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Electrical Wiring

adding ground wire for oven install

11 years ago

hi all,
I am soliciting an advice on how to best solve this problem. We are in California (LA). We are replacing a very old double oven with a new single oven. The old oven was rated 40 amps/240V and was installed correctly (back in the 70s I am assuming) with the right gauge wires and it was a 3 wire install - two hot/one neutral. The new oven has a 4 wires (2 hot, neutral and ground) and the manual explicitly says that "where code allows, 3-wire install is aacceptable). Oven is rated for 30 amps/240V.

Q1. Circuit/junction box is not being moved - we just have the junction box in the wall and the new oven is going into a new cabinet built to accommodate this new single oven. Does California/LA code allow grandfathering of 3-wire installation?
Q2. Regardless of answer yes/no to Q1 - we would like to upgrade to 4-wire eventually. Is it possible to pull additional wire through that conduit (it is a flex metal squidy looking) to make that wire the dedicated ground? We can't really afford to pull a new conduit, our service panel is full and replacing or adding a subpanel is a major expense.

I tried to search through discussions on this site because the question of 3-wire vs 4-wire install comes up often but I got lost and confused. We will hire an electrician to do this of course - I just need to understand the options upfront and make an informed decision on how to proceed.

I really appreciate everyone's advice.
Dax

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Comments (9)

Ron Natalie
11 years ago
Can't vouch for LA, but the California and the NEC still allows the grounded conductor to be used for the equipment ground purposes on existing range circuits.

If there's wire already in conduit, snakey or otherwise, you'll find it very hard to pull in a new wire. Second are you sure that conduit goes all the way back to the panel. Are you sure it's even conduit and not some sort of armored cable?

If you want to run a four wire circuit, it's probably not doable without pulling a whole new length of cable from the panel.
Dks35 Manning
Original Author
11 years ago
thank you, Ronnatalie

I was under the house and this conduit can be traced all the way to the panel and yes, it is an old-style steel flex conduit which is functioning as a ground. It isn't an armored cable.

So if code allows grandfathering then we might be able to install the oven now and think of upgrading later when the budget eases.
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Thanks for the response 'brickeyee'. If your calculations are right, then the manufacturer's power rating for the range of 7.3kW to 9kW @ 240 requires a 40 amp circuit would not work for an oven that was rated @ 8kW. A 40 amp 240V circuit would be limited to 40A*240V*80%=7680 watts. That would be under the 8kW that is in the range 7.3kW to 9kW. Besides, why would the oven have 10 AWG wire coming off it if it were required to be on a 40 amp circuit?
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I'm not sure how can interpret it that way, but OK. I'll try to clarify for you. 300.3(2) Grounding and Bonding Conductors. Equipment grounding conductors shall be permitted to be installed outside a raceway or cable assembly where in accordance with the provisions of 250.130(C) for certain existing installations or in accordance with 250.134(B), Exception No. 2, for dc circuits. Equipment bonding conductors shall be permitted to be installed on the outside of raceways in accordance with 250.102(E). 250.130(C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following:........ 250.102(E) Installation. The equipment bonding jumper shall be permitted to be installed inside or outside of a raceway or enclosure. Where installed on the outside, the length of the equipment bonding jumper shall not exceed 1.8 m (6 ft) and shall be routed with the raceway or enclosure. Where installed inside a raceway, the equipment bonding jumper shall comply with the requirements of 250.119 and 250.148. 1) This is absolutely NOT a receptacle replacement or a "branch circuit extension". It is no longer a dryer receptacle circuit. It is now an existing ungrounded circuit to be used for another purpose. 2) This is not DC so 250.134(B) is irrelevant. 3) This is NOT a bonding jumper as specifically described in 250.102(E) and is obviously longer than 6'. You can try and justify it any way you want. I honestly think you would have an extremely hard time convincing an inspector or qualified electrician that this would be code complaint OR safe, especially considering the use of the circuit.
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You mix "adding" and "replace" in a way that is confusing. You can NOT install countertop receptacles on a circuit that also feeds lighting. Now if your home was old, this wiring is probably grandfathered, but if you are adding a new receptacle you should do it correctly. How did you determine the ground wire was hot? I actually suspect it is floating which is nearly as bad. All the signs indicate that. I suspect you're using a high impedance meter (good for most things, but it will read stray voltages with one leg connected to a grounded conductor and the other either floating or connected to a long unterminated wire). A lot of fluorescent fixtures start intermittently without proper grounds. I bet you reach up and touch the fixture or rub the bulb with your hand, the other ones would fire. I would find all the outlets (lights, receptacles) and any junctions (especially those between the breaker panel) and inspect for more undone or loose connections.
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8/3NM cable always comes with a #10 ground. A #10 ground is appropriate up to a 60A circuit. No, you cannot use a 50A breaker. 40A maximum for #8NM in this installation, and from the oven specs 30A minimum.
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elltwo
11 years ago
If the electrician suggests replacing the circuit with a 30 AMP breaker and new 4-wire cable that can handle 40 AMPS you should be able to accommodate any future needs for not much added cost (it will be a function of length and accessibility). You have to change the breaker anyway and if you have no need for the existing wire then remove it completely and make room in the panel.
weedmeister
11 years ago
If it looks like this, it is Metal Clad cable.

Here is a link that might be useful: picture
weedmeister
11 years ago
It may look more like this, which is armored cable.

They may not look that different, but they are. One allows you to use the metal outside as a ground, the other does not. I think.

You cannot add additional wires to either.

If the installation instructions allow for 3-wire installation, you should be just fine. But if you change anything in the circuit (new outlet perhaps?), you have to update all of it.

Here is a link that might be useful: armored cable
brickeyee
11 years ago
AC (Armor Clad allows the armor as grounding conductor since it has a bonding strip (thins metal strip inside the cable).

MC lacks the strip, so a 'run ground' (a separate wire in the cable) is required.

The resistance of the armor coil can rise to high enough levels to prevent tripping a breaker and turning it into a nice heating element to start in wall fires.

This was the problem with the original 'BX' cable that led to its quick withdrawal from use.

AC replaced it with the bonding strip.
Dks35 Manning
Original Author
11 years ago
yes, the cable looks like the one in the last picture by weedmeister. It is also very thick. I guess I was calling that conduit?
Yes installation instructions allow to do 3-wire install provided local code permits it. As for outlet - it is a junction box, not an outlet and the oven is being hardwired to it.
Ron Natalie
11 years ago
You apparently have BX cable. You do not have an equipment ground nor may you obtain one by "pulling" just a ground wire.

Your options are to use the grandfathering to allow grounding the new unit to the grounded conductor (as it refers to in the 3 wire install) or pulling an entirely new cable.
valisah
8 years ago
I know it's an old post but, on this type of installation the ground wire is twisted together with the white .

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