3 Phase Machinery Wiring

Thread Starter


Joined Jan 25, 2007
I have a couple of machines that I need connected, and I have brought over 3 electricians to give me an estimate. Each electrician is telling me something different in regards to wiring, breakers, and safety disconnects. I was hoping someone can help clear things for me.

1. Where would I find the correct chart for wire ampacity (different websites show different numbers)?
2. Safety Switches, should I use a fusible on not, and why?
3. What fuse size should I use in a safety disconnect (nameplate amps, or X times the name plate amps)?
4. What breaker size is optimal for a given nameplate amperage (% times the nameplate)?

Please, answer only if you know what you are talking about as I am already confused.



Joined Apr 26, 2005
All of the information you seek is referenced in the NEC [National Electric Code] and the NFPA [National Fire Prevention Association] codes.

Wire size is based on the current [amperage] requirements plus a safety factor.

Safety switches, and I assume your talking about the kill switches that will de-energize the equipment, should be used if there will be workers in the immediate vicinity of the machines.

Fuse size is also based on the current [amperage] demand of the machines, as is the breaker size, plus a safety factor.


Joined Jan 20, 2007
To add to Joe's comments... While you have a couple of machines you want wired up, it may be a good thing to do it so that it would comply to local regulations. I don't know what you'rs require, but here you have the supply coming in from the top to a switch to isolate the supply, out the bottom. Then it feeds into the earth leakage again from the top, and then it gets distributed to the different breakers, same way.
If you're going to do it yourself, pls make sure you know exactly what you're doing, otherwise save up and get it done right.

Wiring should be able to carry the max current req with ease, your electrical distributor should be able to give you all the specs you need - then use one guage thicker wire (oh, that's just me). Regarding the breakers, 40A get a 60A breaker, but that's no guideline, the 60A breaker should be able to do 60A. The breakers are there to protect the wiring and the mains supply only, so ie a short happens it would be the breaker that goes, and not the whole state...

Just do it right, a couple of bucks now could and probably would save some tears later.


Joined Jan 21, 2007
I have a couple of machines that I need connected, and I have brought over 3 electricians to give me an estimate. Each electrician is telling me something different in regards to wiring, breakers, and safety disconnects. ......
Let's take one step at a time.
1) What is the motor HP (or KW), Voltage, Full Load Amps, and Service Factor.
2) What style motor starter (if any) do you have?
3) Do you have a motor overload relay, and if so what is the manufacturer and model?


Joined Nov 1, 2006
Howdy, here's my take on your questions based on 33 years in the trade.
1. JoeJester's right about the national electrical code [NEC] having the wire ampacity info. Check with Google for a web site. I presume your machines have motors, if so, check article 430 for info. Also, note that wire ampacity depends on both the type of wire and the insulation on the wire. Some insulating materials can withstand higher temps. [Also read note 3 regarding this.]
2. Do your machines have [fusible] disconnect switches in the control panels already? If so you don't need any more; if not then you need one for each machine. The disconnect switches must be lockable in the off position for the protection of those who work on them. If each machine is on its own dedicated circuit then the disconnects don't need to be fusible. They are sized by both voltage & current ratings; for 240V you need a 250V switch & fuses; for 480V, go with a 600V switch & fuses.
3. According to the NEC, you need to size the fuses by the sum of the full load amps [FLA] of all the motors supplied by the disconnect switch from the appropriate chart in art. 430, not the name plates [although not everyone does this], then multiply this by 125%. If this number doesn't match any existing fuse size then use the next larger standard fuse size. Choose your wire size to match this fuse size. Also, I strongly recommend using dual-element fuses such as [Buss] FRS 20s [for 480V] or FRN 20s [for 240V] since they can handle the brief locked rotor current inrush when the motor first start.
4. Again, if each breaker is dedicated to one machine the same rules apply. If the breaker's in a distribution panel then you still need a lockable safety disconnect switch. Hope this helps.