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Fuse Block Questions


Mark P
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Main panel with fuse block. The main fuse block on the left has 1 60amp and 1 40 amp. The right block for the stove has 1 35amp and 1 40 amp. The panel is rated for 100 amps and the main wire appeared to be at least #4 tinned-copper. Now I know I’ve read you do not add the fuses together – so in this case a 60+40 do not = 100 amps, but I can’t find it in my jumble of a library and need someone to confirm I have not lost my mind, but it is possible. Should not the fuses inside the blocks be the same size?

Thanks

Mark

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Originally posted by AHIS

Main panel with fuse block. The main fuse block on the left has 1 60amp and 1 40 amp. The right block for the stove has 1 35amp and 1 40 amp. The panel is rated for 100 amps and the main wire appeared to be at least #4 tinned-copper. Now I know I’ve read you do not add the fuses together – so in this case a 60+40 do not = 100 amps, but I can’t find it in my jumble of a library and need someone to confirm I have not lost my mind, but it is possible. Should not the fuses inside the blocks be the same size?

Thanks

Mark

The main block on the left should contain two 60-amp fuses. The range block on the right should contain two 40-amp fuses.

There's no real harm in the situation you've found though. In the worst case, the range block's 35-amp fuse might blow when all of the burners and the bake element were on at the same time. That would be a nuisance but not unsafe.

The same is true for the main block. If the load on the entire panel exceeded 40 amps for a while, the 40-amp fuse would blow.

I'd just tell them to install the proper fuses to avoid nuisance trips. (Presuming there are no other problems in the panel, of course.)

- Jim Katen, Oregon

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Thanks for verifying that for me, Jim. Now whenever I come across anything less than 100 amp service I recommend it be upgraded. In this case, it seems possible that 100 amp fuses be installed in the main block, but I’m hesitant to recommend this as there may be other factors I’m not aware of. I’ll report it as 60 amp service, explain that this is sub-standard in today’s world and that a qualified electrician will need to evaluate and upgrade the system.

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Why are you reporting it as a 60-amp service when the service entrance conductors are sized for a 100 amp service?

Look at the rating on the box. If it's rated as a 100 amp panel, and has a #4 copper cable coming into it, it's a 100 amp service. The 60 amp block is only protecting part of what's in the panel. If the box is rated for 60 amps and you've got a #4 copper, than it's a 60 amp service with an improperly sized SEC coming into it from the meter.

Service size is more than just breaker size in the panel.

By the way, that's an obsolete panelboard. You can report it that way and recommended an upgrade based on that alone and I don't think any electrician today would argue with you.

ONE TEAM - ONE FIGHT!!!

Mike

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The reason I plan on reporting it as 60 amp is I go with the smallest rating between panel, service conductor, and fuse/breaker. In this case 60 amp fuse is the limiting factor. The panel is rated 100 w/ what appears to be #4 tinned copper, so I understand those two aspects make it 100amp, but unless the fuses are changed out it is 60amps. At least that is what I believe now, I’m sure more knowledgeable HI will being jumping in this thread to help continue my education, because that is what makes TIJ such a great site.

Speaking of education - you mentioned the panel board is obsolete. I agree, basically because it is old - but I have struggled with making this type comment to my customers because I am not able to explain what exactly is obsolete about it. Not using this box as an example, but if the fuse box has adequate amperage, copper wire w/ intact insulation, is grounded, not over filled, no other significant issues What do I say is obsolete about it? What criteria do I use to determine if a panel is obsolete?

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Originally posted by AHIS

The reason I plan on reporting it as 60 amp is I go with the smallest rating between panel, service conductor, and fuse/breaker. In this case 60 amp fuse is the limiting factor. The panel is rated 100 w/ what appears to be #4 tinned copper, so I understand those two aspects make it 100amp, but unless the fuses are changed out it is 60amps. At least that is what I believe now, I’m sure more knowledgeable HI will being jumping in this thread to help continue my education, because that is what makes TIJ such a great site.

