disconnect

[dd20399]

i have a question,i am wiring a disconnect for my air compressor,220 single phase..i used nomex 2 wire with grd #10 wire 20 amp circuit..compressor is 75 ft away from sub panel..i have a new disconnect for hvac that i used at compressor this disconnect is metal with a switch in it..also has a place for grds to go which would also grd the box itself...my question,the wall i am mounting is tin..like a morton building..is it all right to use grd lugs in disconnect,it would also be grding the whole building through the tin or connect the supply grd with the compressor grd and don't grd the box,i know the box is usually grded but i worry about the tin getting power on it if compressor had a small short

[HayZee518]

building ground

I guess my first question would be "is the building grounded to the electrical panel?" by code each corner of the building structure should be grounded by a driven ground. the bolted up connections provide the rest of the ground system.
why don't you put up a piece of plywood to fasten your disconnect to? what is the nameplate current draw of the compressor motor? you know, a motor draws 125% more than nameplate current when it starts.
the other thing is voltage drop. another code wing ding is that you shall have no less than a 3% voltage drop at the load at 100 feet.
many people when figuring voltage drop only take into consideration the one way path to the load, that is 75 feet [in your case] when they should be figuring a path up to and away from the load. in your case that would be 150 feet.

[dd20399]

thanks for your reply,i don't know code but i suppose i ought to put a grd rod to building,it is a pole building with tin covering,for now i connected the grds together and am not grding the disconnect box..i could drill out the screw hole area and put a piece of wood between it to mount...i took the lid off the disconnect so i could use the switch and not touch the box...i worry more as i get older but still do the same dumb stuff ..again thanks for yor insite

[dd20399]

also,would you run a grd from the panel grd rod to the tin or grd the tin with seperate rod or both

[HayZee518]

morton pole barn

your structure consists of long framing members, a truss ceiling/roof and pre formed "purlins" connecting the verticals to the framing members.
the sub panel should be fed with a four wire cable or underground buried cable.
the neutral in the sub panel should have the neutral bar isolated from case ground [no bonding screw applied]
an auxillary ground strip should be installed with intimate contact with the panelbox case [ground]
the cabling terminates as such - black and red to the sub lugs, the white neutral to the neutral bus and the bare or green ground to the auxilarry ground bar.
the earth ground from a driven ground rod, its cable goes to the auxillary ground strip.
each corner of the pole barn should have a ground rod, eight feet long, being driven full length into the ground and connected by a stranded #4 bare copper wire connected to a lug on a corner bolt or "welded" by an exothermic connection to the steel.
now for the compressor.
a plywood panel would be exceptable for mounting the disconnect to.
the feeder would be a two wire with ground for the 240 volts. bx would be the obvious choice to protect against mechanical damage. you are using romex.
voltage drop - no more than a 3% voltage drop per 100 feet.
you got a length of 75 feet one way this would amount to 150 feet for length.
your choice of #10 is good - fusing at 20 amps.
the remote disconnect need not be fused because the line is protected by the feeder's circuit breaker.
a simple handle type disconnect is ok with no fuses inside.
the wire from the disconnect to the air compressor should be bx cable - again for mechanical protection and the armor would be the equpment ground. 240 single phase does not use a neutral just two hots and an equipment ground.

[dd20399]

i have done everything you have mentioned except the 4 building grds..i have sub box neutral strip seperated and not grd to panel and all recepticals are plastic..with pvc conduit for welder and flex plastic conduit for air compressor...the sub panel wiring comes through the concrete floor and the earth grd was drivin to inside wall before concrete was poured..the only place electrical touchs is the disconnect and without grding the box i should be ok till i find a plastic box that has a switch for 220v..the tin is two tone sides and with the painted stuff i still hesitate on grding the tin,sounds like more for lighting than from electrical power...the stuff would not be a very good conductor...40x30 building...all inside walls are wood..would houses getting metal roofs today have to be grded as well.......thanks for your time and efforts to straighten me out...i read some where lately,that finally made sense to be on neutals on 220v stuff..if it does have 115v circuits in the 220v device,like a cook stove,washer,dryer it needs the neutral..the welders and air comp doesn't need a neutral because it has no 115v in them..thanks again

[HayZee518]

disconnect

The run you have from the sub panel to the compressor, I hope you didn't put the romex inside it. romex cable inside any conduit is a no-no.
it has something to do with heating effect inside the thermal jacket.
for a disconnect switch use the following or anything similar
Square D DU221RB 30A 240VAC Unfused Disconnect
the feeder to the disconnect you said is in pvc plastic. a green stranded wire, #10 should be run with your conductors.
the disconnects equipment ground should be connected to the green ground wire as well as to run a green ground with the circuit conductors to the compressor pressure switch. you mentioned plastic flex - well you got to run an equipment ground with the wires.

[HayZee518]

electrical stuff

might seem like overkill to you, but it may help you if you pick up an NEC code handbook.
other than the regular code book this one has diagrams inside to better explain electrical systems.
expensive, yes, but will relieve any headaches, if you ever get any of your jobs inspected.
the inspectors go by this "bible" for their inspections.

[dd20399]

yes,i did put the romex in conduit from the pressure switch to the disconnect,about 4 ft..from the panel to disconnect is just romex..i'll fix that..your getting good at seeing what i am doing...thanks

[HayZee518]

compressor

don't get me wrong, but short lengths of romex can be installed in short pieces of conduit [any type] for mechanical protection. long lengths is where the no-no comes into play.
Ideally the wires that go into a conduit system are THWN/THHN - This type of wire used to be called TW but with a coating of nylon changes its characteristics to warrant the above designation.
Basically it's a heat resistant thermo-plastic that is gasoline and sunlight resistant.
Printed on its outer jacket is a 600 volt rating, meaning any voltage up to 600 volts can be used with it.
Standard wire gauge sizes apply up to around #8. Anything larger, the designation changes to THW type insulation.