Do circuit breakers in series need to be the same amperage?

[Slowbuilder]

I just had my old Challenger 200a main panel on my house replaced with a new Eaton. While the electrician was here, he was looking at the sub-panel in my shop. The sub-panel is a 100a panel with a 100a service disconnect. It is fed by a 4-wire underground run from a 60a breaker in my (new) main panel, in which all conductors are #6 CU. The shop sub panel is about 50' line of sight from the house main panel, so it is likely about a 70-75' run.

The electrician recommended that I replace the 100a disconnect breaker in the shop sub-panel with a 60a breaker. He said that the two breakers "really should be the same. It is safer.".

Is this required by code? A best practice? An old wives tale?

If it is required, can someone explain why? It seems like the 60a breaker in the main panel will protect the #6cu feed, and will trip when the current draw is above 60a, thus the 100a breaker in the sub-panel will never see anything close to 100a, and will never trip. It will really be just a service disconnect.

I can easily swap the breaker, and it's not a huge expense, but why?

 

[alfredeneuman]

In this case the 100A breaker in the panel is just the required disconnect and is to Code
It still is protected by the 60A breaker in the main panel.

 

[Innovate1]

No reason to swap this. The electrician should know better. The 60A breaker in the main panel provides the over current protection. The main breaker in the subpanel could just be a switch - it just performs as a switch. This isn't a best practice or required by code. I would say it's better classified as an old wives tale. It's perfectly fine the way it is. Your understanding of things is exactly correct.

 

[wyliesdiesels]

Time to get a new electrician. Doesnt know what he’s talking about... the 100a breaker in the sub is not protecting the wire, the breaker in the main is. No need to do what he proposes...

 

[nadogail]

I will bet that the Electrician just happens to have a 60 Amp breaker and will be willing to sll it to you and make the change out for a "Nominal Fee"

 

[The Cobbler]

yeah, I would like to know, "How it's safer"?
either he wanted your breaker or an up-sell

 

[bad_idea]

I'd go with a solid 'FTG'. He doesn't know what he's talking about.

 

[theoldwizard1]

Time to get a new electrician. Doesnt know what he’s talking about... the 100a breaker in the sub is not protecting the wire, the breaker in the main is. No need to do what he proposes...

The only thing you would save is a walk to the house to reset the breaker there if you over load. The 100A in the sub is just a disconnect.

 

[alfredeneuman]

The only thing you would save is a walk to the house to reset the breaker there if you over load.

Not necessarily. If both the breakers are the same rating it's a toss up which one would trip first.

 

[fitter30]

Note by breaker 60 amp supply just for the next person as a reminder

 

[rburke65]

As said, the 100A breaker is just there as a disconnect. He is either confused or …..IDK

 

[Innovate1]

Not necessarily. If both the breakers are the same rating it's a toss up which one would trip first.

And for large overloads like direct shorts it may trip them both. Might even trip them both if they are slightly different current ratings. Not common but it can happen.

 

[nadogail]

A circuit breaker or fuse (in a series circuit will) in theory, trip or blow when the lowest rated device is overloaded and shut the whole circuit down.

 

[Innovate1]

A circuit breaker or fuse (in a series circuit will) in theory, trip or blow when the lowest rated device is overloaded and shut the whole circuit down.

Theory and reality are only theoretically related. For direct shorts the instantaneous current can be hundreds of amps which can blow multiple in series fuses or breakers. Have seen it happen more than once.

 

[rlitman]

And for large overloads like direct shorts it may trip them both. Might even trip them both if they are slightly different current ratings. Not common but it can happen.

A real world example I dealt with had a 20A breaker trip, a 30A upstream of it trip and a 225A upstream of that trip, all simultaneously. The minimum short circuit current required to trip the 225A breaker on the "instantaneous" magnetic was around 900A IIRC.

Theory and reality are only theoretically related. For direct shorts the instantaneous current can be hundreds of amps which can blow multiple in series fuses or breakers. Have seen it happen more than once.

Agreed. The point here is that the current flow in a short is the same everywhere along the entire circuit, and if the circuit has a low enough impedance, the short circuit current may be high enough to trip upstream breakers. Longer wire lengths though tend to reduce residential short circuit currents to the point that this rarely happens.

