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adding ground wire for oven install

Posted by dks35 (My Page) on
Fri, Dec 14, 12 at 17:49

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.

Follow-Up Postings:

RE: adding ground wire for oven install

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.

RE: adding ground wire for oven install

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.

RE: adding ground wire for oven install

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.

RE: adding ground wire for oven install

If it looks like this, it is Metal Clad cable.

Here is a link that might be useful: picture

armored cable

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

RE: adding ground wire for oven install

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.

RE: adding ground wire for oven install

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.

RE: adding ground wire for oven install

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.

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