What Is the Maximum Breaker Size for a Given Cable and Load?

[pyro214]

I'm familiar with the smallest breaker/ cable requirements for a load. Example, a 10A continuous load would require a 10A * 1.25 = 12.5A rated breaker or "the next standard size up". Unfortunately, "the next standard size up" is ambiguous and I'm wondering how large a breaker/cable can be for the load application.

If I have a 5A breaker on a 10A cable, what is the smallest continuous load that can be connected to a 120VAC Power-Supply Unit?

 

[Crazymechanic]

It doesn't matter how thick (aka amperage) the wire is , it matters how many amps your source (wall socket or generator) can supply.Remember smaller cables can have overcurrents if the supply or source is capable of delivering them to the load.
Well not exactly sure about the smallest continuous load though, anyway it can be way smaller than 0.2 A.

Also the weakest part of the circuit will fail first so what's the reason to use a 10A rated cable on a circuit where your breaker is only 5A?

Anyway can you explain what you have in mind in more detail otherwise it is hard to tell what your up to.

Are you trying to run something that has a low current rating but has a high chance of say short circuit is that why you ask the smallest load and the breakers ?

 

[the_emi_guy]

It doesn't matter how thick (aka amperage) the wire is , it matters how many amps your source (wall socket or generator) can supply.Remember smaller cables can have overcurrents if the supply or source is capable of delivering them to the load.
Well not exactly sure about the smallest continuous load though, anyway it can be way smaller than 0.2 A.

Also the weakest part of the circuit will fail first so what's the reason to use a 10A rated cable on a circuit where your breaker is only 5A?

It absolutely matters how thick the wire is. The whole purpose of circuit breakers is to prevent the wires from overheating and burning down the house if an appliance shorts out. This is why we don't use 16AWG on 20A service even though its fusing current is over 100A.

 

[Okefenokee]

"The next standard size up" for a breaker is the next increment of 5. Breakers typically come in sizes like 5, 10, 15, 20. If you calculate that you need a 12.5Amp breaker then go up to the next size of 15. For high current breakers the next size might go from 30Amp to 60Amp depending on Manufacturer.

"The next standard size up" for wire would be a lower gauge number. Are you in the US or Canada? I ask because here we use AWG (American wire guage) as the standard. Each gauge of wire has a certain ampacity (safe maximum of current). Everybody uses the NEC (National Electrical Code) table for ampacity. Here's a short list.

smaller sizes
10 guage
8 guage
6 guage
4 guage
2 guage
1 guage
1/0 (1 ought)
2/0 (2 ought)
3/0 (3 ought)
4/0 (4 ought)
250 KCMIL
300 KCMIL
350 KCMIL
400 KCMIL
biggest

By the way, it's pretty common to use oversized wire with a breaker. Sometimes you do it to eliminate voltage drops if the wire is going to be especially long. Much of the time its because a company got a really good deal when they bough 5,000 feet of 10 gauge wire.

As long as the breaker is bigger than the the load and the wire ampacity is bigger than the breaker you have no worries.

 

[Averagesupernova]

By the way, it's pretty common to use oversized wire with a breaker. Sometimes you do it to eliminate voltage drops if the wire is going to be especially long. Much of the time its because a company got a really good deal when they bough 5,000 feet of 10 gauge wire.

As long as the breaker is bigger than the the load and the wire ampacity is bigger than the breaker you have no worries.

I would not agree with this at all. When running wire a considerable distance it may be necessary to oversize the wire to reduce voltage drop but to run #10 instead of #14 in a residential lighting circuit for instance is just plain stupidity. One of the things that determines the number or wires allowed in a box is the size of the wire. So by oversizing you are complicating things on a number of levels. Working with #10 wires is a lot more difficult than #14. Imagine a homeowner trying to replace a light fixture dealing with wire more suitable to run a central air conditioner unit on a 30 amp circuit than dealing with #14 wire on a standard 15 amp lighting circuit.

 

[Crazymechanic]

@the emi guy

I said it doesn't matter because I thought the OP was talking about some specific supply which then goes down a wire to the load and has a breaker in series.
And even if that would be a house wiring circuit , as long as the wire is withing limits and not too think the breaker will do it's part anyway , I think a breaker should break the line way way before the wires actually burn down because once the isolation has started to burn the breaker can break as much as it wants the house fire is already there.

Even though your answer was good as well.

 

[Okefenokee]

Whether you agree with it or not you will see oversized wires. I used to work for a company that did electrical IR studies.

It doesn't violate any codes and conduits are usually laid out with plenty of fill space left over so that they don't have to do much derating.

