Updated 480 volt machine wiring

[florida-sparkey]

Hello all,
This is an updated and I think corrected idea for checking over.
Thank you!!

Two 3 phase 480 Volt machines- (50Amp air compressor and 13Amp air dryer)
I intend to wire 2 machines from a single 100Amp breaker.
I will go from panel about 2 feet into a small wire way (4x4x24" troffer) with #3 AWG CU. I will use Polaris 3 position connectors and splice two taps to each conductor. #10 AWG CU and #4 are the tap sizes.
The taps will be no more than 3 feel long and will terminate in fused discos with 70Amp and 15Amp current limiting RK1 fusees (MOCPD per nameplates.) (The 100 Amp circuit breaker has a max AIC rating of 34K, the fuse charts show way under 10K of let through at 40K value.)
#8 AWG CU will be used for grounding/ bonding from panel to both disconnects.

On the load side of the disconnects-
Circuit wiring will be reduced to #12 with #12 ground for the 15 Amp machine and #6 with #10 ground for the 50 Amp machine (both runs to machines are less than 20 feet)

All EMT will be bonded for circuits over 151 Volts.
Thank you for the input!!

 

[petersonra]

Hello all,
This is an updated and I think corrected idea for checking over.
Thank you!!

Two 3 phase 480 Volt machines- (50Amp air compressor and 13Amp air dryer)
I intend to wire 2 machines from a single 100Amp breaker.
I will go from panel about 2 feet into a small wire way (4x4x24" troffer) with #3 AWG CU. I will use Polaris 3 position connectors and splice two taps to each conductor. #10 AWG CU and #4 are the tap sizes.
the taps will be no more than 3 feel long and will terminate in fused discos with 70Amp and 15Amp current RK1 limiting fusees (MOCPD per nameplates.) (The 100 Amp circuit breaker has a max AIC rating of 34K, the fuse charts show way under 10K of let through at 40K value.)
#8 AWG CU will be used for grounding/ bonding from panel to both disconnects.

On the load side of the disconnects-
Circuit wiring will be reduced to #12 with #12 ground for the 15 Amp machine and #6 with #10 ground for the 50 Amp machine (both runs to machines are less than 20 feet)

All EMT will be bonded for circuits over 151 Volts.
Thank you for the input!!

Just a few thoughts.

I don't think you can protect a CB with a downstream fuse, even if it is a listed series combination.

the wires seem kind of big to me.

the emt can serve as the EGC so no real need for any green wires at all.

personally, I would skip the little wire way and put the polaris connectors in the CB box.

 

[florida-sparkey]

Just a few thoughts.

I don't think you can protect a CB with a downstream fuse, even if it is a listed series combination.

the wires seem kind of big to me.

the emt can serve as the EGC so no real need for any green wires at all.

personally, I would skip the little box and put the polaris connectors in the CB box.

Thank you for the reply!!
The fuses are the OCPDs for the individual machines. The circuit breaker is in the main panel gear feeding my tap circuit.
The wire way is for making the tap connections.
Pinellas county requires ground wires in all conduits.

 

[petersonra]

Thank you for the reply!!
The circuit breaker is in the main panel feeding the circuit.

I don't know what you mean by this.

The wire way is for making the tap connections.

if there is room, why can't you just make the tap inside the enclosure where the Cb is located?

Pinellas county requires ground wires in all conduits.

Do they have some kind of local amendment for this? I took a quick look and did not see it.

BTW, what do you mean by this?

All EMT will be bonded for circuits over 151 Volts

 

[florida-sparkey]

I don't know what you mean by this.



if there is room, why can't you just make the tap inside the enclosure where the Cb is located?


Do they have some kind of local amendment for this? I took a quick look and did not see it.

I guess I do not understand the protecting the circuit breaker part of your first reply. I am tring to lower AIC to load so I think I am missing your point. I start at the breaker and go to the load. I use the AIC of the breaker because it is quicker than doing all the calculations. I need over current protection at each of the loads and with RK1s the AIC issue is also corrected quick and easy. No??

It is main breaker gear and for the $20 I would rather not have taps inside to deal with. Also I can cut the tap lengths down to just above the disconnects and then pipe straight down into each one.

Yes, it is required in my county.

The circuits are 480 volts, bonding is required per 250.97

 

[Smart $]

I don't know what you mean by this.

100A breaker, 100A feeder

if there is room, why can't you just make the tap inside the enclosure where the Cb is located?

Better yet, land on breaker if lugs are rated for two wires.

BTW, what do you mean by this?

Assured bonding is required over 250V nominal. I think he misquoted the voltage.

 

[petersonra]

I don't think you have a problem with the AIC of either the CB or the fuses. However, the equipment itself has to be able to survive whatever SCC can be put on it. It should have some kind of nameplate on it telling you what its SCCR is. You have to make sure your wiring does not exceed that much SCC. You cannot generally use a current limiting fuse to do that.

You will need to know what actual available SCC is going to supplied to the terminals of the device you are hooking up. That cannot exceed the SCCR of the device you are hooking up. Whatever the OCPD feeding the device happens to be is generally not relevant.

 

[petersonra]

Assured bonding is required over 250V nominal. I think he misquoted the voltage.

don't listed EMT fittings cover that requirement?

 

[florida-sparkey]

don't listed EMT fittings cover that requirement?

Yes if not used in a knockout unless listed, for safety, if I use a knockout, I bond.

 

[florida-sparkey]

100A breaker, 100A feeder


Better yet, land on breaker if lugs are rated for two wires.


