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I have a 40A double-pole breaker feeding 6 AWG aluminum(?) wire going to a NEMA 14-50 receptacle for an existing electric range/oven. We had the oven replaced with a dual-fuel model (Bosch HDI8056U) that has gas burners while using electric for the oven elements. The manual and rating labels all say "30 amp" but that appears to be a minimum value. I can't find any reference to whether or not a 40A circuit is acceptable for this application. The installer did not seem knowledgeable in this area; he just slapped a range cord on it and went on to the next house.

Is the 40A breaker still okay here, or should I swap in one that matches the "30A" value printed on the oven? If I do switch the breaker, do I then have to change the receptacle/cord to a 14-30 to match?

Location: NC, USA.

smitelli
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    1 - Are you guessing aluminum for some reason, or do you know that it is aluminum based on markings on wire or cable? 2 - 14-50 is legal for 40A and 50A. If you switch to 30A then you *must* change to 14-30. However: **What is the make/model of the range so we can really figure this out?** It could get a bit tricky. Most ranges can be hardwired, and that may be preferable for a bunch of reasons, but if you actually have aluminum wires then plug (which almost always has copper wires in the cord)/receptacle (which are available copper/aluminum rated) may be much better than hardwiring CU/AL. – manassehkatz-Moving 2 Codidact Nov 16 '23 at 16:49
  • You don't *need* to use a smaller breaker. Pretty much as long as your load is smaller than the breaker (and wire rated for that breaker), you're fine. – Huesmann Nov 16 '23 at 16:51
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    @Huesmann, I will let others more knowledgeable about code speak more authoritatively than me, but I would guess that what the manufacturer specifies for overcurrent protection for the oven trumps what would normally be acceptable for circuit protection, since an oven likely would be considered a dedicated circuit scenario. The wiring up to the plug might be suitable for 40A, but if the oven specifies 30A, then the cord and oven could easily have an overcurrent situation drawing more than the 30A specified by the manufacturer without tripping the breaker with a higher trip limit. – Milwrdfan Nov 16 '23 at 17:00
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    @manassehkatz-Moving2Codidact Added the model number into the question body. My aluminum guess is based on a small piece of cable jacket that I can see from the crawlspace, which shows the gauge and (as best as I can read) an "AL" marking. – smitelli Nov 16 '23 at 17:09
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    The Product Specification sheet at the link in the question clearly indicates a 30A breaker and "Fixed connection (no plug)". – Jon Custer Nov 16 '23 at 18:31

4 Answers4

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In general, if the installation/operating instructions specify 30A, then using a larger circuit breaker is not fine.

For other types of equipment (heat pumps are typical) the manual may specify both "Mininum Circuit Ampacity" and "Maximum Overcurrent Protection Device" with different values.

Since you say:

The manual and rating labels all say "30 amp"

30A rules. The thing has been tested and listed for failures while connected to a 30A breaker, not for being connected to a 40A or 50A breaker.

So, to do it correctly, you need a 30A breaker, the same wiring (assuming it's actually got a ground wire - there's a whole other mess if dealing with old NEMA-10 and no ground - but if your NEMA-14 is correctly wired you have a ground) and a 30A receptacle (or just skip the receptacle and cord and hardwire the connection in a junction box, which is in many ways preferable.)

Be sure to use a torque driver to tighten connections to the proper specified torque value - that's particularly critical if the wall wiring is aluminum.

You might also drop a note to the LAHJ regarding the installer and their employer Doing It Wrong.

I suppose a potentially acceptable alternative would be to install a fused disconnect behind the stove with 30A fuses. This might cause confusion if the fuse blows, since the breaker would not trip, most likely. If you forgot it was back there, or the next owner didn't know it was, it would be baffling for a while. It's also (IMHO) somewhat less safe than a two-pole breaker where either leg faulting will shut off both poles.

