the trouble call from hell......

[Fulthrotl]

house, circa 1936 ish. hybrid k&t, ungrounded romex.

looks to be remodeled about 20 years ago, by the panel on
the back of the house. 100 amp service. maybe 1,200 sq. ft.

maybe.

house appears to have had extensive "loving hands of home"
work done. grounded decora devices installed in ungrounded
system.

central heating and AC, maybe 5-8 years old.

now, the complaint by the realtor trying to get this to
fly on a short sale, before foreclosure on jan. 01:

everything works "fine" until you turn off the AC, and then
the lights go off. turn the AC back on, and the lights come on.
AC is carrier unit, with honeywell low voltage controller. vanilla
installation, appears professionally done.

turning on the heater doesn't light the lights. only the AC turned
on will light the lights, and according to the homeowner....

"it wasn't a problem in the summer, but now that it is winter, we
would like the lights to work with the heater as well. is that hard
to do? is it expensive?

honest to god, it was so funny, i had to take a video with my
cellphone of me turning on and off the t stat, and the hall light
going on and off.

supplemental information:

in addition to half the lights going on and off, half the plugs do the
same thing, controlled by the t stat heating/cooling switch on the
honeywell stat. but only with the AC, so this must be someone's
"summer home".

when the "switched" plugs are on, plugging in an ideal suretest,
give a voltage drop, at 15 amps, of 38%, with 71 volts rms being
available at a 15 amp load.

ok, all of you gifts to the electrical industry.... what is the problem?



randy

 

[emahler]

house, circa 1936 ish. hybrid k&t, ungrounded romex.

looks to be remodeled about 20 years ago, by the panel on
the back of the house. 100 amp service. maybe 1,200 sq. ft.

maybe.

house appears to have had extensive "loving hands of home"
work done. grounded decora devices installed in ungrounded
system.

central heating and AC, maybe 5-8 years old.

now, the complaint by the realtor trying to get this to
fly on a short sale, before foreclosure on jan. 01:

everything works "fine" until you turn off the AC, and then
the lights go off. turn the AC back on, and the lights come on.
AC is carrier unit, with honeywell low voltage controller. vanilla
installation, appears professionally done.

turning on the heater doesn't light the lights. only the AC turned
on will light the lights, and according to the homeowner....

"it wasn't a problem in the summer, but now that it is winter, we
would like the lights to work with the heater as well. is that hard
to do? is it expensive?

honest to god, it was so funny, i had to take a video with my
cellphone of me turning on and off the t stat, and the hall light
going on and off.

supplemental information:

in addition to half the lights going on and off, half the plugs do the
same thing, controlled by the t stat heating/cooling switch on the
honeywell stat. but only with the AC, so this must be someone's
"summer home".

when the "switched" plugs are on, plugging in an ideal suretest,
give a voltage drop, at 15 amps, of 38%, with 71 volts rms being
available at a 15 amp load.

ok, all of you gifts to the electrical industry.... what is the problem?



randy

the house needs to be moved to the equator?

 

[nakulak]

there is only one hot phase at the main panel ?

 

[Fulthrotl]

there is only one hot phase at the main panel ?

nope....

120/240 single phase, 3 wire service. overhead feed.

 

[480sparky]

Do the switches that control the lights function as intended when the power mysteriously gets turned on by the AC?

(I'm going to suggest that the previous owner had some morbid fear of an electrical fire, and wiring the lights to the contactor in the AC outside so the lights would only work when the AC is running in order to cool down the wiring.)

 

[ultramegabob]

My money says that half of the main breaker is bad, check to see if you have 240v. on the load side of the main. Whats happening is power is being fed backwards through tha AC unit to the other half of the panel when the thermostat calls for the unit to come on.

 

[kbsparky]

Try turning on all 4 burners on high of an electric range. Same thing, the lights will respond. Gas range? Try switching on the dryer. Anything that requires 240 Volts will give the same results.

 

[zappy]

Do the switches that control the lights function as intended when the power mysteriously gets turned on by the AC?

(I'm going to suggest that the previous owner had some morbid fear of an electrical fire, and wiring the lights to the contactor in the AC outside so the lights would only work when the AC is running in order to cool down the wiring.)

Funny! How do you think of this stuff?:-?

 

[480sparky]

My money says that half of the main breaker is bad, check to see if you have 240v. on the load side of the main. Whats happening is power is being fed backwards through tha AC unit to the other half of the panel when the thermostat calls for the unit to come on.

Wouldn't that require the AC to be fed by a circuit before the main?

 

[ultramegabob]

Wouldn't that require the AC to be fed by a circuit before the main?

