Rpc Question

[bpimm]

I just read the code section related to RPCs. Basically, as I see it, you technically need a total of 3 disconnects.

These are always sticky areas up to a lot of interpretation by everyone. At the end of the day the only opinion that matters is that of your electrical inspector.

Thanks Aaron, that clears it up.

 

[Cheeseking]

Hope I didn't start a hornet nest bpimm but ya just make sure you study and follow code requirements. A plug/receptacle could satisfy as a disconnect under certain conditions.

I set up my shop 3ph as follows:

60A 240 1ph breaker off my main load center

6 AWG / 8 for EGC to fused disconnect (safety switch) for the 1ph

To a NEMA starter w/overloads for the phase converter. The push button to start and stop works really nice.


To 7kw phase conv


To 30A 3ph fused disconnect.


To 3ph load center with breakers for each machine.


Twist lock receptacles/plugs at each machine.

All this stuff fit nicely in a corner not too far from the main panel.


Now to add to the complete protection overkill I have yet another disconnect switch and manual starter w/overloads for the lathe motor. . (It was on the lathe already)

 

[Cheeseking]

Took the safety switch off the BP and went with only a twist lock plug/receptacle.

 

[abrace]

No hornets nest, your install looks really nice.

A plug and receptacle can absolutely serve as a disconnect, however both the receptacle and plug need to be specifically rated for the horsepower of the motor..you can't just look at the amp rating and think you are all set. The amp rating is only for resistive loads...motors are inductive. You need to see the HP rating and it needs to meet or exceed your motor. Otherwise they cannot serve as the disconnect.

Most common NEMA pairings max out at 3HP for a single phase feeder and that is with a whopping 50A plug. There is not much in the 5HP range, although some companies like Leviton have some proprietary non NEMA starndard offerings

As soon as you get into the 5HP range as the OP has, the plug/receptacle as a disconnect starts to get more difficult to do.

Below is a link to the Nema HP ratings for plugs. If you have a standard 240V 30A receptacle, that is a 6-30, and it is good for only 2 HP. Its a good chart to keep handy to see if your plug/receptacle combination can actually be considered the 'disconnect' under the code or if you need to do something else.

http://www.leviton.com/OA_HTML/SectionDisplay.jsp?section=66500&minisite=10251

 

[Cheeseking]

Yep. I think the verbiage is found in NEC article 430.109 (F) which permits plug as disconnect means. Of course HP rated to match like you point out.

 

[FOMOGO]

I set up my shop 3ph as follows:



Nice job on your converter setup. I see the in sight rule being served by an inexpensive video camera and monitor. That's just how a twisted mind works. Mike

 

[abrace]

Very true. There is 'safe' and then there is what meets code. Often the code requirements are silly, especially when it relates to our shops. The disconnect rules are written for big industrial/commercial areas where you may have a lot of workers, and you don't want to have someone turn a machine on or circuit on that is being actively worked on by a different worker. In our home shops, I think it is unlikely our wives will come out and fire up the lathe breaker on us while we are working on the circuit...yet the same rules apply.

Don't even get me started about all the stupid AFCI rules and tamper resistant receptacle rules.

There are a lot of manufacturers involved in various ways with the code making panels, and their incentive is to invent new rules so that we will have to buy more products.

That is what 90% of it is all about.

 

[Cheeseking]

bpimm another option to consider is a relay of some sort. That way you can run small gauge conductors (less $$) from the RPC starter to your switch in the shop.

 

[andermma]

hi, you might consider using thhn #8 copper wire in flex or emt instead of romex - emt is not to hard to install and is a lot easier than hard pipe. EMT looks like pipe but is thin wall and can be used indoors and out easy to cut with hacksaw with a fine blade - mount a disconnect near the rpc unit on the feed side at the least so it can be shut off and locked out for service. looks like in the pics above they are using watertight flex to connect to the disconnect.

Mitch

 

[JimDawson]

One thing to watch out for when plugging in a VFD is do it like you mean it. Align the plug, then connect it in a quick motion to minimize arcing. The inrush current is insane on a VFD for a fraction of a second while the capacitors charge. The best way it to kill the breaker, then plug it in, then turn the breaker back on.