Ecnerwal
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    If, for some unknown reason, The #6 AL wire doesn't fit in a 30 AMP breaker, Pigtail some #8 AL to the #6 AL and insert the #8 AL into the breaker. – JACK Nov 16 '23 at 17:38
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    @JACK Yeah, it looks like some breaker brands can't handle larger than 8 AWG on 30A breakers. – manassehkatz-Moving 2 Codidact Nov 16 '23 at 22:50
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    "In general" is not helpful. The NEC has special rules for cooktops and ovens and if you're not going to factor those into your answer, you shouldn't be answering cooktop/oven questions – nobody Nov 16 '23 at 23:09
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    In specific, there isn't a separate electric cooktop here, so following the instructions strictly applies. – Ecnerwal Nov 17 '23 at 00:31
  • Can you quote the code demanding that the 30A rating is treated like an *Maximum Overcurrent Protection* rating? If you have an [MOCP rating](http://www.electrical-knowhow.com/2019/11/sizing-protective-devices-for-air-conditioning.html) on the label, yes, then you must limit the current going through the *device*. Motors and others may fault and draw too much current, e.g. because they are blocked, and will overheat; this must be prevented, hence the *maximum* allowed current. The oven's rating though is not an MOCP, only short circuit protection is needed. – Peter - Reinstate Monica Nov 17 '23 at 09:32
  • Actually, after checking the relevant code section, I believe this is plain wrong. Downvoted until you show me relevant regulation other than in [my answer](https://diy.stackexchange.com/a/287049/48814). – Peter - Reinstate Monica Nov 17 '23 at 13:41
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The National Electrical Code states in article 422 (Appliances), section 11 (Overcurrent Protection), part (E):

(E) Single Non–motor-Operated Appliance.
If the branch circuit supplies a single non–motor-operated appliance, the rating of overcurrent protection shall comply with the following:
(1) Not exceed that marked on the appliance. [No overcurrent is marked.]
(2) Not exceed 20 amperes if the overcurrent protection rating is not marked and the appliance is rated 13.3 amperes or less; or [Appliance is rated 30A > 13.3A]
(3) Not exceed 150 percent of the appliance rated current if the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.

40A < 30A*1.5 -> You are fine, factually and regulatory.

Peter - Reinstate Monica
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    There may be markings on the appliance itself, we don't know. So we go with specifications page. – manassehkatz-Moving 2 Codidact Nov 17 '23 at 14:42
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    @Ecnerwal That is simply not correct, even though it is a common misunderstanding. A device that needs specific overcurrent protection has *two* Ampere values on its plate: (1) "MCA" (**minimum** circuit ampacity**, or what the **circuit** at least needs to be able to deliver; and (2) The "MO[C]P", or **maximum** overcurrent protection (for the **device**). Motors etc. can get into situations where they draw undue and dangerous currents -- dangerous to the device! --, without a short. Even though the circuit may be able to deliver that, the *device* requires protection. – Peter - Reinstate Monica Nov 17 '23 at 15:03
  • The instructions list no need for an overcurrent protection; the 30A are simply the general "rating". The code requires a general protection though for devices like this without specific demands, which is a factor of 1.5. (But yes, if the label on the device has an MOCP rating, that must be observed. I'd assume the documentation must list that as well though, which it doesn't.) – Peter - Reinstate Monica Nov 17 '23 at 15:05
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The Product Specification sheet says "Fixed connection (no plug)" but the installation manual, which includes details for both gas (120V 15A) and dual fuel (240V 30A or 208V 25A) models includes instructions for plug connections and for hardwired connections.

So it may be OK to use this plug/cord/receptacle connected. But it is a $3,000 beautiful machine (dual fuel gets you the best of everything, IMNHO) so why not hardwire it? That avoids the very common problem of mismatched receptacle type (30A? OK, just change the breaker...and now you have a 14-50 receptacle with a 30A breaker and the next machine in 15 years is an induction model that needs more power and gets installed plugin to that without checking breaker size and keeps nuisance tripping (and wearing out...) the breaker until the breaker starts having problems...) and avoids a critical hidden common point of failure.

