Sizing fuses

Well, after a couple of weeks of "family issues", short notice out of town family week-long visit, and work distractions, I'm back on my "Staged RPC". I got the components mounted last night, and some of the control wiring run.

Now I'm wondering how to size the fuses. Seems like it will not be exactly straight forward in this case.

Inside the RPC control box, the power transmission wiring is #8 and the smallest idler relay is rated 7.5 hp (don't recall the amps off the top of my head). Main 220V (L1/L2) input relay is 10hp. So this can all handle some serious juice. Main 220V feed is #8 in 3/4 conduit with a 60A breaker. One idler is a 10hp Baldor HE, the other is a 3hp GE "Industrial" (whatever that means) motor. Primary load motor is a dual speed 7.5/3.75 hp on the lathe. Other load motors range from 1 to 2 hp and 3ph circuit is all #8 in 3/4" conduit.

So, I'm guessing that a reasonable fuse for the 3ph RPC output side is a reasonable margin over the FLA of the 7.5 hp windings. Possibly "slow blow" due to startup current issues on a lathe? And, since it is fed from only 2 lines, maybe a 30-40% increase on the input side?

Also, I have 2 fuse blocks that use the 2" x 1/2" fuses. A 2 conductor for the input, and a 3 conductor for the output side. Other than just checking the usual suspect (McMaster etc.), what's the best place to find these? I haven't checked, but I'm guessing these are probably available at the big box stores as well as local electrical supply too.

I figure the heaters at the machines are the primary "fuse" point, and obviously sized to the expected load. The fuses in the box are just for catastrophic failures in the lines between the machine and the RPC, or within the RPC itself (output and input sides respectively). I don't want to get these sized too close to the actual peaks since I really don't want them to the first thing to pop if something starts going over current.

Thoughts?

Size the fuses to the wiring. They protect wiring, primarily, and cannot help on small motor overloads which are still sufficient to damage the motor if prolonged.

Size the heaters to the motor load. They are intended to "act like a motor" and open the O/L when the motor is "unhappy".

I understand the general goal of "size the fuses to the wiring", or at least to the weakes link (the 7.5 hp rated relays?). But it seems that given the moderate over-kill of the components used, wouldn't it be better to ramp it down more for increased safety margin? These are surplus components after all. So I was thinking maybe 3x30A on the 3ph side and 2x40A on the input. Am I completely off-base on this?

Anyone have a comment on my fuse sizes? The wiring is #8 on the entire power circuit. But I don't really want to fuse it that high (if 2 x 1/2 fuses are even available that high?).

Look up some of the wiring requirements for RPCs on the commercial sales sites. They generally give input wiring sizes and breaker sizes.

According to my handy slide rule a 7.5HP motor @230V requires a NEMA size 1 starter, 60A class K5 fuse or a 40A breaker rated for motors, and #10 min wire.

Thanks for that input. Looks like I'm doing ok.

Here is what I've got.

1) All power wiring is #8 stranded coper. Either 3 or 4 conductors (depending on phase) in 3/4" conduit and inside the control box.

2) Breaker is standard 60A and feeds the phase converter, welder, and plasma (only 1 tool at a time, anywhere in the shop for that matter)

3) The RPC has been adjusted a bit to use a 5hp and 10hp idler independently or concurrently.

4) Relays are rated at or above the largest load motor (all are either 7.5 hp or 10hp rated).

5) The lathe is the biggest load I'll have at 7.5/3.75 hp.

6) Max rated FLA on the lathe is something like 21A IIRC. The 10hp Idler is something like 28 FLA, but will never be loaded.

Now, I still want to check what I'm doing with the fuses.

Question 1:
On the output side of the RPC control box, I have 3 of the 30A standard duty cartridge fuses (2" x 1/2"). This seemed to make sense given that the FLA of the 7.5 hp winding is well below the 30A, which is well below the limits of the components/wiring used. Is this right?

Question 2:
I'm unsure how to figure the 220V 1ph input fusing. The largest 2" x 1/2" fuse I can find is a "Heavy Duty" delayed acting "slow blow" 30A fuse. $7 a pair so a bit steep to just put it in and "lets see if it holds", though I will if it comes to that. So, what do I need there? 30A standard? 30A slow-blow? Search for higher amp, or change form factor?

Can anyone give me some guidance here? What I found by searching Google for RPC manuf wasn't very helpful, only talking about the main circuit capacity (breaker/wire). Searching further in here found a post by 2Slow indicating that startup current could be approximated by Amps = FLA * 1.75 + RPC where RPC was estimated at 5A for the idler. If that's the case, then I'm looking at potentially well into the 40s on startup and these "standard" 30A fuses are likely a waste of time, though they might be fine as long as I don't hang 100lbs of my 16" 4 jaw or something. Never fooled with anything like this before, so I'm a bit out of my depth...

