How many amps on breaker to sub panel?

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Old 02-07-14, 10:16 PM
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How many amps on breaker to sub panel?

Hi,

I had to call an electrician because I had a 200amp main breaker that was failing.

As the electrician was leaving, he said he noticed that the 80amp breaker to my sub panel was on its way out too. He said I could replace that one, and said he recommended upping the amps to about 90 o 100.

How do I know if its safe to up the amps? Would this mean opening up the sub panel and seeing what amp wire goes to it?

The problem is, I can't get an 80 amp in store, and the breaker has just gone. I can get a 90 amp at Lowes so I just need to be 100% sure before I get that.

thanks, James
 
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Old 02-08-14, 04:42 AM
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Who and how did they determine your main breaker was failing? It is highly odd for that to happen, although it can. What size wire is run to your sub panel? You just can't arbitrarily increase the size of breaker. The wire and breaker must be of compatible sizes. Again, how did he determine such a large breaker was "failing"? What brand panel do you have installed?
 
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Old 02-08-14, 08:21 AM
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Its a Siemens, about 22 years old.

I knew the main breaker was failing because it was super hot and smelly, and when the heating came on the smell filled the house. So I flipped it off and called a reputable electrician.

After he had replaced it, he said it had melted the 80amp sub panel breaker that is next to it and he would replace it with 90 / 100amp if he was me, but said I could do that one myself because he was trying to save me an extra callout fee, as he didn't have it in stock
 
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Old 02-08-14, 08:32 AM
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Unless you can determine the size of the wire, wire material, and wiring method, I would just replace it with one with the same rating. 80 amps is not as common as 100 amp breakers sio it is likely an 80 for a reason. If you never had an issue with it tripping there really is no advantage to up the size IMO.
 
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Old 02-08-14, 11:09 AM
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I have installed the 90amp breaker (top-left breaker in the picture) because thats all I could find this weekend, and the electrician had recommended that anyway.

No more lights flickering...
but is it a fire hazard?

The buss was fine but the old 80amp breaker was scorched underneath, so the electrician was right about that needing to be done.

You can see in the picture the wire going in to the 90 amp (formerly 80amp).
There is a red stripe on the top one, but no writing to indicate what the wire gauge is.
I have also opened the 125amp sub panel that it goes to, and there is no info on the wires.

So how can I tell whether that wire can take 90 amps?
i.e. how would the electrician have known?
 
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Old 02-08-14, 11:15 AM
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The red stripe on one of the wires tells me that that cable is aluminum. When you work with aluminum you need to put a no oxidation grease on the connections. It's usually called No-ox. Available almost anywhere electrical supplies are sold.

On that cable, since it appears to be an aluminum service cable, there should be an ID printed on the gray jacket.

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All the connections where the red arrows point should have the No-ox on them.
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Old 02-08-14, 11:15 AM
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The breaker size is based on the wire size. If the wire is too small it can overheat before the breaker trips. The wire size needs to be checked.
 
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Old 02-08-14, 11:28 AM
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Right, the wire was silver in color so it must be Al.
Thanks for the tip about the grease. Only the breaker's bus connection has been lubed so I will re-lube the cables too.

The cable sleeves are actually black, its bad light in the photo. But there's no writing on them at all.

Here is a photo of the wires coming into the sub-panel at the top.
Again - no writing on the sleeves.
 
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Old 02-08-14, 11:32 AM
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The sleeve, or jacket, is not visible inside the panel. The jacket is on the wire outside the panels. Your subpanel looks to be in the wall so you can't see it there. How about at your main panel ?
 
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Old 02-08-14, 11:37 AM
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The wire sizes could be compared against a conductor of a known size if the jacket cannot be seen. Anyone that works in the trade should be able to tell the size.
 
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Old 02-08-14, 07:11 PM
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Thanks for the tip about the grease. Only the breaker's bus connection has been lubed so I will re-lube the cables too.


The intent is not to lubricate anything, the intent is to use oxidation inhibitor compound, many times called grease, to inhibit the aluminum from oxidixing or corroding. No Alox is one name by Ideal, Contax is another name by Blackburn.

Blackburn/Elastimold CTB8 CONTAX Oxide Inhibiting Compound; 8 oz. - Crescent Electric Supply Company
 
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Old 02-08-14, 08:13 PM
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While a good practice there is no requirement to use the No-alox. Also the wires will need to be reterminated since they have already been torqued.
 
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Old 02-08-14, 11:43 PM
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The 90amp siemens breaker came with grease already on the connections.
All the other breakers were fitted by a pro, so I can probably leave them alone.

My bigger concern is that I put the right breaker in
 
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Old 02-09-14, 05:06 AM
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Again, without the breaker being matched to the wire size there is a fire hazard. Rarely is oversized wire run that allows a larger breaker to be installed later.
 
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Old 02-09-14, 07:51 AM
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All the other breakers were fitted by a pro, so I can probably leave them alone
Was that the same alleged pro who replaced a 200 amp main breaker with a 225 amp main breaker. If so anything he said/did is suspect.

If you can't find print on the cable go to the hardware store* and by one foot of #4 THWN, #3THWN, #2THWN. Take them home and strip a couple of inches of insulation off one end of each. Now compare them to the striped wire used in the box.

*Buying at a hardware store instead of a BigBox you may get lucky and they will just give you a piece free off of scrap they have lying around.
 
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