# How can I measure the actual amperage rating of my circuits so I can match it with correct fuses/breakers?

My house has a fuse panel instead of a circuit breaker panel, and as such it's way easier for the wrong ones to be installed. I don't trust that the previous tenant had the right ones in the right sockets. Is there a way I can measure the actual capability of each circuit so I can be sure I'm installing the right amperages?

• All good advice below, though the easy/safe thing is to install 15A fuses everywhere. If you blow any, you can study those circuits to determine actual wire size. Commented Sep 27, 2022 at 14:17
• The size of the wire is the main determiner of the amperage of the circuit. Commented Sep 27, 2022 at 19:21
• The rating on the insulation is the main determiner. #12 THHN is a "90°C rated conductor" up to 30a; it's that just no one likes that. Commented Sep 28, 2022 at 1:00
• The OP's question shows they do not understand the fuse rating relative to the outlet's circuit. As @HotLicks mentioned, it is entirely about the wire size. Over amperage on a wire means it turns hot and can start fires. It is entirely possible to have 10A or even 5A circuits, and it has little to do with what you plug into the circuit. Commented Sep 28, 2022 at 9:24
• You don't say where you are in the world or what type of fuse board you have. Some types of fuse board have (not necessarily obvious) indicators of the fuse or fuse wire sizes you should use. If you post a picture, someone might know. Some of the answers/comments assume you are in the US (as this is a US-focussed site). Commented Sep 28, 2022 at 9:49

... the previous tenant ...

This indicates to me that this is a rental property. Call the landlord and ask him to review to be sure that his property doesn't burn down.

Sure, you can do simple work in a rental without being a licensed electrician - replacing a blown fuse should be one of those things, but 99.9% of work in a rental property cannot (in the US) be done by the tenant or the property owner. You're not allowed to put someone else's life or property at risk.

It should be up to the owner to either:

• know what fuses should go where because he's familiar with the property and knows how it should be set up, or
• get a licensed electrician to figure it out to protect his property and your life.

Fuses are color-coded same as breakers used to be.

• Blue = 15A (#14 wire)
• Red = 20A (#12 wire)
• Green = 30A (#10 wire)

You might take appropriate hobby paint or nail polish and mark the area around each fuse socket with the appropriate color once you have identified the sizes of the wires in the circuit. If a circuit has mixed wire sizes, the smallest size controls.

The breakers are to protect the wiring, not the load. You can generally size by the wire size example for 60C rated wire: AWG 14=15 amps, AWG 12 = 20 Amps, AWG 10 = 30 Amps. It is dependent also on the type of wire. This link will give you the appropriate table. https://www.usbreaker.com/docs/UL489_US-Breaker_Wire_Size_Chart.pdf

The easiest way to size would get an experienced electrician, that person will know the wire size by looking at it. Also they make cheap gauges to check the gauge, many times they are giveaways from some vendors. You need to size the breaker to the smallest wire (highest gauge number) in the circuit so if there is AWG 14 you are limited to 15A. Conversely if you are not comfortable playing a 20 amp breaker (AWG 12 wire) on a circuit a 15 amp is ok. It would be a good idea to note why.

• Understood, but is there a way to electrically measure what the wires are without having to dig out every one of them and put a pair of calipers to each? Commented Sep 27, 2022 at 8:46
• @Atario you could with an amp clamp style digital multimeter (DMM) but you'd have to get into the panel and see if you even had room to get the clamp around the circuit. But even that is a NON STARTER. In almost all certainty the circuit wouldn't be running at fused capacity giving very misleading readings, for example, a circuit with a few lights turned on might only draw 1 -2 amps, yet be fused for 15 amps. All you can do is determine the wire sizes on each fuse and see if it has the right sized fuse. Like Gil said, fuses (and breakers) are there to protect the wiring, not the load. Commented Sep 27, 2022 at 10:42
• I'm going to give an additional "gotcha" scenario. Wires at the panel != the wires throughout a circuit. My house had a 20A breaker with 12 AWG running to it, but there are two sections of 14 AWG on that circuit. Sizing according to what you see at the breaker is not a guarantee by any means. Commented Sep 27, 2022 at 15:58
• There's no practical way to measure the current capacity of a circuit. Commented Sep 27, 2022 at 19:23
• @Logarr: Very true. But if the fuse is too big even for the wires at the panel, then something is definitely wrong - so you can't confirm that it's OK, but you can definitely spot some problems :) Commented Sep 28, 2022 at 10:02