Mike's right. That's a 100-amp fuse panel and the main fuse block is supposed to have only 60-amp fuses installed in it. The range block is not fed through the main block. So, in this particular case, the fuses do, indeed, add up to 100 amps. Next time you see one of these, peek at the schematic; it tells all.

If it helps, think of this panel as the ancestor of later split-bus panels where you might find 2, 4 or 6 "main" breakers, none of which would equal the size of the service.

Speaking of education - you mentioned the panel board is obsolete. I agree, basically because it is old - but I have struggled with making this type comment to my customers because I am not able to explain what exactly is obsolete about it. Not using this box as an example, but if the fuse box has adequate amperage, copper wire w/ intact insulation, is grounded, not over filled, no other significant issues What do I say is obsolete about it? What criteria do I use to determine if a panel is obsolete?

I would not characterize an adequately sized, properly installed fuse panel as obsolete. Such a panel is, however, very rare.

- Jim Katen, Oregon

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Great, this is turning into a real lesson. So if I pull out the left block there will still be power to the right block, but no power to any of the other circuits, and I could go cook some mac-n-cheese in the dark? Would there not have to be a seperate service conductor coming in and connecting to both blocks? The house is empty I may try and get back over and try that out. Not the cooking this, but the other.

From what you can see of this box, would you recommend it be panel be replace due to be obsolete?

I have another inspection to go to but am enjoying this thread, and will be back tonight. If the 104+ heat does not get me first.

Mark

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I remember a Prof in college explaining that fuses were actually better overcurrent devices than circuit breakers because the fuse will fry about 10,000 times more quickly than a circuit breaker will trip. Fuses are much worse as safety devices, however, simply because people monkey around with them. Like so much that we see, it's a matter of not following the instructions . . .

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Originally posted by AHIS

I am not able to explain what exactly is obsolete about it. Not using this box as an example, but if the fuse box has adequate amperage, copper wire w/ intact insulation, is grounded, not over filled, no other significant issues What do I say is obsolete about it? What criteria do I use to determine if a panel is obsolete?

Well, I hope that Mr. Electric is peeking in here today, because I've been using criteria that he taught at a class way back in January of 1997 wherein he told the members of the franchise I used to belong to that fuseboxes and components over 40 years old are absolete and should be replaced.

I figured that if it was good enough for him it was good enough for me. Since then, on that 40 year basis alone I've written them all up and have never had anyone argue the point with me.

ONE TEAM - ONE FIGHT!!!

Mike

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Originally posted by AHIS

Great, this is turning into a real lesson. So if I pull out the left block there will still be power to the right block, but no power to any of the other circuits, and I could go cook some mac-n-cheese in the dark?

Yes. Precisely correct.

Would there not have to be a seperate service conductor coming in and connecting to both blocks? The house is empty I may try and get back over and try that out. Not the cooking this, but the other.

Study this schematic. It's close to the box in your picture. See if you can figure out why you don't need a second set of service conductors.

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From what you can see of this box, would you recommend it be panel be replace due to be obsolete?

Not necessarily, but I wasn't there. I'd certainly recommend getting the transformer out of the fusebox.

My overall recommendation would likely take into account whatever was going on with the panel next to it. If that newer panel were taking some of the burden off the older panel, I might say that the fusebox wasn't really an issue.

- Jim Katen, Oregon

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Originally posted by Bain

I remember a Prof in college explaining that fuses were actually better overcurrent devices than circuit breakers because the fuse will fry about 10,000 times more quickly than a circuit breaker will trip.

That depends on the particular fuses and the particular circuit breakers. Many fuses are slower than circuit breakers.

Fuses are much worse as safety devices, however, simply because people monkey around with them. Like so much that we see, it's a matter of not following the instructions . . .

I'd debate that point. At least with a fusebox you can install safety fuses. There's nothing to stop someone from installing a too-large circuit breaker in a breaker panel. A sufficiently talented fool can burn himself to a crisp either way.

