Wiring 2 5 HP air compressors

[md21722]

Hi,

I picked up 2, 5HP air compressors and am looking to wire them. Per NEC, 28A FLC wire for 125% of FLC. At first I thought of using #10 THHN for each since its rated for 35A @75C and 40A @90C. However, then I started reading that the maximum overcorrect protection device is 30A for #10 "the small wire rule" and also takes into account circuit breakers and other terminals that may not be rated for 90C.

I was also thinking of feeding both off a sub panel using #6 THHN wire 65A @75C and 75A @90C.

So my questions are (a) is the overcurrect protection 240.4(D) not applicable to motor circuits? and (b) is it acceptable to use the 75C rating for the wire?

Then lets say I decide to change the motors out later for 7.5HP. 40A FLC wire for 125 of FLC = 50A. Would that be #8 THHN with #4 to the sub panel?

My owners manual says for 75C copper 30C ambient,
for 5HP use #8 for a single compressor and #6 for a duplex
for 7.5HP use #6 for a single compressor and #4 for a duplex

It isn't clear to me why they call for #8 and #6 on the single compressors but they don't specify wire type. Perhaps they are just providing the lowest common denominator. And wire "can't be too big".

 

[MTW]

The basic rule is under 100A wire must be used at the 60C rating. Over 100A the 75C rating can be used. For the 75C rating to be used, all terminals and devices in the circuit must be marked and listed for that temperature. The 90C rating must never be used for actual rating use, it's purpose is for use in derating, for where wire runs through a elevated temperature or multiple conductors in a conduit.

For motor circuits only. Article 430 takes precedent over 240. Wire is rated at 100%, breakers and fuses on the other hand are only rated for 80% at continuous load. Therefore you must size the protective device at 125% of the continuous load.

MTW Ω

 

[pattenp]

The under 100A or over 100A rule for temperature selection is based on terminal temperature rating of device and is not a basic rule to select wire by. Most if not all newer devices are rated 75 deg. C.

 

[wyliesdiesels]

5HP motor needs 35a rated wire- #10 THHN or #8/2 NM-b.

7.5HP motor needs 50a rated wire- #8 THHN or #6/2 NM-b.

240.4(d) doesnt apply to motor circuits.

Do u plan on running them at the same time?

Yes u can use the 75* c ampacity rating for THWN. NM-b wire is limited to 60* c ampacity.

 

[md21722]

Thanks! Yes I plan on running them at the same time. Does that mean I need #4 to the sub panel because 28 x 125% x 2 = 70A. #6 @75C is only good for 65A?

In the duplex setup, I believe they follow the two motor rule 28A x 125% + 28A = 63A and wire #6 to the line side of both magnetic starters. I could just run one large circuit, #6, to both magnetic starters and then #10 from the load side of mag starter to the motor?

 

[md21722]

Thanks! Yes I plan on running them at the same time. Does that mean I need #4 to the sub panel because 28 x 125% x 2 = 70A. #6 @75C is only good for 65A?

Can anyone chime in here? I'm not too familiar with sizing a us panel for this. Do I need #4 if the compressors go on separate circuits?

 

[md21722]

Duplicate

 

[matt_i]

Seeing as the moment you walk into the shop and call for air, both motors are instantly going to kick on and run until the pressure switches are satisfied. I would feel more confident about using 125% of (28A * 2) =70A putting you out of #6 territory. Or, just wire them separately which is probably going to be cheaper if you size the conduit for 2 circuits' worth of wire and pull THHN down the "trunk" with a single spool of #10.

I have to ask: Do you really have a need for that much air? If these are going to be "ready to run" 24x7 then my theory above doesn't apply.

 

[md21722]

The #10 is cheaper for sure, but takes up 2 extra slots in panel that doesn't have much room to spare. After a bunch more reading online, I believe the answer is yes, you can use #6 either as a feeder to two branch circuit breakers/magnetic starters or you can wire both magnetic starters with #6.

matt_i, I leave the tanks full and switch off the compressors when I'm not using them. I have some tools that really **** down the air. Plus air compressors are downrated for here because of the altitude. I wouldn't mind another 10-20 HP to keep run times lower ;-) If you look at some of the other threads on GJ you will see folks building duplex 7.5HP compressors and stuff like that.. It seems as our cars and trucks have become more powerful, so have our air tools and so they require bigger compressors.

 

[md21722]

On another note, can I use #10 SOOW from the wall mounted disconnect to the compressor and on the compressor or must I use alflex or steel flexible with THHN inside? I've been using SOOW for shop use for the past 30 odd years (along with BX cable). Its tough stuff. I dare you to drive a nail through it but that doesn't mean the NEC likes it.

 

[wyliesdiesels]

Cordage is not to be used in place of permanent wiring so no.