1-Phase VAV on 3-Ph load

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necnotevenclose

Senior Member
Circuiting question during design. I have multiple VAV boxes that are 277/1 with only heat, no motor, same load. My plan is to group a bunch of them and homerun them back to the panel with a 60A/3P 480/V circuit.

My question is how would the guys out in the field alternate these loads? My belief is the guys would treat this as a load balance issue. Basically carry 3-phase through-out the circuit and alternate equipment on phases. Please let me know what you think.

P.S. When I say guys I'm also including the ladies out there.
 

Smart $

Esteemed Member
Location
Ohio
necnotevenclose said:
Circuiting question during design. I have multiple VAV boxes that are 277/1 with only heat, no motor, same load. My plan is to group a bunch of them and homerun them back to the panel with a 60A/3P 480/V circuit.

My question is how would the guys out in the field alternate these loads? My belief is the guys would treat this as a load balance issue. Basically carry 3-phase through-out the circuit and alternate equipment on phases. Please let me know what you think.

P.S. When I say guys I'm also including the ladies out there.
Why use a 3P breaker... you said they are 277V 1?. Run a full complement multiwire branch circuit "home run" off (3) 1P breakers, and spec which VAV is on which circuit on the drawing.
 

necnotevenclose

Senior Member
Smart $ said:
Why use a 3P breaker... you said they are 277V 1?. Run a full complement multiwire branch circuit "home run" off (3) 1P breakers, and spec which VAV is on which circuit on the drawing.

I guess my justification would be to minimize homeruns on the drawings. I would also think it would be more cost effective to run one large feed instead of (3) 20a feeds.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
necnotevenclose,
If you run a 60 amp feeder, how are you going to proivide overcurrent protection for the equipment?
Don
 

Smart $

Esteemed Member
Location
Ohio
necnotevenclose said:
I guess my justification would be to minimize homeruns on the drawings. I would also think it would be more cost effective to run one large feed instead of (3) 20a feeds.
I think you missed my point :D

Think of it as the same as you described originally except the handle tie remove from the 3P breaker (i.e. use three 1P breakers 60A each). This way if one circuit trips, operation of the other two is not affected. Then simply spec on the drawing which VAV is on which circuit (i.e. put the circuit number beside each VAV symbol).
 

sceepe

Senior Member
necnotevenclose said:
Circuiting question during design. I have multiple VAV boxes that are 277/1 with only heat, no motor, same load. My plan is to group a bunch of them and homerun them back to the panel with a 60A/3P 480/V circuit.

My question is how would the guys out in the field alternate these loads? My belief is the guys would treat this as a load balance issue. Basically carry 3-phase through-out the circuit and alternate equipment on phases. Please let me know what you think.

P.S. When I say guys I'm also including the ladies out there.

Why? What are you saving? A couple of neutral wires? Why take the chance the electrician will put them on the wrong circuit. If so you will be trying the prove your design was correct. Just group as many as you can on single pole 20A breakers.

If your determined to try the multiwire branch approach, for clarity you could show the a phase VAV boxes connected together, the B phase boxes, the the C phase boxes. From the last B phase box, show a homerun to the last A phase box. Same for the string of C phase boxes. Now, from the A phase string show a homeun to the panel and indicate 4 wires. Again all your saving is 2 neutrals and 2 runs of conduit from the closest box to the panel.

Also, if you said you were going to a 60A 3P breaker. How did you come up with this? If you are running 60A wire to every box when all it needs is a 20A circuit, your multiwire branch approach is going to be much more expensive than individual home runs of #12 wire.

Lastly, why fused disconnect at each VAV box? They are not motors. They are purely resistive loads (well there is a tiny motorized damper in there somewhere). Anyway, the ones I've seen have disconnects at the box. If the fused disconnects are required for overcurrent protection because of your multiwire branch approach, you are not going to save anything.

P.S. what does the PE who is signing your drawings and overseeing your work think you should do?
 

sceepe

Senior Member
necnotevenclose said:
I guess my justification would be to minimize homeruns on the drawings. I would also think it would be more cost effective to run one large feed instead of (3) 20a feeds.

I just reread this thread, are you saying that you think if you combine 3, 20A single Pole loads you will need a 3 pole, 60 Amp breaker? Please tell me not
 

boater bill

Senior Member
Location
Cape Coral, Fl.
Try to keep the design as simple as possible. KISS is a great design philosophy. If you use 1ph 20 A breakers with a couple of VAV's on a circuit, the guys installing them will not have a problem understanding your design and intent. If I have to explain my design, it is possible to be misunderstood. Less wiring costs will help the contractor significantly by using #12 instead of #6. Keep them in mind. The owner doesn't care how it is done as long as it will work and doesn't cost more to install.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
As far as the drawing is concerned, when you show a single home run symbol with the three circuits (such as 1,3,5), you don?t have to show whether they are going to a three pole breaker or to three single pole breakers. When I show such things on plans, I am expecting the electrician to run a multi-wire circuit (i.e., 4-wire plus ground). As the run is shown on the floor plans, I mark each item in turn with the circuit I intend to power that item (i.e., this VAV is on circuit 1, that VAV is on circuit 5, etc). I am aware that the installer is likely to alter the order. But it is easy for me to lay it out that way, and it will be balanced, if only on paper. This is my practice whether I am laying out VAVs or receptacle outlets or lights or whatever.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Follow the advice and check with the engineer of record for your job. I think a little lunchtime brownbag session may be in order. It's good to try and think outside the box, as Boater Bill said, dont' make it harder then it is.
 

