The simple answer to this question is, "Probably not." It might be---but because most electrical panels are NOT installed by homeowners or handy persons, it is actually pretty rare to find a main panel with out a main disconnect. There are a couple of ways where it might "appear" that it is missing---but is in fact there after all. My goal is to keep this post simple enough that even the highly unelectrifed reader can understand what I am talking about. Real estate agents need to understand the basic principles so that when the inspector call for replacement of what appears to be a perfectly good panel, you can understand why.
The first point I will make is that ALL electrical services to the home MUST have a means of shutting off all the power.
Being able to shut off all of the power is usually achieved by a Main Disconnect Breaker in the electrical service panel---and should be labeled as such. (I am not going to talk about fuses in this post). Where it gets a little confusing is when that main disconnect breaker is in a different location from the panel in the home---like outside the home at the electric meter (as in mobile homes, townhouses, condos and other instances). In these instances the panel in the home is not the main service panel but is instead called a sub-panel or remote distribution panel. This type of panel doesn't "require" a main breaker unless it is in a detached structure. Of course it does no harm to have one and it is done sometimes for added convenience.
The following picture is of a pretty close to correctly wired sub-panel. As a Seattle Home Inspector I love finding panels this nicely wired. This type of panel will not usually have a main disconnect---it will be located at the service meter panel (typically).
There is another type of panel that looks, at first glance, like it might be missing a main breaker. This type of panel is configured such that it takes a maximum of 6 throws to shut all the power off.
This type of Main Service Panel is called a Split-Bus Panel.
The following picture is of a typical split bus panel with its dead-front cover in place. Notice it says, "Service Disconnects" in the center between the upper breakers?
This means that when all those top breakers are turned off---all power in the panel will be off---including the lower breakers.
The next picture is of a split bus panel with the cover off---notice how much it looks like the sub-panel picture above (well except for neatness)?
What is different can best be described by showing the picture with descriptive overlays.
The blue dotted lines are where the power coming into the panel attach to the bus bars. Note that the top six double pole breaker spaces are outlined with blue dotted lines and are numbered 1 through 6. Note how the wires from breaker #2 travel down behind the six spaces and attach (trust me) to the bars for the bottom breakers highlighted in red. Notice also that some of the double pole breakers in the area labeled "Service Disconnects" have been changed to single pole breakers---violating the 6 throw rule. This is common with these panels as more circuits are desired and there just isn't room in the panel for any more.
Here is another panel with only three double pole breakers in the top six spaces but one of them has blue wires that run to the bus bars for the lower circuits.
In this case it only takes three throws to shut off all the power in the panel---still well under the 6 throw rule.
These panels were common into the early 70's and I still find them very often. Most panel manufacturers made such panels----and in many different versions. The code allowed for this type of panel as long as all the power could be shut-off in 6 throws or less---known as the "6-Throw Rule." Inspectors sometimes mistake these split-bus panels for sub-panels and incorrectly call for repairs to the way they are wired. I don't want to go into the differences between how main panels are wired differently from sub-panels, but just understand that they are wired VERY differently and it important for the home inspector to know these differences.
In a split bus panel there will be a section for "6" double pole breakers---one of which is the disconnect for all the breakers located below the 6 double pole breaker locations. Wires will run from that one breaker and be connected directly to the bus bars for the other section of breakers. (Bus bars are the energized metal bars that the circuit breakers connect to.)
These panels are often crowded and no longer have ample space to accommodate modern wiring requirements. Most of the time I am recommending to my buyers to upgrade these panels. Often the size of the service itself is adequate for the house---the panel just lacks to space to add new circuits. Replacing just the panel is almost always going to be considerably cheaper than an whole new service to the home.
Charles Buell, Seattle Home Inspector
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WA State, Home Inspector Advisory Licensing Board