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Old 06-24-2009, 04:54 PM   #1
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crazy behavior with sub-panel


Hello everyone:

I have a 100amp sub-panel that powers 3 adjacent structures in my backyard. (This sub-panel runs off a 200amp main panel that has another 100amp sub-panel that powers the house).

The sub-panel has 7 circuit breakers:

- double pole 50amp that runs hot water heater
- 20amp that powers small refrigerator and occasionally a swamp cooler
- 20 amp that powers 2 ceiling fans and outlet for computer
- 20 amp that powers ceiling light
- double pole 30 amp that powers dryer
- double pole 20amp that powers well pump
- 20 amp that powers chest freezer

Recently the power goes out several times a day - without tripping the circuit breakers - in all circuits except for the following two:

- 20 amp that powers 2 ceiling fans/computer
- double pole 30amp that powers dryer

The power will eventually return on its own. However, I just discovered (and have tested 4 times) that if I turn on the dryer - the power immediately returns to all the circuits. Note that there are no GFCI outlets.

I opened the panel and inspected the wires. Everything looks good there. Also note that this can happen with nothing running except the freezer which I unplugged at one point and it still happened. Does anyone have any idea what is going on? Thank you.

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Old 06-24-2009, 05:02 PM   #2
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crazy behavior with sub-panel


Sounds like a loose hot on one side?
When you turn on a 240v device power flows to the other side

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Old 06-24-2009, 05:09 PM   #3
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crazy behavior with sub-panel


So I should check the hot wires that go into each of the circuit breakers? And I guess, each hot that goes into each receptacle??

Please confirm... it makes sense to me, but just want to be sure.

And, a big thank-you!!!

Last edited by treejep; 06-24-2009 at 05:29 PM.
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Old 06-24-2009, 05:32 PM   #4
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crazy behavior with sub-panel


One leg is out/dead/dying.

When you turn on a 240V load (dryer) the power from A phase jumps over to B phase thru the heating element and all the 120 stuff has power (backfeed).

CAREFULLY check both ends of the hot wires feeding the sub panel.

Turn off all breakers in the sub and you should read 240V between the hot legs and 120 to ground. You will probably only read 120 on one leg.

Lugs, breakers and bus are all subject to burning.

Last edited by 220/221; 06-24-2009 at 05:36 PM.
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Old 06-24-2009, 05:36 PM   #5
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crazy behavior with sub-panel


Check the lugs on the breaker in the main panel that feeds the sub. Also check the main lugs on the sub. With the power off, of course.

With all the breakers turned off at the sub, see what voltage you are getting from the sub panel feeder between each line and neutral, and also between the two lines. If you are getting 0V for one L-N, 120V for the other L-N and 0V L1-L2, then it's a disconnected hot wire.

It could also be that one of the hot wires between the sub and the main panels are damaged.

If you do have a disconnected hot wire to the sub, leave the power to the sub panel turned off until you get this resolved.

Power is probably returning intermittently even when you don't use the dryer b/c the hot water heater or well pump are kicking on. All the current to the circuits that are operating intermittently is flowing through your 240V appliance when it is on. That can at worst cause a fire, or at best ruin the appliances.

If you're not 100% sure that you can solve this, call an electrician.
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Old 06-24-2009, 06:36 PM   #6
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crazy behavior with sub-panel


Scott,

So you are saying that the problem could be

1. a disconnected hot wire from main to sub - in which case i should shut the sub down until problem is resolved

OR

2. disconnected hot wire in sub panel or in one of the downstream receptables/light switch in which case i need to check the hot wires of each receptacle/light switch.

If I can't find anything from number 2 should I assume it's number 1?
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Old 06-24-2009, 06:42 PM   #7
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crazy behavior with sub-panel


Most likely you have a bad underground cable.

What kind of water heater do you have? It's unusual for one to require a 50 Amp circuit.
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Old 06-24-2009, 07:08 PM   #8
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crazy behavior with sub-panel


[quote=kbsparky;292500]Most likely you have a bad underground cable.

before i go and dig up the line to the sub panel could you please answer two questions:

1. is there anyway to test to see if it is indeed a bad cable from main to sub panel?

2. can it be a disconnected or loose hotwire downstream from the circuit breaker? should i look at the hot wire connections for all receptables/light switches on each circuit.

In other words... could the results of the volt test deacribed above be the same for both scenarios?

Last edited by treejep; 06-24-2009 at 07:15 PM.
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Old 06-24-2009, 07:21 PM   #9
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crazy behavior with sub-panel


Assuming aluminum wire: Turn off the power to the subpanel. Remove the feeder wire connections at the subpanel (not at each breaker). Clean the wire and the lugs using a wire brush then reconnect using antioxidant. Torque to the required value. Do the same at the main panel where the wires connect to the breaker.
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Old 06-24-2009, 07:28 PM   #10
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crazy behavior with sub-panel


The EASY way to test for a bad underground is:

1) Check the breaker at the main panel for 240 Volts outgoing.
2) Check the breaker at the sub-panel for 240 Volts incoming.

If you have it outgoing, but not incoming, then the wire is bad somewhere between the 2 locations.

But to FIND the exact spot where the wire is bad is a much more involved procedure.

Test equipment to locate bad underground wires is available -- I have one. But it ain't cheap. I charge a minimum of $120 to locate an underground fault, and that don't include repair costs.

I do not charge anything if I can't locate the bad spot.
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Old 06-24-2009, 07:47 PM   #11
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crazy behavior with sub-panel


thanks, kbsparky

- if it turns out that i have 240 volt incoming then:

do i check hotwires downstream from breakers-- can someone please tell me if this could also be the problem. thanks
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Old 06-24-2009, 07:59 PM   #12
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crazy behavior with sub-panel


To troubleshoot the underground cable, you need a basic understanding of how to use a multimeter.
With the subpanel cover off, check the voltage on top of the main breaker.
You should read 240v between the two hots, and 120 volts to neutral from each hot.
If you do not read 240 volts, stop and go to main panel and test the breaker feeding the subpanel.
If you get 240 volts there the problem may lie in the cable feeding the subpanel.

Breakers and circuits below the main breaker in the sub panel will not affect the problem you have, except what was stated by by 220/221

Be careful!!!!
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Old 06-24-2009, 08:01 PM   #13
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crazy behavior with sub-panel


Quote:
Originally Posted by treejep View Post
thanks, kbsparky

- if it turns out that i have 240 volt incoming then:

do i check hotwires downstream from breakers-- can someone please tell me if this could also be the problem. thanks
NO, they cannot be the problem.
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Old 06-24-2009, 08:04 PM   #14
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crazy behavior with sub-panel


Quote:
Originally Posted by jbfan View Post
NO, they cannot be the problem.

thank you jbfan!!

i know how to use a multimeter, thanks again.

Last edited by treejep; 06-24-2009 at 08:08 PM.
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Old 06-24-2009, 08:05 PM   #15
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crazy behavior with sub-panel


Quote:
Originally Posted by treejep View Post
thanks, kbsparky

- if it turns out that i have 240 volt incoming then:

do i check hotwires downstream from breakers-- can someone please tell me if this could also be the problem. thanks
Not likely. The symptoms you have described indicate a problem with the incoming feeder, not the individual branch circuit breakers.

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