Minimum Neutral size?

[tx2step]

I'm going to be running 2 new circuits to 2 new rooftop HVAC package units. They will be fed 240/120 3-phase delta. The a/c guy says they need a neutral, but only for the control circuit. The MCA is 80A & the MOCP is 100A CB. I'm having to go by memory for the moment, but I think I remember that for a circuit like this I think you can downsize the neutral to as small as the ground called for in 250-122? Is that right? Where can I find the requirements in the 2011 NEC? If I'm right, the neutral would be #8 Cu, right?

I can use #4 CU THHN (85A @ 75 deg) for the phase wires + a #8 neutral in 1" EMT, but am thinking of going to #2 Aluminum (90A @ 75 deg). Each run will be about 75' long. I'd have to upsize the EMT to 1-1/4", right? How would you guys usually do this kind of thing? What's your norm, and why?

Other questions I have:
1. Do most of you use the EMT as the ground (EGC), or do you usually install a ground wire? Why?
2. Is there any problem installing the neutral or the EGC (if I run one) in copper & installing the phase wires in aluminum?
3. At what wire size do you guys start thinking of changing from copper to aluminum, to control costs?
4. Are there any applications that you won't use aluminum on? Why?

Thanks in advance!

 

[augie47]

I'm going to be running 2 new circuits to 2 new rooftop HVAC package units. They will be fed 240/120 3-phase delta. The a/c guy says they need a neutral, but only for the control circuit. The MCA is 80A & the MOCP is 100A CB. I'm having to go by memory for the moment, but I think I remember that for a circuit like this I think you can downsize the neutral to as small as the ground called for in 250-122? Is that right? Where can I find the requirements in the 2011 NEC? If I'm right, the neutral would be #8 Cu, right?!

I think you present an interesting question. The "Neutral = same size as equipment ground" comes from 215.2 as best I know and that applies to feeders. Although it may be a good idea, I know of no such requirement for branch circuits. Some one else may.

I can use #4 CU THHN (85A @ 75 deg) for the phase wires + a #8 neutral in 1" EMT, but am thinking of going to #2 Aluminum (90A @ 75 deg). Each run will be about 75' long. I'd have to upsize the EMT to 1-1/4", right? How would you guys usually do this kind of thing? What's your norm, and why?

Other questions I have:
1. Do most of you use the EMT as the ground (EGC), or do you usually install a ground wire? Why?!

75 % of the jobs I see have an actual conductor as well as the conduit.
I think it makes most electricians feel the job is safer and it's often required on drawings I review.

2. Is there any problem installing the neutral or the EGC (if I run one) in copper & installing the phase wires in aluminum?!

Not that I know of

3. At what wire size do you guys start thinking of changing from copper to aluminum, to control costs?!

Again, just from my inspections, on commercial jobs I rarely see AL smaller than #2. Residential #6 is common.

4. Are there any applications that you won't use aluminum on? Why?!

You may find equipment marked "for Cu conductors only".. I find it fairly often on HVAC equipment. I have also seen folks who absolutely won't allow AL on welder circuits.

Thanks in advance!

 

[Dennis Alwon]

I actually wrote a proposal to basically say that the neutral cannot be smaller then the egc on a branch circuit.

 

[augie47]

I actually wrote a proposal to basically say that the neutral cannot be smaller then the egc on a branch circuit.

for which Code cycle ?

 

[Dennis Alwon]

for which Code cycle ?

The next 2014

 

[Smart $]

I actually wrote a proposal to basically say that the neutral cannot be smaller then the egc on a branch circuit.

What if.... :roll:


... at the load end of the branch circuit the manufacturer tapped the supply ungrounded conductor, within the utilization equipment, for the control circuit and ocp'd it to some low value (which is commonly done by manufacturers) ... let's say 5A. Would you still require the neutral to be no smaller than the minimum size EGC for the branch supply ocpd rating?

 

[Smart $]

I actually wrote a proposal to basically say that the neutral cannot be smaller then the egc on a branch circuit.

Another scenario for thought...

You run a 3? 100A circuit in the same conduit as a 20A mwbc and one EGC. What is the minimum size neutral for the mwbc?

 

[augie47]

What if.... :roll:


... at the load end of the branch circuit the manufacturer tapped the supply ungrounded conductor, within the utilization equipment, for the control circuit and ocp'd it to some low value (which is commonly done by manufacturers) ... let's say 5A. Would you still require the neutral to be no smaller than the minimum size EGC for the branch supply ocpd rating?

Thats why I find the question interesting. I guess if you add a "control circuit" OCP device that makes the grounded conductor part of a feeder and 215.2 applies.,.,.... but, what if we call that control circuit fuse "supplemental" ? Now is the grounded part of a branch circuit and 215 does not apply. If the reasons for 215.2 are valid, then it should hold true for branch circuits also. Hopefully Dennis' submittal will eliminate the question.

 

[Dennis Alwon]

Wording that would take some work but the neutral in that case only needs to be as large as the required egc for that circuit.

 

[hurk27]

I'm going to be running 2 new circuits to 2 new rooftop HVAC package units. They will be fed 240/120 3-phase delta. The a/c guy says they need a neutral, but only for the control circuit.

Not to take your post sideways, but you know on a 4-wire delta you will have 208 volts from the "B" phase to the neutral and will fry any 120 volt loads on it? I would make sure the equipment does not have any "control" circuits on the "B" phase.

 

[jumper]

Not to take your post sideways, but you know on a 4-wire delta you will have 208 volts from the "B" phase to the neutral and will fry any 120 volt loads on it? I would make sure the equipment does not have any "control" circuits on the "B" phase.

Been there, did that.......once. Boss wasn't happy.