Air compressor screw type - questions

At work have Gardner Denver screw type compressor. Nameplate motor data
Volt 480 current 61 amp.
Motor is Toshibe (?? am at home cannot double check nameplate).
Right on the door is legend wye delta start.
Have six leads 123 456 tied to two relays.
OK so I have wye delta start. No big deal no problem works great.
Many of the questions below will be answered I thing when I put a clamp on ammeter in there and do simultaneous voltage checks. Will do volt to ground instead of phase to phase so I have direct comparison to line current. Then will convert volt reading by 1.73 factor. So in wye I should be drawing approx 57% full line current ie 36 amp. Delta should be about 60 or less depending on load and if unit is unloading. Has panel display of pressure. May be able to get load unload status by getting into programming.

QUESTIONS
1. It appears to me that time in wye is too long - about 10 seconds.
2. Conductor feeding disconnect approx 10' (wire length) away is #8. Wire from disconnect to controller is also 8. Assume wire is THHN or MTW. Wire to motor is 6. Just from the visual comparison of building wire to factory wire this branch circuit is undersized. It is just fine for wye assuming 36 amp estimate from full load amp is correct and not considering higher starting current. In delta and IF motor is pulling full current (clamp on will tell me that) then it is clearly undersized. HOWEVER can the wire be downsized due to unloading?? I assume that the only load would then be just spinning the motor and compressor and estimate it to be 20 amp (again clamp on will confirm).
I am REAL LEERY of this but am also considering "dont fix if aint broke".
3. We leave unit running with no one in bldg over the weekend ie supplying leaks. I am arguing to shut it down - don't like unattended equipment running, waste of power, useless wear and tear. If someone has better estimate than mine of 20 amp to spin motor and compressor unloaded and an estimate of load unload time, I would appreciate it. Clamp on should be the final answer of course.
4. What mistaken assumptions have I made, and what am I forgetting or not thinking of??

Dan Bentler

I see 42 have looked at this. Conclude no one is interested or I have all bases covered since they have added nothing.

Thank you all for looking at this.

Dan Bentler

Read it all but not sure I got it all, the wiring from fuses/breakers should be the same as wiring to motor i.e, rated to handle FLA, possibly more. It does not matter what clamp on reading are unless abnormally high, then you have another problem.

Dan if you go to here & post you`ll get a reply.

http://www.electricalknowledge.com/forum/forum.asp?FORUM_ID=2

and yes your wire is to small. It`s like Ron stated it really doesn`t matter what the meter says. You run wire for the current on the motors name plate. What ever is on the name plate times 1.25 is how much current the wire should be good for. Then go to table 310.16 in the code book for the wire size. The circiut should be designed using the weakest link which is usually the terminations which is usually 60 degrees C. I thought you were an electricain in the navy? And just to stay with hte site PLC. If you find it and it`s wrong I too believe you should fix it.

leitmotif said:

Nameplate motor data
Volt 480 current 61 amp.

Click to expand...

I'm interpreting this as a 50 hp motor. If it is now being feed with #8 ga. I hope I'm wrong.

If I'm recalling correctly this should be #4ga..

50HP motor requires #4 wire. NEC 430-52 requires that wire, breakers, starter, etc be sized to handle the full current of the motor, not the load current.


I agree with your concerns about running it over the weekend. For one thing, this is costing your compnay a lot of extra money. If you want to have air-on-demand as soon as the doors open on Monday morning then I suggest you look at a timer to start it up a few minutes before to pressurize the system and refill all the tanks/headers. And fix those leaks! They are costing you huge amounts of money, as in thousands, not hundreds, thousands of dollars. Commpressed air is the most expensive utility in a plant and your true overall efficiency as an energy source is probably less than 10%, so leaks are a killer.


http://www.plantservices.com/articles/2006/132.html

http://www.plantservices.com/articles/2006/124.html

Thank you all.
Believe me if it were mine I would be running MUCH larger conductor. Skimping on wire size to motors just is not a good thing. Don't look at me I did not install it. The boss will probably say (its run for xxx years so it must be OK) BUT even knowing this I will bring it to his attention. That is what I am being paid for.

What had me wondering was the load and unload of the compressor being allowable to decrease wire size due to average current being less.

Making a few assumptions like the unloaded amps is 20 (just to keep the unit rotating) and 75% unloaded time I come up with $20.00 per day to run on the weekend. Probably a very conservative value due to leaks.

The part that gets me is it takes only 5 minutes to bring the system to pressure. Its all OK with me I guess, its their equipment, it is their power bill, if they want to be stupid it is just more job security for me.

Dan Bentler

Dan

NEC 430-52 is about the setting of the branch circuit short-circuit protection based on motor full-load current. I assume the manufacturer of the compressor, based on long experience with their own equipment, correctly calculated and picked the proper circuit breaker protection. Article 430-53(c) basically allows a manufacturer of a group installation (such as a compressor) to determine the size of the short-circuit protection based on his experience with the equipment. This rating must be listed on a nameplate on the machine.

You seem concerned with the motor feeder wire size. The rule you need to meet is NEC Article 430-124: "Conductors supplying motors shall have an ampacity not less than the current at which the motor overload protective device(s) is selected to trip."

For three #6 THHN motor feeder wires in a conduit, and all terminals rated for 75 degrees C, then the compressor motor overloads, for both Delta and Wye windings, must be set for 65 Amps or less. If they are set for 70 Amps then you need larger wire.

For the #8 THHN wires free in air, then they can be rated for up to 70 Amps. Three #8 THHN wires inside conduit or raceway is rated for only 50 Amps.