After Main Panel Protection & Machine Disconnect- 2nd Circuit Breaker or Fuse? Does it Matter?

[Shiseiji]

I've been chastised for not having protection after the panel and SFC. So what to do? Traditional 3P fused switch or the DIN rail mounts?

If a fuse is recommended, what type?
I see miniature 32 amp fuses for DIN rail. What us the value after a 30a panel breaker? Use 20a?

TIA

 

[RJSakowski]

I would opt for a breaker rather than a fuse. The value of the breaker will depend on your motor. There are essentially two faults that can trip a breaker; a catastrophe failure like a short which should trip any sized breaker and something like a motor overload or stall which will cause a current that exceed the breaker rating. The latter will take some experimenting. If the breaker is too small, it will potentially trip whenever the load increases. If too large, it won't trip soon enough, if at all.

Fuses and breakers have a property called i2t which means the trip point is proportional to the square of the current times the time. A high current will trip a breaker fairly quickly while a lower current will take longer to trip. The breakers in my load centers are actually a combination of both. Fuses are usually classified as slow blow or fast blow and it is an either or. You should be able to find that spec for both fuses and breakers.

 

[mksj]

I have no context as to what you are protecting, the wiring, cabinet and SFC? You need to know the application to know the fuse type. Typically breakers protect the wiring, less so the device. Fuses can map closer to the characteristics to the overload point of the device/system you want to protect, and may provide an additional layer of protection. There are many fuse holder types as well as fuses, and but something like a 30A fuse has a different rating for the class of fuse (RK5, CC, J class etc.). Fast blow fuses are often used to protect sensitive electronics, but often require a higher fuse amperage to handle peak surges, time delayed type or dual element typically are used in applications where there may be a very high inrush current load for a short period of time such as motor starting or initial application of power to a transformer.

Placement of the fuse can be in a separate enclosure, such as a power disconnect switch with fusing or in the electrical cabinet, generally after the power disconnect switch. They also may be used to protect specific sub-systems, vs main line fusing. Withing the electrical cabinet one needs to be cognizant of the wiring gauge before and after is supported by the fusing and/or breaker, and the current/voltage ratings of the sub systems. All of the wiring to the overload device must support the amperage rating of the distribution breaker providing power to the electrical cabinet. Dual voltage cabinets, the wiring and fuse most support the higher amperage/lower voltage wiring, in addition to the higher voltage rating. Some breakers are directional as to power in and power out to the device.

Fuse holder and fuse types are usually recommended by the manufacturer for the particular application. Up to 30A, I typically use either CC fusing because the holders are small and fuses cost less, up to 60A I usually use J class. The fuse holders are available in finger safe DIN mount which are convenient. Some of the fuses/fuse holders have indicators as to if the fuse is open.

J CLass 60A fuse holder to protect a 20Hp VFD and accessory MCB 15A breaker for power supply/coolant motor.


CC fusing 30A dual element fuse and sub system breakers

 

[Shiseiji]

I have no context as to what you are protecting, the wiring, cabinet and SFC?

Sorry, yes. Duh. It's a KISS set-up to a 10EE. I was told there should be a final protection before the machine after the phase converters.

Right now I have a two set-ups for powering the lathe. It's a 3hp 3ph AC motor, inrush/start up amperage on each leg is ~ 60 a then down to ~ 11.
Setup 1. 60 amp panel circuit breaker, 2P 2T switch, no protection, to a RPC then straight to the lathe via a plug.
Setup 2. 30 amp panel circuit breaker, a plugged in SPC to the lathe.

A cable from the lathe to plug into either unit, same deal on a Logan 10" with a 3/4 hp 3p, and a Clausing 8511 with a over kill 2 hp 3ph motor. Why? Because it originally had a now unobtainable pre-1954 3/4" shaft and the big motor was pretty much a bolt on. I just don't push it.

I have a 30 Amp 240-Volt 3-Pole Indoor Fusible Safety Switch with Neutral to put on the RPC, basically what I have now with fuses, but then I started to wonder if the micro fuses or CB would be smarter. And which one?

Why both a SPC & RPC? It's some experimentation as a "Very happy to do it" payback to the Practical Machinists 10EE electrical guru to test the SPC capabilities. I don't hog out a lot of material, and it's kinda nice to not have the RPC competing with the loud 10EE motor-generator fan. If you are not familiar, shared shaft 3 hp motor and DC generator. Called a Leonard-Ward drive still used in elevators because they are smooth. A DC motor is isolated from the spindle and gearbox. Up to ~ '49 all were analog. Special built MG units, like mine, were built into the mid 50's all for DOD as much as can be determined. Then came the Works in a Drawer (WiaD) and tubes. I forgot off hand what the next generation was.