Fuses can be designed to open faster than breakers, and high current breakers are often computer controlled to hold off a few milliseconds for downstream breakers to clear a fault. Breaker coordination is its own science.

 

[ycgoat]

I am not trying to research this, especially for residential applications, but there are rules in the NFPA and IBC/IRC codes that address coordination of breakers. These mainly apply to KAIC ratings of the breakers more so than the trip setting. Then there is the UL rating of the breaker which is likely not listed as a switch/disconnect.

can you leave it as is without great fear- YES
Would I leave it or recommend you leave it - No (especially for a $20 breaker)

 

[rlitman]

...Then there is the UL rating of the breaker which is likely not listed as a switch/disconnect...

Actually, it almost certainly is. Breakers not listed as switches or disconnects have been obsolete back since before HACR was a thing.

 

[TractorJeff]

ycgoat..... I doubt the Electrician will sell him that 60amp Breaker for $20. Last one I priced was around $37.

 

[Innovate1]

ycgoat..... I doubt the Electrician will sell him that 60amp Breaker for $20. Last one I priced was around $37.

Plus a "small" installation fee. Absolutely no reason to do anything. It's perfectly fine as is.

 

[dscheidt]

A real world example I dealt with had a 20A breaker trip, a 30A upstream of it trip and a 225A upstream of that trip, all simultaneously. The minimum short circuit current required to trip the 225A breaker on the "instantaneous" magnetic was around 900A IIRC.

I was working on a farm, up on a ladder drilling holes in a beam. halfway through drilling a hole, everything goes dark. guy I was with reset the circuit breaker and the main breaker in the sub in the building we were in, but nothing came back. We spent some time finding someone who could open the building with the main panel in it, and reset the breaker there. I went back to finish the hole; same thing happened as soon as I pulled the trigger. They took the drill away after that, so I never got to look to see what was going on.

 

[rlitman]

I was working on a farm, up on a ladder drilling holes in a beam. halfway through drilling a hole, everything goes dark. guy I was with reset the circuit breaker and the main breaker in the sub in the building we were in, but nothing came back. We spent some time finding someone who could open the building with the main panel in it, and reset the breaker there. I went back to finish the hole; same thing happened as soon as I pulled the trigger. They took the drill away after that, so I never got to look to see what was going on.

It's a times like this that you want a smaller gauge extension cord. Then again, I suspect there was something wrong with that main too.

 

[FredWanaker]

my only thought is that someone opens the sub panel and sees a 100 amp breaker. They assume they have up to 100 amps available so they install a 50 amp circuit off that panel and hook up a device that runs 35 amps. Then they add a 30 amp breaker and a device that pulls 20 amps. Now the circuit is pulling 55 amps continously on a 60 amp circuit. One day they plug in a 12 amp drill off a circuit that panel and the 60 amp pops. With all the work they have done they just swap out the 60 to 100, say "what idiot put in the 60 on a 100 amp circuit, probably only had a 60 that day," and melt the wires the next 20 amp device they add taking the circuit to a continuous 75 amps. People think that way even if something is clearly labeled do not exceed 60 amps on this panel.

 

[mike93lx]

my only thought is that someone opens the sub panel and sees a 100 amp breaker. They assume they have up to 100 amps available so they install a 50 amp circuit off that panel and hook up a device that runs 35 amps. Then they add a 30 amp breaker and a device that pulls 20 amps. Now the circuit is pulling 55 amps continously on a 60 amp circuit. One day they plug in a 12 amp drill off a circuit that panel and the 60 amp pops. With all the work they have done they just swap out the 60 to 100, say "what idiot put in the 60 on a 100 amp circuit, probably only had a 60 that day," and melt the wires the next 20 amp device they add taking the circuit to a continuous 75 amps. People think that way even if something is clearly labeled do not exceed 60 amps on this panel.

Then that is on them for not knowing what they are doing.

Worrying about a dumb next guy is his problem, not yours

 

[FredWanaker]

Then that is on them for not knowing what they are doing.

Worrying about a dumb next guy is his problem, not yours

I guess if you are a stuck on yourself kinda guy who doesn't see a $20 investment as helping someone down the line from hurting themselves or someone else then that statement would hold true. For me, I'd rather pay it ahead and save myself the karma of someone shitting on me cause I **** on someone else.