Oh, and based on that "1.75" conversion factor, *IF* 30A is ok for the output, the the input side would need to be at least 40A (21fla * 1.75 + 5rpc = 41.75). So maybe I need to put a 50A 220V breaker AC disconnect on the input instead of fooling with the little cartridge fuses. Then again, will slow-blow 30A be enough on the output size? If not, then I may just change form factor (breakers?). I really don't want to go to a fuse that is not readily available through common big-box stores because I know that even if I keep a few spares from mail-order, something will happen to blow a couple on some dark and lonely Friday night when there is nothing but Home Depot open until Monday. Then again, maybe I worry too much...

You will need a 60 or 80 amp breaker to supply the combined RPCs from the main panel. I would use a 50A breaker on the output side. Forget fuses. I would also use at least 6AWG wire on the input to the RPC control panel.

I chose #8 for 2 reasons. It is listed on numerous spec sites (such as this) as being rated for 24A "power transmission". I also noted multiple references to being "very very conservative" (see link). It is also, for me, the most readily available and relatively inexpensive. Since the load motor FLA is only around 21 amps, and in-rush/start doesn't really count into that, this seems perfectly adequate for the conduit runs. For chassis wiring it's rated for 73 amps, so I'm pretty sure I'm good there. Even my 10hp Baldor idler seems to have only #10 (might even be #12?) internally...

And I do have a 60A on the panel. I originally typed that post as 60A, then changed it to 50A because I thought I recalled having gone with the smaller size. But it is actually 60A. So, is that sufficient such that I have no need to fuse (or otherwise protect) the input side of the control box?

And on the output side, if the 30A blow (and sounds like they likely will), I'll go to a breaker sub-panel to replace my 4-square junction box on that side. That'll give me more options/flexibility and easy resets if needed; though it will cost a bit more...

Oh, I misread that initially, I see you said "on the input", but I read it to quick and thought you meant throughout. <sigh> I need to slow down and read more carefully I guess. Given that 1.75 conversion factor, that makes sense on the input side. For the "power cord", I do have #6. I figured being bound in a tight sheath it needed the extra capacity. Sounds like I need to keep an eye out for some #6 and rewire the input sometime in the near future. But I'm in no hurry since I can't see ever using that machine at any where near full load. It'll be rare enough that I use half that HP...

Check with your local salvage dealers. Breaker panels sometimes go for little. A 3 phase breaker panel will give you the flexibility to wire in various machines that need 3 phase.

I have been gathering specs and parts for a similar setup to yours with a 5HP and 10HP staged idlers to power equipment. The 5HP is started first. The 5HP is then used to start the 10HP idler when needed. My largest motor is also in a 7.5HP lathe. There is an electrical salvage warehouse about 10 miles from me with piles of equipment and motors. They have made fair deals when I go thru and sort out what I need from untested parts.

Unfortunately, the only local salvage dealer who caries electrical stuff and allows public access is a total asshat with prices that rival new for the privilege of rifling through is filthy piles. And I'm not exaggerating, no organization and I actually have bought new for less than he asked. I no longer go down there at all. However, we have a scrap dealer who gets a LOT of commercial electrical control boxes. That's where I got my contactors, Alen-Bradly switches, and so on. Problem is, these are usually MASSIVE industrial boxes. I'm going to try to go down there next week, but will probably wind up with a small box from HD.

Anyway, I finally finished it tonight. Should be no surprise to you, but you are/were correct. It worked BEAUTIFULLY in all tests, up till the last... The final test was the lathe on 7.5hp and 2200 rpm with a collet chuck mounted. It spun up and was almost settled at full speed (based on sound) in about 3 seconds (guessing), when everything stopped. Some checking readily determined that one of the 30A fuses had popped. But they held up until that final "worst case" test. It's unlikely that anything spinning that fast is going to be much heavier than that collet chuck (rated for higher speed than that), so it's pretty close to "ok" I think. Still, I'll be getting a 40 amp motor rated breaker on that side ASAP, though I have a spare that will allow me to use it on lower speeds in the mean time...

Thanks for the help! I'll see if I can get some pics up in the next few days.

Excellent news! I am glad I could be of some small service. Get those pics posted.

Another test to run is an instant reverse. It does not have to be done at any high speed tho. 2-300 RPM is good.

BTW, I have designed all kinds of industrial control circuits and cabinets, mostly for production machines and hydraulic systems.