Again, TIA

 

[mksj]

Typically each lathe would have a panel breaker/overload device for the load of that machine, as well as a power disconnect switch. You will have a different overload device for each machine. Typically what I have seen when using an RPC with multiple machines w/wo using socket and plugs is to have it feed a sub panel with breakers for the particular load/wiring. But a standard breaker is not a motor overload device, it is to protect the wiring. I do not see the value of using a static converter (SPC) in this application, and a 30A breaker for a 3/4Hp or 2 Hp 3ph motor is high. May be OK into the SPC but would not provide adequate protection for the smaller motors. Hard to say on the 10EE as to the breaker/fuse size/overload protection, but there should be some description in the original manual, I would also think that there is fusing in the lathe control box. I would also want a power disconnect device at each machine. Motors are usually protected by motor controllers/overload relays set to the FLA of the motor, in addition a machine with multiple sub systems may have delayed type fusing for the main power in and then sub-system breakers and/or fusing for each sub-system. Not an electrician, but there are also limits to the plug connection and the Hp rating they are being used for. This has to do more with it being used as a power disconnect device for the motor while running or locked up.

You also need to be careful of how you use a neutral with an RPC, as the voltages are different for the generated wild leg. If you are using it to break out 120VAC at a machine (using the pass through L1 and L2), then that should be on a separate breaker/fusing. What is the neutral being used for in this application?

 

[Shiseiji]

This question on the disconnect came up during troubleshooting and I realized the disconnect being referenced was missing. Flat forehead moment.

The 10EE manual is surprisingly silent on a lot of things. No internal fuses, a main power overload/switch. Manual states "Call the electriction."

Mksj - Shop 220 is wired without a neutral, builder got cheap on me. Four circuits each a wall in the shop, so 4 30a circuits. A 50a circuit for the RPC. All lights are off the closest 120 receptical.

The motor disconnect for the 10EE is too easy, the fused switch is on the bench. The 10" Logan and the Clausing each have a 3p 3t motor switch after the power source, the Logan's is before the old drum reversing switch.

I have a non-fused disconnect before the RPC. I would like to put the RPC "somewhere" to dampen the noise, hence the minimal hard wiring. Plus the need to pull a permit. Not worried about the work, I've pulled 3 permits in the past 18 months, inspectors were pleased each time. Over-kill for the win. I can't remember off hand if the RPC required 8 AWG or 6 AWG. Either way, the cost is pushing me to just leave it next to the panel.

Decision time so I can hard wire a separate leg for each machine, with a breaker or fuse and each machine to have a motor switch.

For the SPC I'm hearing run a hard wired circuit with a breaker or fuse to the SPC, then run a circuit for each for each machine with the motor disconnect after the SPC.

If I'm going to run more than one machine off a SPC, I assume the proper way is to put a second breaker/fuse after the SPC sized for the specific motor.

Did I get the summary correct?

Thanks for everyone's time and input.

 

[tq60]

Location?

First, any power leaving a protected distribution needs to have circuit protection at the source.

The breaker panel, or load center, is the distribution. It is protected as there usually is a breaker on its source.

Breaker does NOT protect the equipment.

Breaker protects the wire and the connection points or outlets if it is not hard wired to something.

For things with a plug, the protection can be any type, but it is best if designed for the load.

The protection can be single at the entry point or distributed at entry point, where there are multiple systems.

Items like VFD are a power source, power source usually need protection at the output connection, but VFD usually has a rollback for soft protection as well as internal fuse.

Motors often have mag starters. These usually provide protection to the motor.

Sent from my SM-G781V using Tapatalk

 

[Shiseiji]

@tq60 Thanks.
All the 3ph plugs/receptacles to the machines are L15-30, 4 conductors X, Y, Z, and ground, rated for 30 amps.
To be a little more clear.
200a at the house distribution center.
100a distribution center in the shop, "professionally" installed and signed off by the State of Washington.

As noted, circuit protection is per the National Electrical Code (NEC), or NFPA 70
The Static and Rotary phase converters manuals also provide recommended conductor size.

NFPA section 430.32 covers motor overload protection. You can access the NFPA on-line pdf version for free.
In "general" the KISS requirement is 250% max data plate amperage. See a 5hp 3ph 14amp below.
Section 455 covers phase converters.
455.20 is covered for a single motor by using the appropriate sized plugs on the 3ph side. A single motor is all I run at once.
455.23 I have covered, the balance capacitors are before the receptacle for the motor plug.





General motor data and recommended motor overload protection can be found here:

https://tools.se.app/motordata/index.html?language=en&country=usa#/

Here is an example for the 3ph, 4.6hp, 14amp motor used on the 10EE rounded up to 5hp.

• Motor FLA - 15.2
• Wire Size (AWG/kcmil) - 14
• Thermal Breaker Size – Amps - 30
• Safety Switch size – Amps - 30
• NEMA contactor – SIZE - 1
• IEC contactor - AC3 Rating - 18
• Dual Element Time Delay Fuse – Amps - 25
• Heavy Duty Switch Type 1 Enclosure - H321N

My wire is way over size, I have 10AWG cable running to my machines.
Comments?