necnotevenclose

Senior Member
I'd like to say thanks for everyone's replies. The approach I'm going to move forward with is to keep the 60A/3ph load, since a 3-phase load can handle more load then a single phase load I figure for clarity I will show the individual circuits as Charles mentioned. I think this will clearly show the field guys the intent of the design.

Again thanks for the replies.
 

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
NECNOTEVNCLOSE,

I hope you read this post again, because I think you are mistaken in your approach. 3 single phase 20A breakers handle the same as 1-3p/20A breaker. The only savings is you can share the neutral between the 3 lines. 3-20A single pole breakers do not, and I repeat, do not equal 1-3p/60A breaker.

20A=20A no matter how you group it.

Somebody, stop the madness.................................

I would be willing to bet that when this hits the field for construction, the Contractor nails it with a change order, and then we'll be back trying to give advice on how he can get out of the mess.
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
kingpb said:
NECNOTEVNCLOSE,

I hope you read this post again, because I think you are mistaken in your approach. 3 single phase 20A breakers handle the same as 1-3p/20A breaker. The only savings is you can share the neutral between the 3 lines. 3-20A single pole breakers do not, and I repeat, do not equal 1-3p/60A breaker.

20A=20A no matter how you group it.

Somebody, stop the madness.................................

I would be willing to bet that when this hits the field for construction, the Contractor nails it with a change order, and then we'll be back trying to give advice on how he can get out of the mess.

I think he is going to put (3) 20A VAV's on each phase. That is why he is using a 60A circuit. 20A * 3 on each phase.

The fusible disconnects at each VAV will be the most expensive part of this. I would prefer to use 20A branch circuits, and eliminate the fusible disconnects at each VAV.

It's worth finding out if the VAV's have disconnects and/or fuses already on them. If they don't, I would probably just use 20A breakers (or breakers sized for each VAV) and put padlock attachments on the breakers. See 424.19 to see if you need a disconnect at each VAV.
 

necnotevenclose

Senior Member
By utilizing an actual 60A/3P rated breaker and not (3) 20A/1P tied together I'm able to then divide the sum total of all the VAV by 831 instead of 277V. Thus I'm able to add more load. So since I'm utilizing this method and I'm tapping off each unit to feed the next I'm utilizing fused disconnects.

kingpb quote "I would be willing to bet that when this hits the field for construction, the Contractor nails it with a change order, and then we'll be back trying to give advice on how he can get out of the mess."

I'm curious how is this a change order there is no code violation And let's assume everything was clear and shown on the drawings. If the contractor is looking to save money they are probably going to do one of two things just make the changes out in the field and as-built it so that they have more money or RFI the engineer asking if they can make the change at which time the engineer will probably ask how much credit the owner will get back?
 

iwire

Moderator
Staff member
Location
Massachusetts
You are making it a major pain in the rear and costly to wire.

I would have to run a 6/4 cable or 6 AWG in conduit to each location.

Than I have to figure out a way to tap this feeder into each fused disconect at each location. More material more labor.

Don't reinvent the wheel run 20 amp circuits to each unit and forget the large cable and fused disconnects.

You intended way will use more copper than multiple 20 amp circuits would.

JMO, Bob
 
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necnotevenclose

Senior Member
iwire said:
You are making it a major pain in the rear and costly to wire.

I would have to run a 6/4 cable or 6 AWG in conduit to each location.

Than I have to figure out a way to tap this feeder into each fused disconect at each location. More material more labor.

Don't reinvent the wheel run 20 amp circuits to each unit and forget the large cable and fused disconnects.

You intended way will use more copper than multiple 20 amp circuits would.

JMO, Bob

I have a hard time believing that you would rather run more 12/4 or #12 AWG in conduit and use more circuit breakers and more spaces in the panel than one run of 6/4 or 6 AWG in conduit with one 60A/3P circuit breaker. I agree that using the fused disconnects and additional taps may be more costly, but will this really increase the labor cost? Also can you explain "Than I have to figure out a way to tap this feeder into each fused disconect at each location".

My thought on how this would actually get wired in the field is the electrician would provide a main branch line with 6 AWG in conduit or MC then a tap would be made at each VAV box after that tap #12 could then be used as long as it is within 25ft.

Thanks again to everyone for their replies. Please keep them coming!
 
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