Voltage Drop

[bigdinbassett]

Hi Folks,

Went into the shop today and no lights.

Switch tested good.
Breaker tested good.
Checked voltage at the feeder panel = 240V
Checked voltage at the sub panel = 240V

When I turn either of the bottom breakers ON in the sub panel, the voltage in the sub panel drops from 240V to 120V. All other breakers have no effect.

I've been scratching my head for a few days and can't seem to figure it out. Any insight would be appreciated.

Thanks!

 

[Dennis Alwon]

Sounds like you have a bad phase coming into the building

 

[LarryFine]

Welcome to the forum. What do you instruct?

We're only 3.5 hours apart. If you can't get anyone closer, contact me.

 

[bigdinbassett]

Sounds like you have a bad phase coming into the building

In the sub panel I have 240V until I turn on the bottom branch circuit.

 

[bigdinbassett]

Welcome to the forum. What do you instruct?

We're only 3.5 hours apart. If you can't get anyone closer, contact me.

Thanks Larry! I instruct industrial automation and instrumentation.

 

[LarryFine]

I asked because this site is for professionals in the electrical field, and we're not supposed provide DIY guidance.

 

[Dennis Alwon]

In the sub panel I have 240V until I turn on the bottom branch circuit.

Yes you will read the correct voltage until a large load is on. You need to contact an electrician.

 

[bigdinbassett]

Thanks for the help. I'll contact an electrician. Should I turn off the breaker that feeds the sub panel?