Hello all.
Recently my main central AC unit just up and stopped working. After some basic checking I found it wasn't getting power at the contactor. I traced up to the box on the side of my home and found it was reading 0v as well, even though the breaker wasn't tripped.

SO after resetting the breaker and getting the same result, I guessed the breaker might be bad. I decided to open up the service panel and take some readings.

Here's where it gets weird for me...
All breakers on the right side of the box (generally 20 and 30 amp breakers) read just under 120v.

I go to the left side - which contains the breaker feeding my central AC - and none of them read over a volt (this I tested the same way as the right side). However, every outlet that I tested on the circuits fed by those breakers reads just about 120v, EXCEPT the centeral AC, which is still getting nothing.

So is the answer a faulty breaker where the AC is concerned, or am I reading wrong to begin with?

 

If you have any unused slots on the right side, transfer any breaker from the left side to the right & take a reading.

 

Welcome to the forums!

Voltage is nominal. +/- 10% of nominal voltage is considered OK. IE: 120 volt can be 108 - 132 volts and still be within tolerance.

Normally panels are set up so every other breaker as you go down is on a different leg. Example:
AA
BB
AA
BB

Not:
AB
AB
AB
AB

So your readings sound a little odd. When you are testing, between what two points are you testing between? Hot to ground/neutral? Hot to hot? It sounds possable you may have lost a leg, do you have any other 240 volt appliances (stove, oven, water heater) and do they work?

 

I have tested hot to neutral on the left side for +/-120v readings and hot to hot for +/-240v. Readings are in the nominal range. Left side, I tested both ways and this still resulted in extremely low voltages on my auto-ranging multimeter.

I feel like I'm doing something wrong as my bathroom GFCI breaker, for example, gets this same low volt reading yet tests fine 120v at the actual outlet. >

My water heater, stove, and dishwasher...I believe are all 240v and work. If not, at least one of them are. I'll have to double check and take a picture.

 

Are you putting one probe on the breaker terminal screw and the other probe on the neutral bar.

 

yep, doing this for both sides.

 

You mention you tested the left side twice, is that a typo?

Can you post a picture of your panel with the cover off?

 

Yep, I apologize. I tested the right side for those readings, then the left side when I read low voltages.

Here are the pictures - the bottom left breaker is the outdoor AC unit (probably not relevant).

 

This appears to be a back fed subpanel. Is that correct? Is this a GE panel?

 

Yes, looks like you have a nice mixture of breakers there GE, Siemens/ITE, Cuttler Hammer BR. (Not really) Panel appears to be a Siemens/ITE.

Anyway, I can see that your panel is set up as I posted previously with each leg every other breaker going down. Why you are not getting any voltage on the entire left side does not make any sense because the breaker on the right side is on the same bus so you should get voltage.

Make sure you are taking your readings from the same point. Put one prob to the neutral bus jumper (marked) and then the other to the terminal screw of the breaker. Make sure you put the prob on the screw with the black wire on the GFCI/AFCI breakers on the top left. Also check between the two terminal screws of the two pole breakers, you should get 240 volts.

 

It is a little different from European panels, but if one of the service fuses has blown, the readings could be as described?

dsk

 

I'd try checking the voltages again with a different meter. When you say all the voltages on the right side of the panel are near 120 volts you are checking the voltage from both legs. When you say all the voltages on the left side of the panel and none of them read over a volt, you are again checking voltage on both legs. There is something wrong with either the way you are checking voltages or with your meter. Get a different meter and start checking at the main lugs, you should get 240 volts across the main lugs. Then check each main lug to neutral, you should get 120 volts on each.

 

If you have a digital meter change the battery. If reading still of try a different meter as suggested. Use an analog meter not digital.

 

No. On ours one leg dead will show as half the spaces dead on both sides. That is why this is so puzzling.

 

Retracted answer as I did not see something else that someone else posted.

 

I had suggested that you move one of the breakers from the left side, to the right & maybe do it the other way around as well. Did you do that or do you think that it's a worthless idea?