100 amp or 200 amp fused service panel

[Sam Morris]

Just checking to be sure, I pulled these fuses out of a Square -D fuse type electrical panel. Would this be a 100 amp or 200 amp fuse panel ?? Im saying 100 amp ??

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[Jerry Peck]

Just checking to be sure, I pulled these fuses out of a Square -D fuse type electrical panel. Would this be a 100 amp or 200 amp fuse panel ?? Im saying 100 amp ??

To clarify some terminology: The rating of the fuses in the main service disconnect is 100 amp for each leg of the service, which makes for a 100 amp rating for the main service disconnect ... which is your question.

However, that does not address the actual question of: "Would this be a 100 amp or 200 amp fuse panel ??"

The rating of the panel is unknown. You would need to look at the label on the panel to find out what the rating of the panel is.

The rating of a service is the lesser of: the service entrance conductor rating; the main service disconnect rating; the panel rating.

And ... the requirements are that: the panel rating be the same as, or higher rated than, the service disconnect rating (fuses ratings); AND that the service entrance conductor ratings be the same as, or higher rated than, the service disconnect's overcurrent rating (fuse ratings).

The overcurrent device ratings are required to be the lowest rated of those components.

[david shapiro]

I think Jerry's point about the feeder and bus rating, while quite accurate, is almost certainly irrelevant to this situation: at least the main looks OEM and thus, as was pointed out, the bus would be suitable. The firehose analogy is fine, but the point really is that the 100 A breaker is unlikely to serve as an overcurrent device, but is quite all right as a local disconnect.

There is, however, plenty wrong with the installation. Given how many cables seem to not be secured within a foot of the panel, and possibly the thermosplastic-insulated multiwire circuits without handle ties (a requirement only since local adoption of the 1981 NEC), it has the appearance to my weak eyes of having a history of unpermited or not seriously inspected circuit additions. I'd want to check not just the feeder but all the branch circuits for overfusing.

And missing a cover? I'd be concerned about possible contamination of bus and breakers. If a licensed electrical contractor took on the task of fixing what's wrong, it would be a major job--starting with creating an accurate circuit directory (think spreadsheet) for his or her replacement for the subpanel.

[Jerry Peck]

I think Jerry's point about the feeder and bus rating, while quite accurate, is almost certainly irrelevant to this situation: at least the main looks OEM and thus, as was pointed out, the bus would be suitable.

My reason for pointing out the rating of the main bus with what looks like an OEM breaker was, and still is, that the label needs to be looked at for information.

The label gives a lot of information, and the more labels one reads, the more comfortable one gets reading the labels and finding the information one wants.

Such as main bus rating; which terminal bars are suitable for neutral and ground or neutral only; breaker tab rating (not usually an issue for 70 amp and under branch or feeder breakers, but could be for 100 amp breakers); location of ground bonding screw or jumper when needed, and where it is when it's not supposed to be there; etc.

Looking at an electrical panel label can be intimidating the first few times, then they get easier to read and find what one is looking for (don't need to read it in the same detail the electrician is reading it - don't need to know torque for the various terminals, etc.

[david shapiro]

My reason for pointing out the rating of the main bus with what looks like an OEM breaker was, and still is, that the label needs to be looked at for information.

The label gives a lot of information, and the more labels one reads, the more comfortable one gets reading the labels and finding the information one wants.

I quite agree, Jerry, especially in this case where there's a disccnnect ahead of the panel. On a consult last week, I noticed a tandem/piggyback CB in a 42-space QO panel. you know and I know it's not kosher. But I also showed the customer a photo of the schematic, indicating that it was listed only for use with one CB per space.

Now the caveat. With crowded service panels, you may need to move wires around in order to get a view of the label and schematic. I've been a master sparky for 40 years and even I don't like shoving conductors around in order to take a peek. Unfortunately, I may need to do so in order to get the basic information so I can contact the manufacturer--presuming they're still in business.

- - - Updated - - -

My reason for pointing out the rating of the main bus with what looks like an OEM breaker was, and still is, that the label needs to be looked at for information.

The label gives a lot of information, and the more labels one reads, the more comfortable one gets reading the labels and finding the information one wants.

I quite agree, Jerry, especially in this case where there's a disccnnect ahead of the panel. On a consult last week, I noticed a tandem/piggyback CB in a 42-space QO panel. you know and I know it's not kosher. But I also showed the customer a photo of the schematic, indicating that it was listed only for use with one CB per space.

Now the caveat. With crowded service panels, you may need to move wires around in order to get a view of the label and schematic. I've been a master sparky for 40 years and even I don't like shoving conductors around in order to take a peek. Unfortunately, I may need to do so in order to get the basic information so I can contact the manufacturer--presuming they're still in business.

[David Bertrams]

AND that the service entrance conductor ratings be the same as, or higher rated than, the service disconnect's overcurrent rating (fuse ratings).

Assuming the application is residential you might decide to apply the code and multiply the service disconnect overcurrent rating by .87 to determine the service entrance conductor rating.

[ROBERT YOUNG]

Just checking to be sure, I pulled these fuses out of a Square -D fuse type electrical panel. Would this be a 100 amp or 200 amp fuse panel ?? Im saying 100 amp ??

Sam. The 'main disconnect' is fused 100 amps.
Don't double the ampacity due to both legs being fused with cartridge fuses listed at 100 amp max.

Typically/usually the panel label, on the back of the enclosure front, lists enclosure ampacity.
Any service panel/enclosure has a maximum ampacity rating.

Don't get losts in the narratives Jerry and David are clarifying. This can go on for some time. No offence intended gentlemen. You will catch on the longer you stay as I did.