  • Replace the 40A breaker with a 30A breaker
  • Install a proper 40A rated wire whip instead of the cord/plug
  • Remove the receptacle and connect the wires

The one catch is that you have aluminum coming from the breaker and copper from the range. So you need a proper way of connecting 6 AWG aluminum to 10 AWG copper. The Ideal 65 Purple doesn't handle large sizes. Polaris connectors may be your best bet:

Polaris

Depending on breaker brand (and you must use the correct type for your panel), you may need Polaris (or similar) connectors to pigtail 8 AWG aluminum or 10 AWG copper wire to the 6 AWG aluminum wire in order to fit on the breaker.

manassehkatz-Moving 2 Codidact
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    Why is this downvoted? That "product specification" sheet looks suspiciously like a document created for contract submittals packages. My instinct is to treat it as the actual specification. Under a philosophy of "follow _all_," you would follow the intersection of specification and instruction, i.e. hardwire it. – popham Nov 16 '23 at 22:41
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    Did you *read* the instructions? They **do not** say "30A breaker" - they say "**a minimum of ... 25 or 30 A". The NEC has special rules for cooktops and ovens and if you're not going to factor those into your answer, you shouldn't be answering cooktop/oven questions. – nobody Nov 16 '23 at 23:09
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    @nobody Yes, NEC has special rules for cooktops and ovens. A lot of that has to do with *shared* circuits between multiple devices and with load calculations. In this case, it is a single device. More importantly, while the installation manual has a lot of vague stuff like "minimum of...", the [Product Specifications page](https://media3.bosch-home.com/Documents/MCDOC03184968_HDI8056U.pdf) very clearly says **Circuit breaker (A) 30 A** – manassehkatz-Moving 2 Codidact Nov 16 '23 at 23:39
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    I'm no expert of code, so this is not an answer, but: In my book, the 30A is a minimum to inform the customer that they won't get away with less. And in an application like this it does not protect against unspecified overcurrent situations but simply against shorts. One reason is that the oven typically has three independently usable heating elements (top/bottom heat and broiler) which, when operated alone, theoretically may have defects which make them pull triple the current when operated alone, so 30A protects exactly against nothing. – Peter - Reinstate Monica Nov 17 '23 at 08:09
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Change the breaker to 30A. The instructions are part of the Listing, and the NEC requires following them.

Sometimes you have to figure out how to comply with multiple code sections, here at least 3 have to be considered. Section 422 deals with appliances:

NEC 422.11(E) Single Non-Motor-Operated Appliance. If the branch circuit supplies a single non-motor-operated appliance, the rating of overcurrent protection shall comply with the following:

(3)Not exceed 150 percent of the appliance rated current if the overcurrent protection rating is not marked and the appliance is rated over 13.3 amperes. Where 150 percent of the appliance rating does not correspond to a standard overcurrent device ampere rating, the next higher standard rating shall be permitted.

You also have to go back to the beginning, section 110 Rules for Electrical Installations:

110.3(B) Installation and Use. Equipment that is listed, labeled, or both shall be installed and used in accordance with any instructions included in the listing or labeling.

UL/CSA/ETL Listings require installing and using as instructed in any enclosed instructions. Page 11 of the instructions shows a table, says 30A for 120/240v ranges. 30A is less than the maximum of Section 422, so only a 30A breaker complies with both Sections 422 and 110.

The receptacle can remain a 50A:

NEC 210.23(B)(1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.

A 50A receptacle is not less than 30A, so satisfies the reqirement when it is a single receptacle on a branch circuit.

There is some confusion about the receptacle rating, eyes are drawn to an irrelevant table, 210.23(B)(3). The table for years was titled "Receptacle Ratings for Various Size Circuits", it shows a 30A breaker only allows 30 receptacles. But the table doesn't apply! Section 210.23(B)(3) only applies to circuits with multiple outlets. The 2023 edition of the NEC makes this more clear, the name on the associated table has been changed to clarify to now read "Receptacle Ratings for Circuits Serving More Than One Receptacle or Receptacle Outlet".

