7.5HP Air Compressor 208V Ground-fault/Short Circuit protection and motor circuit conductors.

J

jklr

Member
Location
Miss
Occupation
Electrician
Hello,

I've got another motor calc that I have done and just want to make sure I am interpreting the code correctly.

We are going to be swapping out a old compressor for a new one. Details below.

Old compressor and circuitry.

5HP
23FLA
40c
RPM 1750
Ser Fac 1.15

#8 thhn on a 40amp qo standard trip beaker.


New compressor
7.5HP
FLA 33
40c
Rpm 1750
SF 1.15

According to 430.248
7.5HP = 44amps @ 208v
430.22 44 x 1.25 = 55Amps
Table 310.16 75c. 6 awg thhn

430.52.... Using Standard trip QO breaker.

Do we use the nameplate for the FLA or 430.248 FLC for the breaker?
If we use FLA then 33 x 300(non time delay)= 99 MAX (100) and 33x1.25 41.25 MIN (45) (per table 240.6)

Or if FLC 44 X 300 = 132 amp MAX
44 x 1.25 = 55 amp min

About a 200 ft run, VD 33amps at 208v 3.08% VD.

Plan on coming out of panel with 3/4 emt then switching over to 6/2 MC above grid ceiling then go up to the roof perlins and strap MC for about 130ft inside the garage then set a box and then go down the wall to disco, then flex over to compressor. I had planned on using a 45 amp breaker if we use fla or a 55amp breaker if we use flc for 430.52.

The motor has its own thermal overload.

Thanks!
 
Motor conductor size and branch circuit short-circuit and ground-fault protection are based on the values found in the Article 430 Tables.

Also the code only uses the term Full Load Current (FLC) and does not use the term Full Load Amps (FLA). However it uses two different FLCs...the one from that table for the sizing of the conductors and branch circuit protection and the one from the nameplate for sizing the overload protection.
 
don_resqcapt19 said:
However it uses two different FLCs...the one from that table for the sizing of the conductors and branch circuit protection and the one from the nameplate for sizing the overload protection.
Click to expand...
Nameplates aren't used for circuit sizing because replacements may have different current requirements.
 
don_resqcapt19 said:
Motor conductor size and branch circuit short-circuit and ground-fault protection are based on the values found in the Article 430 Tables.

Also the code only uses the term Full Load Current (FLC) and does not use the term Full Load Amps (FLA). However it uses two different FLCs...the one from that table for the sizing of the conductors and branch circuit protection and the one from the nameplate for sizing the overload protection.
Click to expand...
Thank you for the clarification!
 
Ok now the kicker. They want to put the new compressor down in the oil change pit. The pit has multiple tanks for holding new and used oil. The only work performed in the pit is oil changes and there is no mechanical ventilation. I am going to start looking into Article 500s, but at first glance it is somewhat confusing determining whether the area is classified class1 div 1 or class 1 div 2 or if any areas are unclassified.
 
jklr said:
Ok now the kicker. They want to put the new compressor down in the oil change pit. The pit has multiple tanks for holding new and used oil. The only work performed in the pit is oil changes and there is no mechanical ventilation. I am going to start looking into Article 500s, but at first glance it is somewhat confusing determining whether the area is classified class1 div 1 or class 1 div 2 or if any areas are unclassified.
Click to expand...
See post 7.

You don't classify the area. Go up the food chain. An AHJ... Anyone but you.
 
jklr said:
430.52.... Using Standard trip QO breaker.
Click to expand...
Just for your edification:
You say you are using a BREAKER, then you are referring to percentages (300%) which is for FUSES (non-time delay). There is a different column in 430.52 for breakers (Inverse Time are standard thermal-mag breakers), calling for 250% max., not 300%.

So 250% of 44A FLC = 110A as the MAXIMUM breaker size, not 132A.

Also note that for the conductor size (125% of Table FLC), that is the MINIMUM, you can go larger (and may need to if there is distance).
But for the breaker size, 250% of Table FLC is the MAXIMUM. You can go smaller if you want to, but going smaller does raise the possibility of nuisance tripping, so I always try to go as large as allowable.
 
Would your standard 60 gallon shop air compressor found at most hardware stores use 430.248? or 430.249?
What would be the conductor size and max OCPD for a 7.5 hp air compressor motor where min/max is not indicated?

Jap>
 
jap said:
Would your standard 60 gallon shop air compressor found at most hardware stores use 430.248? or 430.249?
Click to expand...
249 is for 2 phase power. That is exceedingly rare, there are only a couple of old old locations where is it still in use, as in a few blocks in downtown Philadelphia and a couple of towns in upstate New York. The rest of us can ignore that.

jap said:
What would be the conductor size and max OCPD for a 7.5 hp air compressor motor where min/max is not indicated?
Click to expand...
Min/max? Not sure what you mean by that.

The rules as stated above remain the same, based on the motor nameplate. Note though that you do NOT use the marketing material or stickers on the tank, they lie about the “HP” values. The motor should have a nameplate on it that shows the REAL electrical data, but what they will do to cover up the marketing lie about HP is to show the watts (or kW) value. Watts / 746 = HP (or kW / .746).
 
Jraef said:
The rules as stated above remain the same, based on the motor nameplate. Note though that you do NOT use the marketing material or stickers on the tank, they lie about the “HP” values. The motor should have a nameplate on it that shows the REAL electrical data, but what they will do to cover up the marketing lie about HP is to show the watts (or kW) value. Watts / 746 = HP (or kW / .746).
Click to expand...

What determines when to go by the nameplate or the NEC Table on a standard 1ph Motor?

Jap>
 
wwhitney said:
See 430.6.

Cheers, Wayne
Click to expand...

Yes, but, "other than motors built for low speeds" "Less than 1200 RPM's" "High Torque" or Multispeed motors,,, Wouldn't the Table be the "go to" for most of the common motors you would run into?

Wouldn't the standard Air Compressor motor fall into the "table" category and not the "nameplate" ?

I guess 430.6 A1 exception 3 is what brings you back to using the Nameplate amperage in this instance.

JAP>
 
jap said:
Wouldn't the standard Air Compressor motor fall into the "table" category and not the "nameplate" ?
Click to expand...
A good presumption, but the HP would come from the nameplate rather than the marketing.
jap said:
I guess 430.6 A1 exception 3 is what brings you back to using the Nameplate amperage in this instance.
Click to expand...
Are air compressors listed? If not, then it would not apply.

Cheers, Wayne
 
wwhitney said:
A good presumption, but the HP would come from the nameplate rather than the marketing.

Are air compressors listed? If not, then it would not apply.

Cheers, Wayne
Click to expand...

I wouldn't think your standard air compressor would be considered low speed, high torque or less than 1200 rpm.

Jap>
 
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