 

[mike93lx]

I guess if you are a stuck on yourself kinda guy who doesn't see a $20 investment as helping someone down the line from hurting themselves or someone else then that statement would hold true. For me, I'd rather pay it ahead and save myself the karma of someone shitting on me cause I **** on someone else.

Doing the right thing and worrying about what someone down the line may do out of their own ignorance are not the same thing.

You can spend all kinds of money thinking it may idiot proof something. It may help, it may not

 

[wyliesdiesels]

I guess if you are a stuck on yourself kinda guy who doesn't see a $20 investment as helping someone down the line from hurting themselves or someone else then that statement would hold true. For me, I'd rather pay it ahead and save myself the karma of someone shitting on me cause I **** on someone else.

with that logic, why not idiot proof everything and spend gobs of money doing so?

 

[FredWanaker]

I

Doing the right thing and worrying about what someone down the line may do out of their own ignorance are not the same thing.

You can spend all kinds of money thinking it may idiot proof something. It may help, it may not

We've been in this house for 32 years, often I find things I did 20 - 32 years ago and haven't a clue why or how I did that something back then, but because I always follow best practices I can usually guess why I did something one way or another. Do you remember the size and type wire you put in a specific conduit 25 years ago at a specific location? Would you have wired a 100 amp sub-panel for a 60 amp breaker, or would you have wired it for the 100 amp breaker in it 25 years ago? Think hard, which way did you wire it?

Give an example of how things can get messed up. I added power in a shed 25 years ago. I did not add ground rods because the permitting folks here and their code didn't require it. The inspector didn't require it. The subpanel instuctions did not require it under these conditions. BUT, I once got royally flamed in this forum or another electrical forum because someone noticed there was no grounding rod in a photo and everyone jumped me on it so I followed the advise and added grounding rods. Recently had another inspection here for something else - what did the inspector say when he walked by the shed and saw the new shiny rods and wires? "Why did you do that, we don't require it here and prefer it not be done because we have so little lightning but I won't write you up on it."

 

[mike93lx]

Do you remember the size and type wire you put in a specific conduit 25 years ago at a specific location? Would you have wired a 100 amp sub-panel for a 60 amp breaker, or would you have wired it for the 100 amp breaker in it 25 years ago? Think hard, which way did you wire it?

Remembering doesn't matter. Look at the insulation and measure the wire if needed.

I don't need to think hard. When I materially change something, I veryfy what I am working with.

 

[Terry D]

my only thought is that someone opens the sub panel and sees a 100 amp breaker. They assume they have up to 100 amps available so they install a 50 amp circuit off that panel and hook up a device that runs 35 amps. Then they add a 30 amp breaker and a device that pulls 20 amps. Now the circuit is pulling 55 amps continously on a 60 amp circuit. One day they plug in a 12 amp drill off a circuit that panel and the 60 amp pops. With all the work they have done they just swap out the 60 to 100, say "what idiot put in the 60 on a 100 amp circuit, probably only had a 60 that day," and melt the wires the next 20 amp device they add taking the circuit to a continuous 75 amps. People think that way even if something is clearly labeled do not exceed 60 amps on this panel.

People should be educated in electric before adding circuits to a panel just because it has space. Sounds to me that the people you describe have no business being in that panel.

 

[alfredeneuman]

Do you remember the size and type wire you put in a specific conduit 25 years ago at a specific location?

Yes.
I had enough #2 Cu THWN left over from another job to do a 100A service change (a waste of money but I already had it). It had an 1-1/2" Rigid mast that was 16' long (2 story). It was in Santa Ana, CA.
....but that was only 22 years ago.

 

[The Cobbler]

my only thought is that someone opens the sub panel and sees a 100 amp breaker. They assume they have up to 100 amps available so they install a 50 amp circuit off that panel and hook up a device that runs 35 amps. Then they add a 30 amp breaker and a device that pulls 20 amps. Now the circuit is pulling 55 amps continously on a 60 amp circuit. One day they plug in a 12 amp drill off a circuit that panel and the 60 amp pops. With all the work they have done they just swap out the 60 to 100, say "what idiot put in the 60 on a 100 amp circuit, probably only had a 60 that day," and melt the wires the next 20 amp device they add taking the circuit to a continuous 75 amps. People think that way even if something is clearly labeled do not exceed 60 amps on this panel.

you know what they say about "assume"

 

[EngineerNate]

If you open up a panel and hook 6awg copper up to a 100A breaker you're bringing whatever bad happens next on yourself.

