Circuit Ampacity

Alwayslearningelec said:
How are 8 AWG good for 90A?
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The rules for HVAC, or hermetic compressors, is different than the normal ampacity tables. The MCA of the unit is 50A, so 50A/#8 conductors are used. the Max OCPD is 90A. The 90A breaker is not to protect the wire in this case, it's just for short circuits and ground faults. The compressor has overload protection that will trip out for overloads.
 
Here is another nuance. The schedule you showed stated "max fuse size." That means that fuses not a breaker must be the protection for the motor. Only the conductors from the disconnect to the motor (defined as the branch circuit) can be sized per NEC 430 which allows the #8's. The wire from the breaker to the disconnect are feeders and they must be sized per 240 for feeders.
 
Strathead said:
Here is another nuance. The schedule you showed stated "max fuse size." That means that fuses not a breaker must be the protection for the motor. Only the conductors from the disconnect to the motor (defined as the branch circuit) can be sized per NEC 430 which allows the #8's. The wire from the breaker to the disconnect are feeders and they must be sized per 240 for feeders.
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We've had this discussion before. I'm talking about the branch circuit. I don't remember if we had a consensuses or not.
 
Alwayslearningelec said:
How are 8 AWG good for 90A?
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The 90 amp device is only providing short circuit and ground fault protection. The overload protection if provided at part of the AC equipment. There is little difference in the instantaneous trip points of 50 amp device as compared to a 90 amp device. The instantaneous trip part of the device curve provides the short circuit and ground fault protection.
Short circuit and ground fault protection can only be provided at the supply end of the circuit, but the overload protection can be at any point on the circuit. For this type of installation the overload protection is at the load end of the circuit.
 
Strathead said:
The schedule you showed stated "max fuse size." That means that fuses not a breaker must be the protection for the motor
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The circuit does have 90 amp fuses, but I would have to see the actual equipment nameplate to say only fuses are permitted. The table was from engineering drawings, and it is pretty rare that you find an actual nameplate for new equipment that only shows a fuse as the protective device.
 
Strathead said:
Only the conductors from the disconnect to the motor (defined as the branch circuit) can be sized per NEC 430 which allows the #8's.The wire from the breaker to the disconnect are feeders and they must be sized per 240 for feeders.
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In this application the fuses are in a bucket in a distribution panel, so the feeder is the distribution panel bus. There is a non-fused disconnect at the equipment.
 
Strathead said:
Only the conductors from the disconnect to the motor (defined as the branch circuit) can be sized per NEC 430 which allows the #8's.
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This is not correct. 240.4(G) refers to Article 430 Parts 2-7. Part 2 is "Motor Circuit Conductors" which does not differentiate between branch circuits and feeders. Part 4 is "Motor Branch-Circuit Short-Circuit and Ground-Fault Protection" which covers SCGF OCPD sizing when the conductors are sized per Part 2. And Part 5 is "Motor Feeder Short-Circuit and Ground-Fault Protection" which covers SCGF OCPD sizing when the conductors are sized per Part 2.

So any conductor supplying at least one motor may be sized per Article 430 Part 2 and protected against SCGF according to Article 430 Parts 4 and 5.

Cheers, Wayne
 
augie47 said:
If is nameplate is MCA =50 and MOCP = 90 then he is fine with a 50 amp conductor but if his nameplate MOCP is 70 and he has a 90 at the panel he would need a 90 amp conductor to the disconnect.
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OP says unfused switch at the unit. He needs whatever size fuse that is going to be used/allowed at the distribution board. The thing we don't know, unless I haven't gotten to it yet is what the MOCP is for the unit. 70 to 90 sounds about what one might expect it to say if MCA is 50 though.
 
wwhitney said:
Sorry to have been unclear.


OK, but in this case the OP is not in the field (he's an estimator) and has provided incomplete nameplate information for the unit. So the two primary possibilities are that the unit is marked MOCP 70A or MOCP 90A, and there are two different conclusions for these two cases.

Cheers, Wayne
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So run the specified 2 AWG conductors even though is seems overkill unless it is a really long run, and estimate for fuses shouldn't be too big of a deal, 70, 80, 90, 100 likely all same price in most cases. Let the installers select the proper fuses when they can see what nameplate calls for.
 
don_resqcapt19 said:
The circuit does have 90 amp fuses, but I would have to see the actual equipment nameplate to say only fuses are permitted. The table was from engineering drawings, and it is pretty rare that you find an actual nameplate for new equipment that only shows a fuse as the protective device.
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I agree but it does exist.
 
Little Bill said:
We've had this discussion before. I'm talking about the branch circuit. I don't remember if we had a consensuses or not.
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I believe there were people who chose to ignore or spin the code because they don't like it, and it may be an unintended product of the codes, but...
The definition of a feeder and a branch circuit are pretty clear. The codes we are citing for the conductor size are for branch circuits. If you have a fused disconnect that is the last means of disconnect. The conductors feeding it are a feeder. Find something that says that feeder can be downsized and it can be. I am not aware of a code that says that.
 
Strathead said:
Find something that says that feeder can be downsized and it can be. I am not aware of a code that says that.
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430.62(A) applies to a feeder, and when you apply that rule to a feeder that services one piece of equipment, the feeder and branch circuit conductors end up being the same size.

Note that 440.3(A) that specifically said the rules in 430 apply unless modified by a rule in 440, was deleted from the 2023 code. However it was deleted only because of a style manual rule, not for any technical reason. 90.3 makes the provisions of 430 apply to 440, if 440 has not specifically addressed that issue.
 
Strathead said:
The definition of a feeder and a branch circuit are pretty clear. The codes we are citing for the conductor size are for branch circuits.
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No, they are not. Part II of Article 430, and in particular 430.22, 430.24, and 430.25, applies to any conductor, be it a branch circuit, a feeder, or a service conductor.

Then 430.52 and 430.53 cover OCPD sizing for branch circuits; 430.62 and 430.63 cover OCPD sizing for feeders; and 230.90(A) Exception 1 covers OCPD sizing for service conductors.

Cheers, Wayne
 
Strathead said:
Here is another nuance. The schedule you showed stated "max fuse size." That means that fuses not a breaker must be the protection for the motor. Only the conductors from the disconnect to the motor (defined as the branch circuit) can be sized per NEC 430 which allows the #8's. The wire from the breaker to the disconnect are feeders and they must be sized per 240 for feeders.
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Seeing the backend of how those tables are created, there is a very significant chance that the HVAC intern typing it in does not know the difference between a fuse and a circuit breaker.
 
tjdilly said:
Seeing the backend of how those tables are created, there is a very significant chance that the HVAC intern typing it in does not know the difference between a fuse and a circuit breaker.
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If I were bidding the project and I saw this in the mechanical table, I would assume that this engineering firm is requiring fused disconnects for the equipment, unless it was already being fed from a fused motor control center. So I would be checking the electrical for that, or just go ahead and quote fused.
 
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