Tap Conductors

[Rpack22]

Currently doing a job at a middle school where the electrical engineer has listed taps from a 100 amp breaker from a MSB. Load is 3 different condensing units all fed from their own disconnects, one alone on a 40 amp ocpd, the other two (50A and 40A MOP) are being tapped off of a 100 amp breaker. The EE has listed that #6s shall be used for the tap conductors and it shall not exceed 10 feet. In my mind this is wrong because the total calculated load is 90A (240.21(1)(a)) there for if it’s under 10 feet of tap conductor the wire would have to be sized to 90A. If it were over 25 feet, it could be sized according to their respective disconnecting fuses (50A and 40A).

The way I see it being done is; feeders are #3, tap conductors not over 25 feet but over 10’ are #6 for 50 amp fuses and #8 for 40 amp fuses landing in their respective disconnects. Does this sound about right?

 

[infinity]

What is the MCA for each unit?

 

[Rpack22]

Correction, I think it should be a #1 feeder with #6 (50A disconnect) and #8 (40amp disconnect) all taps over 10’.

 

[Rpack22]

What is the MCA for each unit?

MCAs for the tapped loads are 40 and 34

 

[infinity]

Interesting question. The taps are over 10' so the conductor size is a minimum of 1/3 of the 100 amp OCPD. That's means a tap conductor of at least 33 amps to each switch. The MCA's are 40 amps and 34 amps so the conductors need to be at least 40 amps and 34 amps. What is the MaxOCPD size for each unit?

 

[Rpack22]

Ahhhhh it seems I misread the 240.21(1)(a). The total calculated load on the circuits supplied by the tap conductors is not 90. It’s the individual branch circuit fed by the taps so basically 50 and 40 on separate taps. Cool.

So 10’ or more up to 25’ remains the same in regards to this application. Should be #1 feeder and the taps should be #6(50A) and #8(40A). This is the way I see it being done. Let me know if I’m right lol

 

[jaggedben]

The description is not very clear. What conductors are coming off the 100A breaker and where are the taps being made exactly?

That said, I don't think 10ft tap conductors are ever required to be larger than 25ft tap conductors; it's the opposite.

 

[Rpack22]

Interesting question. The taps are over 10' so the conductor size is a minimum of 1/3 of the 100 amp OCPD. That's means a tap conductor of at least 33 amps to each switch. The MCA's are 40 amps and 34 amps so the conductors need to be at least 40 amps and 34 amps. What is the MaxOCPD size for each unit?

MOP is 50A and 40A

 

[Rpack22]

The description is not very clear. What conductors are coming off the 100A breaker and where are the taps being made exactly?

That said, I don't think 10ft tap conductors are ever required to be larger than 25ft tap conductors; it's the opposite.

Currently on the plans from the EE it shows a #2 feeder with #6 taps not to exceed 10’. I think the way I listed it above (with taps outside of the switchgear that supplies them) is the way to go. Took some revising but I got there haha

 

[infinity]

Currently on the plans from the EE it shows a #2 feeder with #6 taps not to exceed 10’. I think the way I listed it above (with taps outside of the switchgear that supplies them) is the way to go.

That sounds code compliant. Are the taps landing in a fused disconnect or a circuit breaker? The tap conductors could be sized for the MCA of each unit but then the OCPD would need to be sized for the MCA also which may cause other problems.

 

[jaggedben]

The #2 (assuming copper) is not a tap, because it is protected by the 100A breaker. In fact with copper it could be #3. It is also evidently sufficient for the calculated load.
The individual taps with 6awg are compliant landing on 40 and 50 amp overcurrent devices.

 

[Rpack22]

That sounds code compliant. Are the taps landing in a fused disconnect or a circuit breaker? The tap conductors could be sized for the MCA of each unit but then the OCPD would need to be sized for the MCA also which may cause other problems.

How can I be a #2 feeder since the OCPD is 100A or less?

 

[infinity]

How can I be a #2 feeder since the OCPD is 100A or less?

#2 THHN has an ampacity of 115 amps @75° C. If all of the terminals are rated for 75° C it's a 115 amp conductor. #3 would be 100 amps under the same conditions.

 

[jaggedben]

#2 THHN has an ampacity of 115 amps @75° C. ...

As long as we're talking about copper. Aluminum is 90A.

 

[Rpack22]

#2 THHN has an ampacity of 115 amps @75° C. If all of the terminals are rated for 75° C it's a 115 amp conductor. #3 would be 100 amps under the same

The way I’m seeing it;

75 degree system
#3 feeder with #6 and #8 taps

60 degree system
#1 feeder with #6 and #8 taps

 

[infinity]

The way I’m seeing it;

75 degree system
#3 feeder with #6 and #8 taps

60 degree system
#1 feeder with #6 and #8 taps

Even at 60° C #2 would have an ampacity of 95 amps. Next size up rule would allow the 100 amp OCPD.