That unit nameplate says 208-230/460v 50Hz
Not that +20v and +10Hz should kill it, but 460 @ 36A, with the 480v it's approx 38A.
PF is not that great, can be corrected to take a little burden off the xfrmr.

Is the source power 50 or 60Hz?

460v 50Hz motor running on 480v 60Hz power seems ok for basic motors, but will the rest of the jet system be ok running from 480v3ph 60Hz?

 

No OCPD on the 480v3ph banch ckt? Isnt that a violation of NEC? The disco is a non-fuse type?

And for clarity, is it blowing fuses on xfrmr energize (secondary no-load), or when you apply motor load on 480v?

What's the Amp rating of the waterjet system? Is 480v3ph balanced on load?
Any known PF for the waterjet system? Is 75kva robust enough? If the PF is not so good, PF correction caps may help things.

O/C for transformers has a number of interesting twists and turns. One of them being secondaries not requiring O/C. In the setup that exists, operating too close to the service capacity, energizing the trans or starting a 30 Hp motor, will likely take out a quick fuse. The trans is more than adequate to start a 30hp motor, and large enough to be under utilized, penalizing the PF. As the motor is likely the substantially largest load in the machine, it will be generally balance, with a typical motor kind of PF. That could all change if something was wrong, but there's no indication of that.

 

O/C for transformers has a number of interesting twists and turns. One of them being secondaries not requiring O/C. In the setup that exists, operating too close to the service capacity, energizing the trans or starting a 30 Hp motor, will likely take out a quick fuse. The trans is more than adequate to start a 30hp motor, and large enough to be under utilized, penalizing the PF. As the motor is likely the substantially largest load in the machine, it will be generally balance, with a typical motor kind of PF. That could all change if something was wrong, but there's no indication of that.

OCPD is on secondary side of every service. OCPD sits between secondary and branch or feeder ckt.
208secondary feeding a step-up, I would expect OCPD on the secondary side of the step-up.
A fused disco is disco+OCPD.

One trick to energize slowly, is to use a 3ph variac to slowly increase the voltage up to line voltage, then switch in the lines, then switch out the variac. A pita, but doable.

Blowing a 100A fuse (one phase) seems to suggest the other two lines get near 100A. Inrush seems a bit high to me to energize a 75kva dry iron core. With a amp-clamp (on inrush setting) 'you' can perhaps see how high the amps really are before the fuse blows.

 

That unit nameplate says 208-230/460v 50Hz
Not that +20v and +10Hz should kill it, but 460 @ 36A, with the 480v it's approx 38A.
PF is not that great, can be corrected to take a little burden off the xfrmr.

Is the source power 50 or 60Hz?

460v 50Hz motor running on 480v 60Hz power seems ok for basic motors, but will the rest of the jet system be ok running from 480v3ph 60Hz?

The name plate says 60hz even though it may look like 50hz. The source power is 60hz.

 

delete.

 

The name plate says 60hz even though it may look like 50hz. The source power is 60hz.

It sure does, I saw the 50Hz next to the voltage, my bad.

 

I think you need a 600A amp clamp with decent inrush function. Excitation (in milli-sec time) can be as high as 10x normal rated. 75kva (0.89 PF) is about 100A/ph.

 

Here's my final review/input.

75,000 x 1/208 x 1/1.732 = 208.15 = 208
NEC is 125% rule
1.25 x 208 = 260A ocpd (fuse)
Ampacity of the feeder wire needs to be 260A(min), hence #300 copper.

inrush however can be about 6x of rated, 6 x 208 = 1,248A !!

However, the OCPD and wire size can be smaller, but you won't get full rated kva, and, might run into nuisance tripping with inrush.

If the service is limited to 208v 100A, then that's only half rating of the 75kva xfrmr.

Limited service power, plus inrush on the 75kva step-up, looks like an issue that can't be solved. Can the service support 125A fuses? Are wire ampacity 100A(min).

