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I just fixed a hard-to-find short circuit in my home wiring. I had heavy gauge aluminum wires running from two 40 amp circuit breakers to bring 220V to my electric cooktop. My guess is that those were the original wires from 40 years ago. Those wires were spliced to copper 10G wires in a junction box and then they ran to another junction box near the cooktop where they were spliced again. One of the aluminum /copper connectors melted in that middle box. This was very hard to diagnose.

Question: The cooktop label says 7KW for 120V/240V; 5.3KW for 120V/205V. Someone suggested that I should replace those 40A breakers with 30A breakers if the the appliance can run on the 30A as it would be safer. Can I just take 7000 divided by 240 = 29Amps? Will that be okay? Or do I have to take into account RMS voltage and current? I don’t remember whether these ratings are in peak to peak. I think my cheap meter shows peak to peak, not RMS.

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Rather than replacing the breaker, you should fix the actual problem. This document (PDF) from the Consumer Product Safety Commission, describes a few safe ways to handle aluminum wiring in a home.

Aluminum wiring itself is not dangerous. The danger with aluminum wiring is with how it's spliced/terminated. If connections are not done properly, it can lead to overheating and potentially a fire. If terminations are done properly, aluminum wire can be, and is, used without incident.

Replacement

The most obvious recommendation, is to simply replace all the aluminum wiring with copper. However, that is not always the most practical, or cost effective way to handle the situation.

Pigtailing

If the wire cannot be replaced, the CPSC recommends a few alternative methods to terminating/splicing aluminum wire.

COPALUM Connectors

Using a special connector and tool, joining aluminum wiring to a short piece of copper wire (pigtail) can be a safe solution. This method may require professional assistance, because special tools and knowledge are required.

AlumiConn Connectors

If COPALUM connectors are not available, AlumiConn connectors may also offer a safe connection. AlumiConn connectors are basically insulated terminal block, which allow a copper pigtail to be connected to the aluminum wire. CPSC recommends that this repair also be conducted by a licensed Electrician.

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    This is clearly the right solution -- bring in an electrician to re-bond all of the aluminum junctions (due to the critical safety aspect, this is not something I'd do myself). Even if it were ok to replace the 40A breaker with a 30A breaker, it would be a marginal safety improvement at best if there are flaky joins in the cable. – Johnny Nov 24 '15 at 17:16
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The main additiona risk with aluminium wiring VS copper wiring is a greater chance of bad joints, especially if the correct termination methods are not used. Once a joint starts going bad it heats up which causes more thermal stress and makes the problem worse.

A smaller breaker will not help much. The bad joint is in series with the load, so (assuming a resistive load) it actually reduces the current drawn. The only time a smaller breaker might help with a burning up joint is if the burning up joint leads to a partial short and by some fluke that partial short draws enough current to trip the 30A breaker but not enough to trip the 40A breaker.

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root mean square or peak, 30A or 40A. don't worry about this at all. i see this problem all the time. aluminum wiring is an accident waiting to happen. aluminum from 40 years ago should have been replaced 39 years and 364 days ago. aluminum mechanically, thermally and dielectrically fatigues with every passing year, and it becomes exponentially more so when used in a high current application like supply wire to an electric range or oven. when copper is connected to aluminum in a junction box, you get galvanism taking place, and the aluminum oxidizes and degrades further. that is why the connection failed. it will do so again, and its just a function of time as to when and how. do yourself a favour and replace the line and the junction boxes. its not hard to do if you are handy, and not too costly to get an electrician in to do it. if it saves you and your family's lives and home, its the best solution.

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    There's no problem with aluminum wire, the problem is with bad splices/terminations. – Tester101 Nov 24 '15 at 16:49
  • yes thanks.. i know about the issues of aluminum/copper splicing. those splices were done 20 years ago with a special "Polaris" brand connector that allows both wires to be screwed down seperately. over the years one of them failed and i just replaced it (those damn connectors are $20 a throw!). i am probably not going to rip up my walls to replace the alum with copper BUT i was asking how to probably size a breaker for a stove that is labeled as " 7KW for 120V/240v , 5.3KW for 120V/205V". – Cool Pontiac Nov 24 '15 at 18:09
  • fair enough, but having seen lots of these installations over the years, they always seem to have to be repaired again and again and again. if you do insist on keeping the aluminum, think of it like this. if the aluminum is old, downgrade the max current rating. if it has a lot of bends in it or mechanical abrasion to the cabling, downgrade the rating. for every junction box (even when correctly done and airsealed), downgrade the rating. so if you have 220v (measured) at 40amps, that gives you 8800 W, or 8.8kw.... – personal privacy advocate Nov 24 '15 at 18:43
  • continuing...but this is for a perfect situation. there are things like power factor, type of wire, condition of wire, etc to consider. if your voltage is lower, your current will be higher. so given all that, you need to consider safety. your 40amp circuit will just handle your stove in a perfect situation, but you don't have that. if you drop to a 30A breaker, it will not be able to handle the load at all, and will trip when you are using the stove at max load. if you go to new copper, it can use a 40A breaker safely and everything is new, safe and working. – personal privacy advocate Nov 24 '15 at 18:48
  • Downvoted due to lack of evidence to support grand claims about aluminum. – Aloysius Defenestrate Nov 25 '15 at 15:08

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