NoSparksPlease
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  • No where does the Code say "you shal comply with sales brocheres", often those are out of date. – NoSparksPlease Nov 17 '23 at 15:10
  • No, you can **NOT** have a 50A receptacle on a 30A circuit. Basically a single receptacle needs to match and multiples have some flexibility. But only within specific constraints. Which are (circuit -> 1 receptacle/multiple receptacles): 15 -> 15/15, 20 -> 15/15or20, 30 -> 30/30, 40 -> 50/50 (because there is no 40A-specirfic receptacle so per code 14-50 is used instead of the non-existing 14-40, etc.), 50 -> 50/50. 210.23B1 is really about "if you have a 20A circuit and only one receptacle it must be a 20 and not a 15". – manassehkatz-Moving 2 Codidact Nov 17 '23 at 15:18
  • Ugh, I really wish the receptacle wasn't in play here. It's surface mounted with the cable coming up through the floor. I popped the cover off and there is literally zero slack in there to rearrange wires, add a j-box or pigtail. – smitelli Nov 17 '23 at 15:26
  • @manassehkatz-Moving2Codidact The NFPA has changed their layout, if I can still find the proposal for changing the name on the table I think you will see that my interpretation is from the accepted change proposal. – NoSparksPlease Nov 17 '23 at 15:26
  • Per code there is always supposed to be enough length on the cable to swap things out. But things aren't always done right. The existing box **is** a j-box, by definition. And splices for a hardwired connection should be no worse in terms of box fill than the receptacle (which would be replaced with a blank cover). – manassehkatz-Moving 2 Codidact Nov 17 '23 at 15:30
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    The NEC says the breaker can be **150% of the rated current** (rating is 30A here, so the breaker may be up to 45A) if no overcurrent protection rating is marked on the appliance (422.11(E)). – Peter - Reinstate Monica Nov 17 '23 at 15:30
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    @manassehkatz-Moving2Codidact Sorry, I'm not finding the NFPA Doc that includes public comment. I do find in https://docinfofiles.nfpa.org/files/AboutTheCodes/70/70_A2022_NEC_P02_FD_PIReport_rev_2of2_rev_1008.pdf page 408 where the accepted change was proposed David Hittinger writes " Second, an installer could install one single 30 amp receptacle on a 15 amp circuit. There are no requirements against that... since it is protected by an overcurrent device" Hittinger is a member of CMP-2. – NoSparksPlease Nov 17 '23 at 17:19
  • @NoSparksPlease The problem with a 30A receptacle on a 15A circuit, or in this instance a 40A/50A receptacle on a 30A circuit, is that it may seriously confuse future users. Example: Uses 40A/50A receptacle, changes to 30A breaker - everything safe now; 10 years from now does a heavy-up and panel move - electrician sees 30A breaker and extends that run with 10 AWG cable; 5 years later new owner decides they want to get rid of gas and puts in an induction cooktop/electric oven which requires a 40A circuit - sees 40A/50A receptacle and says "wow, that's great - must be the breaker was downsized – manassehkatz-Moving 2 Codidact Nov 17 '23 at 17:35
  • because the dual-fuel was supposed to be on a 30A circuit" **doesn't look at the wiring between the old and new panel locations and simply puts on a 40A breaker**. And **now** there is a real problem - which would not have happened with a 30A receptacle. – manassehkatz-Moving 2 Codidact Nov 17 '23 at 17:36
  • @manassehkatz-Moving2Codidact I see what you believe to be a critical issue, but what Code section backs you up? I see where is says "not less than" and I think that paragraph allows receptacles not less than. What additional code sections restricts me? – NoSparksPlease Nov 17 '23 at 18:29
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    @manassehkatz-Moving2Codidact In your example the last person changing the breaker should be able to see the #10 size of the panel change extension and know to not put it on a larger breaker. – NoSparksPlease Nov 17 '23 at 18:33
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    @NoSparksPlease I know. But people do stupid things, especially if they don't check with DIY SE first :-) – manassehkatz-Moving 2 Codidact Nov 17 '23 at 18:40