Posted: 9/27/2010 12:51:56 PM EDT
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I need to wire up my new air compressor. The manufacturer recommends a 40 amp breaker (seems I could get by with a 30 amp but that's what they recommend), which means I need to run 8/3. Manufacturer's sheet says that it has running amps of 24 and start up of 60 amps.
Here's the specs on the 5hp motor: V 208-230 A 23.50 - 21.50 SFA 27.10 SF 1.25 I have a run of 20' and then a run of 28' from the panel. I can either go to conduit after the 20' run or go up a gable and back down. At the end, there would be a 4-5' run of conduit down to the compressor either way. I was going to end with a receptacle and use a 50amp dryer plug and cord to connect the compressor to the electrical service. I'd prefer to go up the gable for aesthetic reasons but can live with the conduit if necessary. My question(s) are: A. When I switch to conduit, do I (can I) use stranded 8/3 THNN (that's what I saw on the spool at Lowes) or should it be solid wire? B. In the panel, do I just cap the neutral of the 8/3 or connect it to the neutral bus? C. For the switch on the compressor, they say to "hook one hot lead to 1L1 lug, hook other hot lead to 3L2 lug". Looking at the switch, which lugs are they talking about? Ground would go to ground. D. What size conduit to use? 1/2" or 3/4" E. Should I get a 4-wire range cord and just cap the neutral at the compressor? Or save some money and use a 3-wire? 
Compressor switch cover info. 
Compressor switch right-side with wires coming from magnetic starter. 
Compressor switch left-side 
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Read the NEC article on elctric motors, Articele 430.
The breaker size and wire size do not match the same way as in other circuits. 120% of the mortoprs full load running current determines the wire size. The breaker is than allowed to be increased to allow th emotor to start (up to 350% of the running current). The motor contains thermal overload oprotection and cannot overload the feeder. Watch out for voltage drop at starting current though. It can prolong the starting time enough to cause long term damage., especillay if the er are a lot of on/off cycles. The motors horsepoer is used to determine the running current using an NEC table, NOT the motors nameplate current. |