Metal Clad grounding question

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idoc4u

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Could you please help me understand a wiring issue I'm having? I have #12 hot and neutral running from a new GE 20amp breaker to a metal junction box. From the jb I have 12/2 armor clad running behind drywall (old work) to a metal box with a 15amp outlet. If I connect the copper (green) ground from the metal clad (which is aluminum corrugated sheathing) to the junction box on one end and to the receptacle ground screw on the other end, the breaker trips. If I simply cap off the ground on both sides, I have no problems. I thought you are supposed to always ground metal clad to the junction box and outlet. Is the aluminum clad acting as the ground? I have run circuits like this in the past and did ground the metal clad to the jb and the receptacle ground screw without an issue. What am I not understanding? Thank you!
 

wwhitney

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Yes, you need continuity of the EGC (ground).

AC cable and MC cable are two different types. So you have MC cable from a breaker to a junction box, and AC cable from the junction box to the receptacle box?

In AC cable the armor is the EGC. The armor should have a shorting strip alongside the inside of the spiral armor to ensure that the armor is of sufficiently low resistance. Very old AC cable without the shorting strip does not provide a good EGC; if you have that, it would be best to replace it.

I'm not sure I follow the narrative completely, but my first thought would be a problem with the old work AC cable, perhaps a hot to armor short somewhere along its length or at one of the terminations.

Cheers, Wayne
 

Sylvan

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Hi Sylvan!

I noticed you offered awesome help to another member. I am new here with a water heater question. Could you help?

I have two new water heaters in a duplex (one in each respective basement)
-Model: Rheem 50gal Heaters (Purchased at Home Depot)
-Electric heaters
-Expansion Tanks
-Dehumidifiers running 24/7

These two heaters were installed, filled, and worked the past seven (7) months. The heaters haven't gotten much use, we are still doing rehab and units are not occupied. Ten (10) days ago, Basement Waterproofers had to CUT the water supply lines to the heaters to move them away from the wall and install sump pumps. When they cut the water supply lines, water leaked atop the units.

Today, (10 days later) I notice rust in BOTH Electrical Junction Boxes located on the heater lids. The styrofoam underneath the junction box absorbed the rust color as well. (Photo Album) Is this because the water sat on the lid and seeped into the unit? Or is this a defect that's been rusting for months from within?

The contractor argues this rust is old and splashing from cut water lines never caused a problem before.

Your help is truly valuable. Thank you for any advice or suggestions!

-Christian

Christian see the answer I sent you as
 

idoc4u

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Yes, you need continuity of the EGC (ground).

AC cable and MC cable are two different types. So you have MC cable from a breaker to a junction box, and AC cable from the junction box to the receptacle box?

In AC cable the armor is the EGC. The armor should have a shorting strip alongside the inside of the spiral armor to ensure that the armor is of sufficiently low resistance. Very old AC cable without the shorting strip does not provide a good EGC; if you have that, it would be best to replace it.

I'm not sure I follow the narrative completely, but my first thought would be a problem with the old work AC cable, perhaps a hot to armor short somewhere along its length or at one of the terminations.

Cheers, Wayne
Wayne, thank you so much for the reply! Let me clarify my situation. I have 1/2" EMT running from the breaker. In the EMT are hot and neutral #12 wires. The 1/2" EMT connects to a junction box. From the junction box, I have Armorlite 12/2 aluminum clad cable going to a new 15amp receptacle. If I attach the new AC ground to the junction box and ground screw on the receptacle, the breaker trips. If I simply cap off the ground with wire nuts on each end, the receptacle operates correctly. When I test the receptacle, the tester verifies that the ground is closed and the outlet is functioning correctly in all ways. I assume the circuit is being grounded properly as a result of the cable armor and or the emt? Why would attaching the ground cause the breaker to trip? I thought it was protocol to use the ground with aluminum clad cable? Thanks again...........Tony
 

wwhitney

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OK, so you have EMT and MC cable, no AC cable. Is the breaker a GFCI or AFCI breaker?

The info so far sounds like an EGC to hot short that involves the MC cable EGC. (Or if a GFCI or AFCI breaker, a neutral to EGC short). You should disconnect all 3 wires in the MC cable at both ends, confirm that your breaker holds, and then check the resistance between all 3 pairs of conductors in the cable (black-white, black-green, white-green). They should all be infinite (or many megaohms if you have a megohmmeter). If you get low resistance on any pair, your MC cable section is bad and you should replace it.

If not, connect up all 3 wires on the MC cable in the junction box, but leave all three wires capped at the receptacle box. Check if the breaker holds. If not, then you either did something wrong making up the connections, or your MC cable has a fault in it you didn't find with the ohmmeter. Double check your connections, and if there's no other way for there to be a short, replace the MC cable.

Finally, if everything has held so far, sounds like the receptacle--replace it and try again. Make sure that no exposed hot or neutral conductor is touching any metal box when you make up the connections.

Cheers, Wayne
 

idoc4u

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Yes, you need continuity of the EGC (ground).

AC cable and MC cable are two different types. So you have MC cable from a breaker to a junction box, and AC cable from the junction box to the receptacle box?

In AC cable the armor is the EGC. The armor should have a shorting strip alongside the inside of the spiral armor to ensure that the armor is of sufficiently low resistance. Very old AC cable without the shorting strip does not provide a good EGC; if you have that, it would be best to replace it.

I'm not sure I follow the narrative completely, but my first thought would be a problem with the old work AC cable, perhaps a hot to armor short somewhere along its length or at one of the terminations.

Cheers, Wayne
Yes, you need continuity of the EGC (ground).

AC cable and MC cable are two different types. So you have MC cable from a breaker to a junction box, and AC cable from the junction box to the receptacle box?

In AC cable the armor is the EGC. The armor should have a shorting strip alongside the inside of the spiral armor to ensure that the armor is of sufficiently low resistance. Very old AC cable without the shorting strip does not provide a good EGC; if you have that, it would be best to replace it.

I'm not sure I follow the narrative completely, but my first thought would be a problem with the old work AC cable, perhaps a hot to armor short somewhere along its length or at one of the terminations.

Cheers, Wayne

OK, so you have EMT and MC cable, no AC cable. Is the breaker a GFCI or AFCI breaker?

The info so far sounds like an EGC to hot short that involves the MC cable EGC. (Or if a GFCI or AFCI breaker, a neutral to EGC short). You should disconnect all 3 wires in the MC cable at both ends, confirm that your breaker holds, and then check the resistance between all 3 pairs of conductors in the cable (black-white, black-green, white-green). They should all be infinite (or many megaohms if you have a megohmmeter). If you get low resistance on any pair, your MC cable section is bad and you should replace it.

If not, connect up all 3 wires on the MC cable in the junction box, but leave all three wires capped at the receptacle box. Check if the breaker holds. If not, then you either did something wrong making up the connections, or your MC cable has a fault in it you didn't find with the ohmmeter. Double check your connections, and if there's no other way for there to be a short, replace the MC cable.

Finally, if everything has held so far, sounds like the receptacle--replace it and try again. Make sure that no exposed hot or neutral conductor is touching any metal box when you make up the connections.

Cheers, Wayne
Wayne, the 20amp GE breaker is a standard breaker. The metal clad in the Armorlite is aluminum. My testers show that the ground is correct, not open and not reversed. The receptacle functions as normal without the ground attached, but trips the breaker when the ground is connected. I don't understand why the outlet functions normally, if there is a short. I'll try your recommendations. Thank you again..............Tony
 
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