Fuse size/classes. Help!


Senior Member
I need some education on fuse classes, meaning physical size and form factor.

(Tried uploading pic but the new forum software seems not set up right, anyway...)

I bought some 175A fuses not realizing that they can come in multiple form factors? I bought RK5 fuses, they are about 9" long, blade end to blade end. They are too big.

Turns out the fuses I'm trying to replace are about 7" long (blade end to blade end). The barrel is about 4" long and 1.5"-2" diameter.

I'm not easily finding what type of fuse this is. RK1 seem to be too small at 175A, or do I need fuse adapters??

The equipment is an FPE switchboard from the late 70s.

Please help! :dunce:


Senior Member
Thanks. The equipment is 240VAC.

It looks like I misread another chart I looked at and RK1 is the right size. Does that seem right? Wasn't able to find an indication on equipment.
Class RK1 and RK5 fuses are the same physical size, but Class RK1 fuses are more current-limiting than Class RK5 fuses. This means Class RK1 allow less damaging energy to pass through them while opening (blowing).

It's the rated voltage and current that alters the dimensions for RK1/RK5 class fuses. RK1 and RK5 of the same voltage and current rating will be the same dimensions. It sounds like you may need a 250 volt fuse whether RK1 or RK5 instead of the 600 volt you have.
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Senior Member
Ok, I'm catching on here.
So how do I choose between RK1 or RK5? I couldn't find an indication of the existing fuse class.
I was hoping someone more knowledgeable would give some input, I think it's pretty simple but I'm not seasoned in this area so there's a chance I miss something.

See the crude selection guide on page one of the first link I posted, either will likely do for a switchboard. There are some things you should consider but usually won't make a difference with a switchboard, including what exactly is the fuse protecting and it's short circuit current rating? was there an arc flash study done? And as with any protection, it would be good to check selective coordination with the other devices in the system.

RK1 is the safer choice, it looks like the instantaneous for Bussman's i'm looking at is about 14X (2.5KA) so if there's no doubt you are below that then it will hold. But RK5 under normal situations is usually fine as well, the biggest difference is the instantaneous current, the Bussman i'm looking at looks like 28x (5KA). Do you have one large inductive load on the switchboard?

If there was an arc flash study done then they likely used the specific fuse rating. RK1 will reduce let through current, so safer choice of the two if it will hold. RK1 10KA fault has 4KA RMS let through, RK5 is 7KA RMS, 15 peak

Here is a link about the two different current limiting characteristics of Bussman's fuses, there are specific charts for RK1 and RK5.
http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Fuse_Let_Through_Charts_Current_L imitations_Charts.pdf

Here are time current curves for Bussman RK1 and RK5s in your current range.


Here is a time current curve of a 175amp breaker if you were curious and more familiar with breakers.
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Staff member
RK5 fuses are significantly less expensive, so if you don’t need the added current limiting capability of RK1, it would be a waste of money to use them.

In Bussman terminology, FRN-R is RK5 vs LPN-R for RK1 (240V rated). If something is old enough to have used FPE gear, RK5 is probably fine. Low Peak (LP) RK1 fuses came out after FPE was basically defunct.