Breaker size vs cable Amperage? 2 8GA aluminum wires to a 60A breaker?

Question

I am in the planning stages of installing a new sub panel in my garage.

I'm looking at what's coming in: 4 modern, 8GA aluminum wires. Two hot, one neutral, one ground.

From what I see online, an 8GA aluminum wire can carry about 40A safely. However, I see on my main panel that the dual pole breaker feeding the garage (both hot wires) is rated 60A.

I'm curious about this discrepancy. Does this mean I can actually use up to 60A of current (~80% of 80A maximum) in the garage, or is this breaker too large for the two aluminum wires heading to my garage? I would have thought I can actually use only 30A, even though there are two hot wires coming in.

Thanks.

Answer 1

First of all, you need to understand 120V/single breaker vs. 240V/double breaker. The current is set for each wire. So a 60A double-breaker gets 60A on each wire, for a total of 60A @ 240V or (theoretically, if all balanced) 120A @ 120V. It doesn't let you use "30A wire" and double it to 60A. There seems to be a lot of confusion about this from people who don't understand how the US 240V/120V system works. Almost everything larger than 20A will be a double-breaker, 4 wire (hot, hot, usually but not always neutral, ground) circuit. When we refer to a "30A circuit", that is immediately understood to mean "30A @ 240V, 2 hot". If a circuit is only 240V (water heater) then neutral is not needed. If a circuit is 240V/120V (clothes dryer) then it needs a neutral.

Next we get to wire size. Wire size generally varies based on:

• Type of wire/cable - wire in conduit can often carry more current than wires inside NM cable (a.k.a., Romex)
• Copper vs. aluminum - Aluminum typically needs to be one size larger to carry the same current as copper. In addition, aluminum is (for historical reasons) not usually used for 15A or 20A circuits but is very common for larger circuits and especially for feeders (utility to main panel, main panel to sub panels).
• Temperature rating - The minimum is usually 60 C, higher allowed for 75 C (which generally is allowed for breakers (so panel to panel) and some other devices and for 90 C (much less common).
• Continuous use derate - For continuous use (there are formal definitions - but this includes things like water heaters, EV charging, HVAC), you only get to use 80% of the wire/breaker rating - e.g., if you need to actually use 40A then you have to use 50A rated wire and breaker. But people confuse this sometimes (and there are some exceptions...) to think if they have a 40A actual continuous load needs only 40A wire but a 50A breaker (wrong, and doesn't scale to 60A anyway).
• While breakers are supposed to, usually, be equal to or smaller than the wire size, there is an exception for rounding - e.g., if you have 55A rated 6 AWG NM copper then you can use a 60A breaker because 55A breakers are not available. (but that doesn't apply for 8 AWG aluminum rated for 40A)

Charts are available, such as this one that list allowable current.

8 AWG aluminum gets 40A at 75 C, and 45A at 90 C. Unlikely the terminations are rated for 90 C - even if that was the case the rounding would only get you to 50A. So no matter how you slice it, this is no good.

Bottom line: Unless you replace with larger wires (6 AWG = 50A breaker, 4 AWG = 65A breaker - not available so use 70A breaker, 2 AWG = 90A breaker), you are limited to (a) a 40A breaker and (b) continuous loads should be a maximum of 32A (40 x .8).

Note that the sub panel (in the garage) can have a main breaker that is larger than 40A - even 200A would be fine because it functions just as a shut off switch/disconnect and not to protect the feed wires.

Answer 2

I am guessing this feeder is being recycled from an old branch circuit dedicated to a large motor load. Given that it was abandoned and that size, I would guess an ancient 5 SEER whole-house air conditioner.

Most motors are special. They have onboard overload protection (which has an advantage, as it can sense motor temperature, which is what it's all about). Since the motor provides the overload protection, the breaker amps have some flexibility, and can be increased to prevent nuisance trips. That's how you get a 60A breaker on 40A wire.

Note that NM and TW type wire is only 35A, and they do make 35A breakers so no "round up" rule.

Yes, assuming you have neutral AND ground separately in that cable, you are all set to wire a 120/240V subpanel.

Your setup would be sufficient to run two 120V circuits in the garage along with Power Sharing EV charging for two EVs sharing 30A. That means replenishing 200 miles a night (dynamically according to the cars' needs) assuming the first car arrives say at 7pm and the last car departs at 8am (13 hours to charge).

Answer 3

If it's run in conduit, not as a cable, and lands (typical for breakers) on 75°C rated terminals, 8 AWG Aluminum is rated for 40A. Some types of cable (SE) may also be acceptable at 40A, others (NM) are not.

Whoever connected it to a 60A breaker was violating code, almost certainly. (There are some weird rules around motors and breaker sizes that probably don't apply here.)

You either replace the wires with larger ones, (easy if in large enough conduit) or replace the feed breaker (in the main panel) with a 40A breaker. You are allowed to "plan for" up to 32A of use (the "80% rule".) If they enter the subpanel on a breaker, that breaker can be any size larger than 40A (it's serving the purpose of a "local disconnect" - a 200A breaker is just fine, since the 40A breaker in the main panel protects the wires.)

The breakers on the subpanel do not need to add up to less than 40, or 32 (not possible in the US breaker market.) The 32A load you plan for is from a load calculation, NEC Article 220 load-calculation for entire house and there might easily be 100A of branch breakers hanging off a 40A sub-panel without violating anything. Likewise, if you have limited understanding of the US system, you might not realize that the 40A @ 240V feed equals 80A @ 120V if the branch breakers are correctly balanced between the hot legs.