Need analysis - 3 phase wiring - a/c condenser

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hardworker

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Commercial 3 phase a/c condenser wiring comes directly off main 3 phase panel without any breaker. This is an old panel that does not have breakers. It only has 1 set of large fuses. This a/c wiring is connected in the main panel after it is fused. It then goes directly from the main panel to a disconnect box on #6. The main and this disconnect are only about 1 foot apart. This disconnect is fused with time delay 60 amp. The load comes off the disconnect on THHN or THWN AWG #8. It runs in conduit about 100 ft to the outside to a non-fused disconnect at the unit.

The condensor has a minimum circuit amp rating of 38.9

What do you think about all of this, especially the #8 carrying the load?
 
First off make sure that the #6 conductors and the fused disconnect meet one of the tap rules in 240.21(B).

Second the #8 THHN/THWN has an ampacity of 50 amps at 75 degrees C so that would be adequate ampacity to supply a MCA of 38.9 amps.

440.4(B) requires manufactures of multimotor/combination A/C units to be marked with a minimum circuit ampacity that is already calculated in accordance with Part IV of Article 440, so it is not necessary to add 125% to the MCA.

Make sure that the 60 amp fuses don't exceed the maximum fuse size marked on the A/C unit.

Chris
 
This unit is about 35 years old. It did not state maximum fuse size on the plate. What about the 60 amp fuses protecting the awg #8 ? Are they not too heavy of an amp rating?
 
hardworker said:
Commercial 3 phase a/c condenser wiring comes directly off main 3 phase panel without any breaker. This is an old panel that does not have breakers. It only has 1 set of large fuses. This a/c wiring is connected in the main panel after it is fused. It then goes directly from the main panel to a disconnect box on #6. The main and this disconnect are only about 1 foot apart. This disconnect is fused with time delay 60 amp. The load comes off the disconnect on THHN or THWN AWG #8. It runs in conduit about 100 ft to the outside to a non-fused disconnect at the unit.

The condensor has a minimum circuit amp rating of 38.9

What do you think about all of this, especially the #8 carrying the load?
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I can't tell how certain things are done but what you mentioned so far sounds OK to me. I understand you have a feeder tap that is about 1 foot long, you did not give all details as to how this tap is connected but sounds like it may be acceptable aside from maybe method of connection - you did not say what size the large fuses are but they can be 10 times the ampacity of the conductor for a tap 10 feet or less - #6 75 degree conductors could be tapped from 650 amp overcurrent device.

You say the unit has a MCA of 38.9 and is fed with #8 THHN/THWN which is good for 40 amps if you have 60 degree terminals or 50 amps if you have 75 degree terminals either way it is more than 38.9 so it is ok.

Marked MCA on these units do not need additional derations because the manufacturer has already included them in the marked values. If you have a high ambient temperature or multiple conductors in the raceway you will need to do these derations to your conductors but not to the MCA.

The 60 amp fuse size will depend on what the unit nameplate calls for overcurrent protection before we determine if it is alright but I am guessing it will be acceptable
 
#8 at 75C is rated 50 amps but I bet it is just 60C so you have 40 amps. 100 feet is a long way to go on #8 for 38 amps but with the VD calculator it is Okay. If the max OCPD calls for 60 aamps then the #8 is fine. Or what Kwired said.....:) Posted at the same time
 
Thanks for all of this, but there is one part I do not understand. Please explain to me, because I want to know.

How can #8 be protected by a 60 amp fuse? It seems to me, it should have a 40 or 50 amp fuse.
 
hardworker said:
Thanks for all of this, but there is one part I do not understand. Please explain to me, because I want to know.

How can #8 be protected by a 60 amp fuse? It seems to me, it should have a 40 or 50 amp fuse.
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Take a look at Part III of Article 440 for sizing the branch circuit short circuit and ground fault protective device.

Basically because the A/C compressor has additional overload protection in the A/C unit itself the breaker or fuse is only providing short circuit and ground fault protection for the A/C unit and branch circuit conductors. That means that the general overcurrent protection rules don't apply and you use Part III of Article 440 to size the OCPD for A/C circuits.

440.22(A) states that the OCPD can be size at 175% of the motor compressor rated load current or branch circuit selection current whichever is greater.

Chris
 
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