No, if one half of the main is bad, you will get 120 going into the AC motor and since the other leg is dead, the voltage will travel backwards through the feeders and AC units breaker powering the bussbar with 120v, both bussbars are now being fed with the same phase, so you will read 120 to neutral off of each buss, but 0 volts from buss bar to buss bar.

 

[zappy]

My money says that half of the main breaker is bad, check to see if you have 240v. on the load side of the main. Whats happening is power is being fed backwards through tha AC unit to the other half of the panel when the thermostat calls for the unit to come on.

I'm just trying to understand, so bear with me. So if everything in the house works fine until you turn on something that's 240V. That means your only getting 120v. at the main breaker? Then how does everything else in the house that's 120v. work when the 240v. stuff is off? There all on one phase?

 

[ultramegabob]

I'm just trying to understand, so bear with me. So if everything in the house works fine until you turn on something that's 240V. That means your only getting 120v. at the main breaker? Then how does everything else in the house that's 120v. work when the 240v. stuff is off? There all on one phase?

yes, everything that is staying on must be all on the same phase.

 

[480sparky]

No, if one half of the main is bad, you will get 120 going into the AC motor and since the other leg is dead, the voltage will travel backwards through the feeders and AC units breaker powering the bussbar with 120v, both bussbars are now being fed with the same phase, so you will read 120 to neutral off of each buss, but 0 volts from buss bar to buss bar.

Brain damage on my part. I missed the word 'half' in your first post.

Never mind.

 

[ultramegabob]

The problem could be before the main breaker, but the first thing I would check is the voltage on the load side of main, that will tell you what the problem is real quick, then you can track down where the problem actually is whether it is a bad main, connection in the meter base, or a break in the utilities line somewhere.

 

[zbang]

FWIW, just reading the first post made me think similar thoughts- backfeed though a 240v device. Could be a bad main, bad PoCo drop, or from the sound of things, bad bus connection in the panel (any suspicious burn marks inside?).

 

[Fulthrotl]

My money says that half of the main breaker is bad, check to see if you have 240v. on the load side of the main. Whats happening is power is being fed backwards through tha AC unit to the other half of the panel when the thermostat calls for the unit to come on.

ok.... if a 220 load is backfeeding thru the dead half of the house wiring,
then all the branch circuits in parallel are providing low enough resistance
to allow the compressor motor to run without blowing fuses? that one's iffy, as
the house was empty, with about three 60 watt incandescent lamps
in parallell that were affected. the receptacles had no loads across them
so would not have an influence.

so, three lamp filaments would be able to run a 20 amp 240 volt load
in series, while not dropping the voltage so far as to cause the motor
to draw excessive current from low voltage and blow the fuses?

the math looks like this.... a 65 watt lamp has a resistance of 15.5
ohms. three of them in parallel would be a 5 ohm resistance, equal
to about 2,500' run of #10 copper.

would that compressor run on a 2,500 foot branch circuit without
blowing the fuses because of a low voltage situation?

well, if i get asked back out to repair the problem if/after escrow
closes, i'll look at the main first thing. your theory looks like the
best place to start, and i'm inclined to agree with it. the extremely
high voltage drop measured at the receptacle makes sense if it is
measuring thru a motor in series, and light bulbs in parallel.

truth be told, when i looked at it, and the general condition of
the house, with all the creative interpretations of the NEC,
i'll be happy to do a complete rewire of the house, including
replacing all of the circuit breakers, for $7,000.

i'm not really interested in owning this can of worms for the
price of a circuit breaker, and a two hour service call, y'know
what i mean?

thanks for your feedback.....


randy

 

[marcerrin]

If half the main was bad, the 240v a/c wouldn't work.........

Does the panel have a main, or is it a split buss?

 

[Sparky555]

The funny ones are usually lost neutrals. I wouldn't touch a patch job on this.

 

[480sparky]

ok.... if a 220 load is backfeeding thru the dead half of the house wiring,
then all the branch circuits in parallel are providing low enough resistance
to allow the compressor motor to run without blowing fuses? .........

Why would two loads in series automatically blow a fuse?

 

[ultramegabob]

If half the main was bad, the 240v a/c wouldn't work.........

Does the panel have a main, or is it a split buss?

that is true, if you believe what a Realtor and or a homeowner says about how and when the problem started:roll: I could be wrong, and this could be a really oddball funny reason for this to be happening, but my troubleshooting experience leads me to think it is a lost leg of power. I dont usually put a lot of faith in what customers say about how and when electrical problems start, they tend to throw you off track more than they help....