Feeder vs Branch Circuit?

[anothaparson]

The customer has replaced a 5 HP Air compressor with a 7.5 HP and after wiring it up he wasn't sure why the breaker kept tripping.

He has 2 Air compressors side by side with seperate Disconnect with 2-pole 30a circuit breakers. They are fed with seperate conductors in a 3/4" EMT about 100' between the Main Panel and the disconnects. The DM's are within 3'.

The Main Panel is single phase @ 240v.
The new Air compressor has a service Factor 1.15. and a FLC = 40a

I went through all my calculations and determine that inorder to keep within a 3% Voltage drop and 80% derating of between 4-6 cond. in a conduit that I would need to size my conductors at #6 THHN.

I want to size my Overcurrent Protection.

Are the conductors from the Main Panel considered the feeders and would the disconnect be considered the Branch Circuit?

The original installation had a 2-pole 30a circuit breakers at the panel and a 2-pole 30a CB at the disconnect wired with #10 THHN and the Air compressor lasted 15 years.

Voltage drop of 100' distance in this installation is acceptable but not when include 4 current carrying conductors in the same raceway.

I feel like i'm over thinking this and I should just remove the 1 of the #10 THHN use the other as an equipment ground and pull in 2 #6 THHN and replace the 2-pole 30a breakers with 2-pole 40a Circuit breakers.

Thank you,
Arthur

 

[mcclary's electrical]

The customer has replaced a 5 HP Air compressor with a 7.5 HP and after wiring it up he wasn't sure why the breaker kept tripping.

He has 2 Air compressors side by side with seperate Disconnect with 2-pole 30a circuit breakers. They are fed with seperate conductors in a 3/4" EMT about 100' between the Main Panel and the disconnects. The DM's are within 3'.

The Main Panel is single phase @ 240v.
The new Air compressor has a service Factor 1.15. and a FLC = 40a

I went through all my calculations and determine that inorder to keep within a 3% Voltage drop and 80% derating of between 4-6 cond. in a conduit that I would need to size my conductors at #6 THHN.

I want to size my Overcurrent Protection.

Are the conductors from the Main Panel considered the feeders and would the disconnect be considered the Branch Circuit?

The original installation had a 2-pole 30a circuit breakers at the panel and a 2-pole 30a CB at the disconnect wired with #10 THHN and the Air compressor lasted 15 years.

Voltage drop of 100' distance in this installation is acceptable but not when include 4 current carrying conductors in the same raceway.

I feel like i'm over thinking this and I should just remove the 1 of the #10 THHN use the other as an equipment ground and pull in 2 #6 THHN and replace the 2-pole 30a breakers with 2-pole 40a Circuit breakers.

Thank you,
Arthur

From the panel to the disconnect is a feeder. From the disco to the unit is a branch circuit

 

[anothaparson]

Thank you for your response!

 

[jdsmith]

The customer has replaced a 5 HP Air compressor with a 7.5 HP and after wiring it up he wasn't sure why the breaker kept tripping.

He has 2 Air compressors side by side with seperate Disconnect with 2-pole 30a circuit breakers. They are fed with seperate conductors in a 3/4" EMT about 100' between the Main Panel and the disconnects. The DM's are within 3'.

The Main Panel is single phase @ 240v.
The new Air compressor has a service Factor 1.15. and a FLC = 40a

I went through all my calculations and determine that inorder to keep within a 3% Voltage drop and 80% derating of between 4-6 cond. in a conduit that I would need to size my conductors at #6 THHN.

I want to size my Overcurrent Protection.

Are the conductors from the Main Panel considered the feeders and would the disconnect be considered the Branch Circuit?

The original installation had a 2-pole 30a circuit breakers at the panel and a 2-pole 30a CB at the disconnect wired with #10 THHN and the Air compressor lasted 15 years.

Voltage drop of 100' distance in this installation is acceptable but not when include 4 current carrying conductors in the same raceway.

I feel like i'm over thinking this and I should just remove the 1 of the #10 THHN use the other as an equipment ground and pull in 2 #6 THHN and replace the 2-pole 30a breakers with 2-pole 40a Circuit breakers.

Thank you,
Arthur

Maybe I'm asking a stupid question, but in this case why does it mater if you call it a feeder or a branch circuit? What's the electrical issue?

 

[LarryFine]

Arthur, welcome to the forum!

I bet you could run a single set of conductors in that conduit, feed a sub-panel at the load end, and supply the two units that way.

 

[Dennis Alwon]

Interestingly, based on the code wording, the wire from the panel to the disco is a feeder ad from the dico to the a/c is a branch circuit. Now, make the disco a nofused disco (no OCPR) and the entire circuit is a branch circuit.

 

[kwired]

The customer has replaced a 5 HP Air compressor with a 7.5 HP and after wiring it up he wasn't sure why the breaker kept tripping.

He has 2 Air compressors side by side with seperate Disconnect with 2-pole 30a circuit breakers. They are fed with seperate conductors in a 3/4" EMT about 100' between the Main Panel and the disconnects. The DM's are within 3'.

The Main Panel is single phase @ 240v.
The new Air compressor has a service Factor 1.15. and a FLC = 40a

I went through all my calculations and determine that inorder to keep within a 3% Voltage drop and 80% derating of between 4-6 cond. in a conduit that I would need to size my conductors at #6 THHN.


I want to size my Overcurrent Protection.

Are the conductors from the Main Panel considered the feeders and would the disconnect be considered the Branch Circuit?

The original installation had a 2-pole 30a circuit breakers at the panel and a 2-pole 30a CB at the disconnect wired with #10 THHN and the Air compressor lasted 15 years.

Voltage drop of 100' distance in this installation is acceptable but not when include 4 current carrying conductors in the same raceway.

I feel like i'm over thinking this and I should just remove the 1 of the #10 THHN use the other as an equipment ground and pull in 2 #6 THHN and replace the 2-pole 30a breakers with 2-pole 40a Circuit breakers.

Thank you,
Arthur

Questions?

Are the 5 and 7.5 hp compressor motors actually NEMA standard motors or are they special purpose rated motors? Many air compressors will state an inflated horsepower when if you look at the motor and the actual current rating of it it does not match up to what that horsepower would normally draw on a regular motor - it is actually a limited on demand horsepower rating and not a continuous rating. These motors you need to size conductors according to their namplate amperage and not the tables in 430.

If they are motors that fall under T430.248 then 5 hp = 28 amps x 1.25=35 minimum ampacity of supply conductors. Even if you can use 75 degree conductor ampacity your 10AWG is too small if you have more than three current carrying conductors in the raceway on the original install, and the 7.5 hp can not be on a 10AWG even if it is the only circuit in the raceway.

As for the breaker tripping, he may be lucky that the 5 hp motors held during starting. I usually put 5 hp single phase motors on at least 50 amp breaker to ensure they do not trip during starting, you could easily put them on a 70 amp breaker, any larger than that would require the 70 amp breaker not being able to hold during starting.

Number of conductors in the raceway will not effect voltage drop. Actually it will some, because of temperature, but that is not a variable I have ever seen in a voltage drop calculation normally used for field purposes.

If you wish to install a feeder for a 5 and 7.5hp motor, you need a minimum feeder ampacity of 78 amps (40 x 1.25 + 28 = 78), if you are able to use 75 degree conductors you would need minimum of 4AWG. The feeder breaker could be 125 amps - see 430.62.