Assured bonding is required over 250V nominal. I think he misquoted the voltage.

Thank you for the reply!
It is main breaker gear and for the $20 wire way, I would rather not have taps inside the panel to deal with. Also I can cut the tap lengths down to just above the disconnects and then pipe straight down into each one from the wire way.

 

[florida-sparkey]

I don't think you have a problem with the AIC of either the CB or the fuses. However, the equipment itself has to be able to survive whatever SCC can be put on it. It should have some kind of nameplate on it telling you what its SCCR is. You have to make sure your wiring does not exceed that much SCC. You cannot generally use a current limiting fuse to do that.

You will need to know what actual available SCC is going to supplied to the terminals of the device you are hooking up. That cannot exceed the SCCR of the device you are hooking up. Whatever the OCPD feeding the device happens to be is generally not relevant.

I may be lost, AIC and SCCR are basically the same thing are they not? One is service related and one is equipment related but basically the same.
by using current limiting fuses I am reducing the fault current that could be applied to the equipment am I not? I did not look at the data plate but I assume the SCCR is 10K or higher. By using the fuses, I have reduced the short circuit current to under 10K. No??

 

[Smart $]

don't listed EMT fittings cover that requirement?

Yes, when used in equal-trade-size KO. Not readily certifiable where installed in oversized concentric or eccentric KO's; definitely not when using reducing washers; etc.

 

[Smart $]

Thank you for the reply!
It is main breaker gear and for the $20 wire way, I would rather not have taps inside the panel to deal with. Also I can cut the tap lengths down to just above the disconnects and then pipe straight down into each one from the wire way.

What do you mean by main breaker gear???

If breaker lugs are rated for two wires, you just land both tap wires in the breaker lug, and run wires directly to their respective disconnects. They're still considered taps, there's just no feeder wire.

 

[florida-sparkey]

I guess I do not understand the protecting the circuit breaker part of your first reply. I am tring to lower AIC to load so I think I am missing your point. I start at the breaker and go to the load. I use the AIC of the breaker because it is quicker than doing all the calculations. I need over current protection at each of the loads and with RK1s the AIC issue is also corrected quick and easy. No??

It is main breaker gear and for the $20 I would rather not have taps inside to deal with. Also I can cut the tap lengths down to just above the disconnects and then pipe straight down into each one.

Yes, it is required in my county.

The circuits are 480 volts, bonding is required per 250.97

More presicly, The system is 480 / 277 volts or 277 volts to ground

 

[florida-sparkey]

What do you mean by main breaker gear???

If breaker lugs are rated for two wires, you just land both tap wires in the breaker lug, and run wires directly to their respective disconnects. They're still considered taps, there's just no feeder wire.

I agree your idea is a good one. In my case, I do not know the lug rating of the existing circuit breaker. Also, maby laughable, but I can make one hole in the cabinet (actually use the existing one.) One 1" 90 at top of cabinet to wire way and then straight shots from there into each disco mounted just below. I have to shut the whole building down while in the gear so less time in there is better.

 

[Smart $]

I agree your idea is a good one. In my case, I do not know the lug rating of the existing circuit breaker. Also, maby laughable, but I can make one hole in the cabinet (actually use the existing one.) One 1" 90 at top of cabinet to wire way and then straight shots from there into each disco mounted just below. I have to shut the whole building down while in the gear so less time in there is better.

Your raceways can be the same if you want. The only difference is no 'bugs' in the run.

 

[florida-sparkey]

Your raceways can be the same if you want. The only difference is no 'bugs' in the run.

Great point!! went right over my head.
I need to consider 6 current carrying conductors in the raceway but I should be able to keep it under 24"
I need to find out about the rating of the existing breaker lugs.

 

[Smart $]

I may be lost, AIC and SCCR are basically the same thing are they not? One is service related and one is equipment related but basically the same.
by using current limiting fuses I am reducing the fault current that could be applied to the equipment am I not? I did not look at the data plate but I assume the SCCR is 10K or higher. By using the fuses, I have reduced the short circuit current to under 10K. No??

Sorta kinda.

AIC is a OCPD safe-operation rating.

SCCR is an equipment safe-operating rating.

Neither can be less than the calculated available short-circuit current where they are installed.

Using fuses will limit the ASCC downstream, but not the enclosure it is in.

 

[florida-sparkey]

Sorta kinda.

AIC is a OCPD safe-operation rating.

SCCR is an equipment safe-operating rating.

Neither can be less than the calculated available short-circuit current where they are installed.

Using fuses will limit the ASCC downstream, but not the enclosure it is in.

This seems to fit my looser understanding. I am assuming the main service gear and the breaker are correctly sized and then using the maximum AIC of the breaker as my applied current to the fuses without reducing for wire resistance. The wires do extend from the discos to the equipment and that would further reduce the current already limited to less than 10K by the fuses per the fuse charts.
Is my logic on point?

 

[GoldDigger]

This seems to fit my looser understanding. I am assuming the main service gear and the breaker are correctly sized and then using the maximum AIC of the breaker as my applied current to the fuses without reducing for wire resistance. The wires do extend from the discos to the equipment and that would further reduce the current already limited to less than 10K by the fuses per the fuse charts.
Is my logic on point?

You need to keep in mind that the AIC capacity or the current limiting (fuses or wire length) MUST be upstream of the low AIC device. Current limiting downstream of the low AIC device does not protect against a fault in wire in between the two points.

This is just the opposite of the situation with motor loads where the overload (as opposed to short circuit) protection can be downstream of the protected wires.