NEC rules and physics of iron cores, dictate the issue.

Even if you went 208v, the nameplate shows 77A rated, then you have to add in the inrush factor. Not sure what that factor is for that equip, but at 4x it's now up over 280A. 77 x 1.25 = 96.25, hence the 100A fuses are ok, just need a slow type.

My speculated guess, probably better/easier to run equip on 208v vs trying to deal with inrush issue of the step-up.

 

Thank you so much for all of your help. Sounds like it might be best to run it at 208. I'm going to double check with the manufacturer just to confirm and then I'll go for it. I really appreciate everyones input!!

 

I suggest that it would be very hard to start a 30hp 208 motor over a fast acting 100a fuse.

 

would switching it to a time delay rk5 fuse like you mentioned help? Not sure how I could configure that fuse into the current setup as the Boltswitch fuse holders don't seem to accommodate such a fuse.

 

There are a number of delay fuses available. I'd look through the Boltswitch literature to see what options are available with the disconnect you have. You might be best to call a local supplier and ask what is required to accommodate a delay fuse in that particular device.

 

would switching it to a time delay rk5 fuse like you mentioned help? Not sure how I could configure that fuse into the current setup as the Boltswitch fuse holders don't seem to accommodate such a fuse.

NEC might not let you use slow fuse if the service is used for other than your equip.
But yes, if you run the equip on 208v, the fuses need to be "motor rated", or a slow/delay type.
Then you probably also need proper 100A 3ph tied-handle breaker to the equip.

 

Just running some numbers here:
208VAC @ 100A = 20800Kva
[edit] 20800Kva @ 480VAC = 43.3•••A
20800VA @ 480VAC still = 43.3•••A [end edit]
And that's assuming 100% efficiency. Looking a little out of spec to this untrained eye.

 

#1 is not 150A. 1/0 is 150A copper or 120A aluminum.

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Maybe you're thinking of special circumstances ???
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Maybe you're thinking of special circumstances ???
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View attachment 258265

90C rated lugs? Sure, if you can find or already have that. Wire alone is not the only factor in NEC book for picking correct wire.

 

I would recommend

1. Swapping the fuses as suggested ("motor", "slow", or "delay" type, keep them 100A).
2. Might need a breaker change too, not sure what type is there now.
3. Run equip on 208v3ph.

 

Just running some numbers here:
208VAC @ 100A = 20800Kva
20800Kva @ 480VAC = 43.3•••A

And that's assuming 100% efficiency. Looking a little out of spec to this untrained eye.

no, 208 x 100 = 20.8 kva = 20,800 va
20,800 watts @ 1PF
PF is not really a factor for xfrmr, because the nameplate accounts for PF, the PF is in the design. Hence, if the xfrmr is rated 20.8kva, then it can deliver 20,800 watts to a 1PF load. Nameplate says what the device will give on a 1PF load.

I also suspect, jet equip is not sucking in 77A when operating. That 77A likely has fudge factor in it, which helps guide wire and breaker sizing. It's always good to have 50A available to a 2A device, lol. I would run some temp wire to the equip and amp-clamp to see how the equip starts and runs, then go back and size breaker and wire accordingly.

 

I would recommend

1. Swapping the fuses as suggested ("motor", "slow", or "delay" type, keep them 100A).
2. Might need a breaker change too, not sure what type is there now.
3. Run equip on 208v3ph.

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I agree for the most part .........
But when the Machine is running, ( on 208-Volts ),
there's still only ~20 to ~30-Amps left over to run everything else in the Shop.
A service up-grade is still needed.
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I agree for the most part .........
But when the Machine is running, ( on 208-Volts ),
there's still only ~20 to ~30-Amps left over to run everything else in the Shop.
A service up-grade is still needed.

I suspect the equip is not running at 77 or even 70A. 1st amp-clamp it to get real #'s, go from there.
Upgrading to 200A service may or may not be ez or cheap. I can't see what's there from here.