Anyone who's that ignorant or incompetent shouldn't be allowed to work on anything upstream of a 15A socket.

 

[Slowbuilder]

Wow, thanks for all the responses and insights. I'm not sure I'll spend $37 on a 60a breaker, but a label that says "Max load not to exceed 60a" might be a indication to future me (or someone else) to actually look at the feeder wires... even though I know I should (today).

 

[Innovate1]

my only thought is that someone opens the sub panel and sees a 100 amp breaker. They assume they have up to 100 amps available so they install a 50 amp circuit off that panel and hook up a device that runs 35 amps. Then they add a 30 amp breaker and a device that pulls 20 amps. Now the circuit is pulling 55 amps continously on a 60 amp circuit. One day they plug in a 12 amp drill off a circuit that panel and the 60 amp pops. With all the work they have done they just swap out the 60 to 100, say "what idiot put in the 60 on a 100 amp circuit, probably only had a 60 that day," and melt the wires the next 20 amp device they add taking the circuit to a continuous 75 amps. People think that way even if something is clearly labeled do not exceed 60 amps on this panel.

And people used to put a penny under screw in fuses. Doesn't mean I need to somehow try to prevent that. Trouble is people who don't know what they are doing are sometimes very creative. A label about the feed rating is not really needed but a plus if you want to do it. But you say some people will ignore that label too so I think that leaves us at the point of people messing around with wires and changing breakers in a panel should know what they are doing.

 

[Innovate1]

Choices in panels available with 60A mains are pretty limited and have a small number of breaker spaces. So even if you wanted to match the subpanel main with the feed breaker rating and wanted a decent number of spaces you would be stuck with back feeding a breaker as the main or changing out the main breaker in a larger panel (a lot more than $20) or maybe special ordering one. Seems like a lot of trouble to go through to fix a non-problem. In the OP's case it sounds like the subpanel main is a backfed breaker since he said he could swap it without too much cost but I didn't see it directly stated.

 

[TTTTTT]

After reading this entire thread I was thinking like everyone else, just keep it as you have it now. If your wire is 6cu, sure your 60 Amp breaker will protect everything. But in reverse what if someone sees the 100 Amp in the garage and thinks they have 2cu going out and replaces 60amp house with 100 Amp breaker. Then you're screwed. Of course switching house breakers, you should see the difference that there is only 6cu not 2cu when switching but **** happens. On this note I see your electricians concern.

 

[Terry D]

After reading this entire thread I was thinking like everyone else, just keep it as you have it now. If your wire is 6cu, sure your 60 Amp breaker will protect everything. But in reverse what if someone sees the 100 Amp in the garage and thinks they have 2cu going out and replaces 60amp house with 100 Amp breaker. Then you're screwed. Of course switching house breakers, you should see the difference that there is only 6cu not 2cu when switching but **** happens. On this note I see your electricians concern.

You could change the breaker size to a larger one on every branch circuit in your panel. This is why, only experienced people should be in a panel.

 

[EngineerNate]

As has been said several times. If someone after you comes in and does a bunch of boneheaded stuff there's nothing you can do to stop them.

Anyone who puts a 100A breaker on 6 awg copper is incompetent or ignorant enough to do any number of dangerous things that might kill them or burn the house down no matter what you do.

 

[alfredeneuman]

Anyone who puts a 100A breaker on 6 awg copper is incompetent or ignorant enough to do any number of dangerous things that might kill them or burn the house down no matter what you do.

Anyone that says that a 100A breaker on 6 awg isn't allowed is incompetent and ignorant of motors, A/C compressors and welders.

 

[EngineerNate]

Anyone that says that a 100A breaker on 6 awg isn't allowed is incompetent and ignorant of motors, A/C compressors and welders.

Show me in the code where that applies to 6awg feeding a subpanel.

If you're changing breaker sizes feeding a general use subpanel based on special use ratings... I don't know what to tell you.

We can lawyer the off the cuff phrasing I used there all night but the point stands.