Plug Fuse Type

Hello, all, and a fuse question for a practicing electrician (or anyone
in the know):
Assuming a residential branch circuit is fused for 125 volts, 30 amps
and the fuse box accepts the rejection base ("S") fuse, how
interchangeable are the 30 amp "D" dual element time delay (has the
spring loading) and the 30 amp "SL" time delay types? Thanks for your
time and comment. Sincerely,
Reply to
J.B. Wood
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The fuses themselves are identical in their operation. The only difference is the Type D screws into the standard Edison base fuse holder, while the SL fuse needs the base adapter screwed into the Edison base first, then the fuse screws into the adapter.
Reply to
Rich.
Hello, and many thanks for the prompt reply. Unless I'm mistaken, the types of plug fuses that I referred to have identical footprints; i.e., they are both rejection base types. Either one could presumably be screwed into the 30-amp rejection receptacle in the fuse box. (There is a type D TC fuse with an Edison base but that's not the one I meant)
So the question seems to be the difference between having or not having the "dual" time delay capability. Sincerely,
Reply to
J.B. Wood
Dual element fuses are usually intended for motor loads. They have an instant element that will respond to short circuit faults rated at more than the nominal fuse size and a thermal element that will cook off if you have a sustained overload. It allows the fuse to sustain the start current without blowing but it will let go if that overload continues. You can go to the Bussman web site and see the curves for the various fuses.
Reply to
gfretwell
Time delay fuses with the spring blow faster than dual element fuses. Is that what you're looking to find out?
Reply to
Rich.
Hello, and I think that might be backward. The dual element type seems to have the spring. Sincerely,
Reply to
J.B. Wood
Hello, and thanks for the detailed follow-up. All this started because the S-type 30 Amp plug fuse (with the internal spring) blew (I had a shorted-out lamp fixture). My local hardware store only had the SL 30 amp S-type and substituting that appears to work but judging from your comment it may be subject to blowing more quickly than the original given certain conditions. Sincerely,
Reply to
J.B. Wood
It won't blow any quicker for general use, such as lighting, TV, etc. It's more apt to blow if you have a heavy motor load such as a window A/C on the circuit, but not smaller motor loads like a vacuum.
Reply to
Rich.
An important question here is what is the load served by this 125V 30 amper e fuse. Thirty Ampere circuits that are operating at 115V nominal are quit e rare. The only type of load such a circuit should be installed to serve is a load that is wired directly to the branch circuit or a National Electr ical Manufacturers Association (NEMA) pattern NEMA 5-30R receptacle. The m inimum wire gauge for such a circuit is American Wire Gauge (AWG) Size Ten copper (10AWG/Cu) or Size Eight Aluminum (8AWG/Al). Neither Fifteen or Twe nty ampere receptacles may be installed on such a circuit. If the circuit is actually a full phase 230V nominal voltage circuit then such a fuse coul d only be installed in a common operating lever, simultaneous opening, fuse d switch. I two such fuses are used to supply a nominal 230 volt circuit f rom a non switched panel board then the circuit is installed in violation o f the listing and labeling of the panel board and thus in violation of the US National Electric Code.
Reply to
Tom Horne
I wondered that to Tom. There is a scary thing about S adapters. The SA-30 that takes a 30, will also take a 20 and I see them on 12ga circuits all the time in old houses around here. More times than not, they have 30a fuses in them.
Reply to
gfretwell
You should not be using a 30 amp circuit for general lighting. Reason, lamp fixture cords can easily burn and start a fire before that fuse blows. 15 amp is recommended for general lighting circuits. If you have motors on your 30 amp circuit, you should replace that present fuse with a delayed one (30amp), if it blows.
Reply to
Buffalo
Hello, and I reside in a small condo unit that is in effect a spruced-up (except for electrical) apartment building. Beyond the electric meter for my unit there are two cartridge-type fuses, one circuit goes through that 30 amp non-tamp (rejection base) fuse we've been discussing and the other circuit goes through a 20 amp non-tamp fuse that feeds a single 120 volt outlet that is apparently intended for a window A/C. IOW, if that 30 amp fuse blows all power except for the A/C is out within my appartment. There are no 240 volt outlets anywhere in my apartment. There is also a small wall-recessed two-fuse panel inside my apartment that further divides what comes off that 30 amp circuit. Sincerely,
Reply to
J.B. Wood
That makes sense if this was a place built around the before WWII and it is tiny. The general lighting load of a place around 1000 square feet is going to be 3500 va and you have 3600va from that fuse. Of course these days you would also need a dedicated 20a bathroom circuit, 2 kitchen circuits and maybe even a laundry circuit if there are no laundry facilities in the building.
We use more electricity than they planned on in the 30s.
Reply to
gfretwell
J
What fuses are installed in that "two-fuse panel inside my apartment that further divides what comes off that 30 amp circuit"? The reason that I ask is that one of them should have opened on that fault in your light fi xture (Luminaire). If one of them did open but you simply had no challenge in finding an identical replacement then that solves the mystery. But if the fuse in that panel did not open then it is not properly sized or it has been bypassed in some way. You don't want to let this one go. Your safet y is at stake here.
Reply to
Tom Horne
If he has 2 20s in there, it would be quite easy for you to blow a 30 feeding both without blowing either 20. This setup is far from "coordinated protection". That 30 could have just been old and on the edge of popping anyway. I would not over think it. Replace the fuse and see how it goes.
Reply to
gfretwell

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