I just figure that electricians must sometimes have extra #12 wire laying around their work truck when all they needed was #14.

An electrical company's bidder once told me that they had a weird customer who insisted on doing his whole house in #12. I bet it was expensive but it wasn't against code.

 

[Averagesupernova]

Whether you agree with it or not you will see oversized wires. I used to work for a company that did electrical IR studies.

It doesn't violate any codes and conduits are usually laid out with plenty of fill space left over so that they don't have to do much derating.

I just figure that electricians must sometimes have extra #12 wire laying around their work truck when all they needed was #14.

An electrical company's bidder once told me that they had a weird customer who insisted on doing his whole house in #12. I bet it was expensive but it wasn't against code.

It should be against code to do something like that. Here is the reason: When a house is wired, you can oversize the wires but the end fixtures (lighting for instance) will still be required to protected with a 15 amp breaker. Now we have ALL number 12 wires entering the service panel. If the panel is replaced or circuits added it is too easy to accidentally oversize the breaker based on the wire size. It is certainly understandable that oversizing will be required on long runs but this would hardly be the case in a residence. It is bad enough that I see 15 amp receptacles on 20 amp kitchen circuits ALL THE TIME.

 

[Wolfpack83]

Absolutely nothing wrong with 15 amp duplex receptacles on 20 amp circuits. You are thinking about a single 15 amp receptacle on a 20 amp circuit which is not allowed.

 

[psparky]

I'm familiar with the smallest breaker/ cable requirements for a load. Example, a 10A continuous load would require a 10A * 1.25 = 12.5A rated breaker or "the next standard size up". Unfortunately, "the next standard size up" is ambiguous and I'm wondering how large a breaker/cable can be for the load application.

If I have a 5A breaker on a 10A cable, what is the smallest continuous load that can be connected to a 120VAC Power-Supply Unit?

You generally ask for the highest continuous load...not the smallest. The smallest continuous load woiuld be an open switch. Or if it has to be a device, how bout an LED? It has super small load, but not sure how that helps with figuring any wire sizes or breaker size.

You first size the breaker to the load (how many full load amps). Then size the wire to code by the breaker size. If there is a fuse, you would first size the fuse to the load, then the breaker, then the wire size. If it's a big breaker out of a factory substation, you would then need to set all the trip settings. For example, you want the smaller breakers to trip before the main breaker in substation. Also, the breakers will need to be set NOT to trip during first few cycles of in-rush current.

 

[Averagesupernova]

You generally ask for the highest continuous load...not the smallest. The smallest continuous load woiuld be an open switch. Or if it has to be a device, how bout an LED? It has super small load, but not sure how that helps with figuring any wire sizes or breaker size.

You first size the breaker to the load (how many full load amps). Then size the wire to code by the breaker size. If there is a fuse, you would first size the fuse to the load, then the breaker, then the wire size. If it's a big breaker out of a factory substation, you would then need to set all the trip settings. For example, you want the smaller breakers to trip before the main breaker in substation. Also, the breakers will need to be set NOT to trip during first few cycles of in-rush current.

I think you missed the point of the question on 5 amp breaker with 10 amp wire. Pyro asked a question, maybe in a non-conventional context, about how large of a continuous load can go on an existing breaker/feedwire combination. The rule is 80% of the breaker size. As far as calculating the breaker and wire size for a given load, it isn't always as simple as the above 80% rule and then size the wire to the breaker. Here's why: Consider a continuous load that is a considerable distance away from the source. This alone will require a size or two increase in wire size. If the equipment has suitable overcurrent protection internally or at the disconnect then there is no reason not to increase the size of the breaker at the head end as well. Sub-panels are put in all the time that will not likely ever come to 80% of the rating of the panel.

 

[psparky]

I think you missed the point of the question on 5 amp breaker with 10 amp wire. Pyro asked a question, maybe in a non-conventional context, about how large of a continuous load can go on an existing breaker/feedwire combination. The rule is 80% of the breaker size. As far as calculating the breaker and wire size for a given load, it isn't always as simple as the above 80% rule and then size the wire to the breaker. Here's why: Consider a continuous load that is a considerable distance away from the source. This alone will require a size or two increase in wire size. If the equipment has suitable overcurrent protection internally or at the disconnect then there is no reason not to increase the size of the breaker at the head end as well. Sub-panels are put in all the time that will not likely ever come to 80% of the rating of the panel.

Yes, the question was asked in an unorthodox manner.

Sure we need to consider voltage drop over long runs. Let's not forget D-rating with multiple cables in a conduit. May need to up the wire size for that as well in addition to the voltage drop increase in cable size.