 120 VAC system that uses a neutral instead of just using 240 VAC system like Europe. We use 120 VAC because A. Edison was in a war with Tesla about electrical distribution. Edison lost because he was completely wrong and wanted to use DC. We run 765,000 VAC in transmission lines to lower amperage and wire size then just drop VAC as necessary thru transformers. Most people die of electrocution from a 120 VAC circuit that has lost a path to ground (neutral).

 

???????????
Wire per the vendor diagrams. I have had gas valve open with no safeties because of loose or disconnected 24vt grounds, solid state controls intermittently not function and other fail because of inadequate grounding.

 

If this transformer is going on a unit that has heating with flame rectification to sense the flame, polarity of the transformer and which side gets grounded is critical.

So you always fire off the heating to be sure you don't/won't have a call when it gets cold.

 

The solid state flame rectifier uses a simple electronic current relay, when it senses current flow from the flame rod to anywhere it proves pilot. If you deliberately ground out the flame rod the relay circuit senses a failure and opens. Clean the rod and pilot hood

 

Doesn't the current NEC call for the secondary to be grounded?

 

Special for you , it's grounded at the chassis.

 

Yes, in most all cases.
NEC "Circuits under 50V may have to be grounded. (sec. 250-5)". "If supply system operates ungrounded, then secondary must be operated grounded regardless of voltage."

Wiring by mfg has to be certified by an "approved agency" UL, ETL, CUL....

 

I suggest an experiment: go get a ground rod, some #6 ground wire, a couple of clamps. Then go out by your 230 VAC condensing unit and connect L2 (common) to ground rod, throw the disconnect and prove your theory.

 

Im thinking the only times Ive seen the 24V side grounded to the frame .... is on furnaces for flame rectification

On a Heat Pump I dont see any benefit

 

If the common side of the transformer is connected to the chassis, then the entire chassis becomes Common. If...a gas valve coil, for instance, goes to ground then the valve has found a new current path for 24 Vac.

 

Another related question:
I am going to use a 24v (120v primary) transformer to connect to a NEST thermostat to control a relay.
If I do not ground one leg of the secondary then I do not have a 24v for red. If I want to connect R, W, C on the nest how should I ground this?
Can I ground one leg of the secondary to the ground wire coming from the breaker panel providing the 120V for the primary?

 

I did test this carefully using a 3 amp fuse and grounded one leg of the secondary and it worked. the voltage still reads the same 24v and now I have a common to give it to C wire and a 24v to give it to the R.
(which leg I chose? I read the voltage from each leg first and chose the leg that has the smaller volt to ground.)
I also read the amp that is pulled from the grounded leg to the ground wire of the 120v side and it is pulling about 0.01 Amp (without any load)and 28v , it is while the primary pulls about 0.07 amp and 125v.
I hope this is helpful. I appreciate if anyone can elaborate on this more scientifically or if there is any other points to consider.

 

Flame rectification polarity and phasing

View attachment Flame Rectification Polarity and Phasing.pdf

 

Hi BBeerme,
It "Flame Rectification Polarity and Phasing.pdf" is such a good informative document, you shared.
***Thanks dude.***
I was feeling, I was inventing the wheel. thanks for clearing that up.

I further set it up and measured the voltages again. I read:
120v on primary,
24v on secondary
93.3v between primary hot to secondary hot,
0.04A : clamp reading on the wire grounding both primary neutral and secondary com, jumped and to the ground of the incoming power line, then I added a 1kΩ resistor on the line and read less than 1µA.
again used a fuse 5A just to be safe.

 

The funny thing is the date stamp on that file I just posted was several years before my first post on this thread. Just sayin'. Probably was too busy with work.

 

Looked like those good old documents. Love those old documents.
where did you get it from?

 

Time stamp on my file is 2017, too long ago to remember. I just saved it because I knew it was true, but difficult to explain it, especially to some manager or know-it-all who doesn't like to be told they are wrong.

 

I had a gas valve knock the poo poo outta me one day in a hot attic.

The furnace had a dial type fan/limit switch that was wired to break the low voltage circuit, but someone had neglected to break the tab, so it was being fed 120 volts and I assume sending the same through the low voltage circuit.

I was setting up to look/check the heat exchanger and laid my arm on the gas valve. KaPow, it bite me hard ! ! !

After thinking about why, I got the meter and checked the voltage, yup, 120 on a 24 volt valve. It's a wonder the place had not blown up yet !

 

I had a gas valve knock the poo poo outta me one day in a hot attic.

The furnace had a dial type fan/limit switch that was wired to break the low voltage circuit, but someone had neglected to break the tab, so it was being fed 120 volts and I assume sending the same through the low voltage circuit.

I was setting up to look/check the heat exchanger and laid my arm on the gas valve. KaPow, it bite me hard ! ! !

After thinking about why, I got the meter and checked the voltage, yup, 120 on a 24 volt valve. It's a wonder the place had not blown up yet !

This problem is common. People testing and find line voltage at the T'stat. Or taking a hit like you did.
Honeywell f**k up this time.
I wonder if anyone ever took a really serious hit.