- Jim Katen, Oregon

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  • 3 years later...

My apologizes for reopening a very old thread, but the pictures and diagrams herein are most illustrative of my question. My thanks to the original posters!

I'm trying to understand the electrical system in my friend's old mountain cabin. There's been extensive electrical work done to the place over the years. I'll provide as much detail as possible, then ask my question at the end of the post.

There's a new circuit panel in the kitchen. And there's a Main/Range fuse block in the utility room that powers a water heater and the lights for that part of the house. The kitchen circuit panel has a double-throw 50 amp breaker labeled "Utility Room." Turning OFF the "Utility Room" breaker in the kitchen circuit panel removes power from the utility room.

Looking at the Main/Range fuse block... there are wires connected to all 4 terminals on top (the 2 designed for supply and the 2 designed for a range). The wires to the 2 Supply terminals are old. The wires to the 2 Range terminals look new. The water heater is powered from the bottom terminals in the normal fashion. There is no range in that part of the house.

Here's my hypothesis and question:

Is it possible that the 2 wires connected to the Range terminals are actually supply lines from the kitchen circuit panel? And that the 2 wires connected to the Supply terminals are unused? (Where the ends of those latter 2 wires are is another question.)

Looking at Jim Katen's above-posted schematic, this looks possible, given the (what I'll call) "backplane" connections between the Main and Range sides of the fuse block.

Thanks very much.

-Jake

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. . . There's a new circuit panel in the kitchen. And there's a Main/Range fuse block in the utility room that powers a water heater and the lights for that part of the house. The kitchen circuit panel has a double-throw 50 amp breaker labeled "Utility Room." Turning OFF the "Utility Room" breaker in the kitchen circuit panel removes power from the utility room.

I don't mean to be a butt, but I'm pretty sure that you mean "two-pole 50-amp breaker" and not "double-throw 50-amp breaker." A double throw breaker is very rare in a residential setting.

Looking at the Main/Range fuse block... there are wires connected to all 4 terminals on top (the 2 designed for supply and the 2 designed for a range). The wires to the 2 Supply terminals are old. The wires to the 2 Range terminals look new. The water heater is powered from the bottom terminals in the normal fashion. There is no range in that part of the house.

Here's my hypothesis and question:

Is it possible that the 2 wires connected to the Range terminals are actually supply lines from the kitchen circuit panel? And that the 2 wires connected to the Supply terminals are unused? (Where the ends of those latter 2 wires are is another question.)

From your description it sounds like the fuse box, which used to be the service panel for the house, is now a sub panel. It's certainly possible that someone ran the new feeders to the top of the range block and is backfeeding the panel that way. If that's the case, then the old feeders are also backfed, and energized. If someone just clipped them off in a wall somewhere, that could be a problem.

To find out for sure, someone could pull the range fuseblock and test the terminals to see what's still hot. That should tell you right away where the power is coming in.

If the original feeders are abandoned, I think I'd disconnect them and bring the new feeders in the way they were meant to come in. Then switch the water heater over to the range fuse block and put in properly sized fuses (you might need an adapter). This would give you a single disconnect for the water heater and free up some space in the lower part of the panel.

Also, be aware that the fuse panel might be a 60-amp design, not the 100-amp design in my schematic. In the 60-amp version, the range block is fed from the *load* side of the main fuse block. Look for a schematic on the panel to be sure.

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Thank you, Jim Katen.

Yes, I suppose I mean "two-pole 50-amp breaker" -not- "double-throw 50-amp breaker." Appreciate the terminology correction.

One detail I forgot to mention: the cabin's fuse panel schematic shows the Range Block being fed at its bottom from the Main Block's top. Thus, I suppose it is a 100-amp panel as its schematic is identical that way to the schematic in this thread.

Next time I'm there I'll pull the Range Block and see what's what.

Thanks again.

-Jake in Durham, NC

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