AHU with dual circuit indoor

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hhsting

Senior Member
I have AHU indoor unit Model FV4CNF002 that has dual circuit schedule on plan show following:
208V single phase
MCA: 36.2A
MOP: 60/25A


Plans show only one disconnect at the indoor unit with two separate branch circuits from two separate branch circuit breaker going to same one disconnect.

Two questions:

1. I have never seen this type of unit with dual circuits. Has anyone seen this type of unit? Why dual circuits what is going on?

2. Would it need two separate disconnects at the unit instead of one?
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
I have AHU indoor unit Model FV4CNF002 that has dual circuit schedule on plan show following:
208V single phase
MCA: 36.2A
MOP: 60/25A


Plans show only one disconnect at the indoor unit with two separate branch circuits from two separate branch circuit breaker going to same one disconnect.

Two questions:

1. I have never seen this type of unit with dual circuits. Has anyone seen this type of unit? Why dual circuits what is going on?

2. Would it need two separate disconnects at the unit instead of one?
Pretty typical. Looks like you need an optional accessory for a single circuit application. It's sold this way because the supplemental resistive heating element is optional.
e0625ce5ce4f536fe5b3e3977c224cb8.jpg


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hhsting

Senior Member
Pretty typical. Looks like you need an optional accessory for a single circuit application. It's sold this way because the supplemental resistive heating element is optional.
e0625ce5ce4f536fe5b3e3977c224cb8.jpg


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Ok but on plans one disconnect is provided for two branch circuits see below:

e28148a4095997e98f33f5eff3840266.jpg


Below is the schedule provided on plans 5 th one from bottom MOP 60/25A:

98ae1ae9c099bba1a134a120892baed4.jpg



Does NEC 2017 require two disconnects or one disconnect at indoor unit? Also which MOP belongs to heater and to the unit?
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Ok but on plans one disconnect is provided for two branch circuits see below:

e28148a4095997e98f33f5eff3840266.jpg


Below is the schedule provided on plans 5 th one from bottom MOP 60/25A:

98ae1ae9c099bba1a134a120892baed4.jpg



Does NEC 2017 require two disconnects or one disconnect at indoor unit? Also which MOP belongs to heater and to the unit?
How could you have a single disconnect for two circuits? I suppose you could use a 4 pole disconnect to disconnect all ungrounded conductors on two 240v circuits.

You have two options. 1. Use the single circuit accessory which has overcurrent protection built in, or 2. Run two individual branch circuits. I've never seen the latter done though.

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hhsting

Senior Member
How could you have a single disconnect for two circuits? I suppose you could use a 4 pole disconnect to disconnect all ungrounded conductors on two 240v circuits.

You have two options. 1. Use the single circuit accessory which has overcurrent protection built in, or 2. Run two individual branch circuits. I've never seen the latter done though.

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Designer has available from system 208V single phase Not 240V.

He is running two individual branch circuits from Panelboard called AM to the one disconnect on post #4.

My question is would two disconnects are require if two individual branch circuits are run?
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Also:

MOP = [2.25 x (Rated Current of Largest Motor)] + (Other Motor Loads) + (All Heater Loads)

If you have 2 separate circuits, one for motors and one for heaters, you'll have two MOPs. The larger number would obviously be the heater load(s).

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Designer has available from system 208V single phase Not 240V.

He is running two individual branch circuits from Panelboard called AM to the one disconnect on post #4.

My question is would two disconnects are require if two individual branch circuits are run?
Yes. You have to be able to disconnect all ungrounded wires in the AHU for servicing.

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hhsting

Senior Member
Also:

MOP = [2.25 x (Rated Current of Largest Motor)] + (Other Motor Loads) + (All Heater Loads)

If you have 2 separate circuits, one for motors and one for heaters, you'll have two MOPs. The larger number would obviously be the heater load(s).

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So red rectangle two attachments below I see heater amps column and has dual circuit L1,L2,L3,L4 they dont seem to be one circuit for heater and one for unit? Seems like all four wires seem to share loads between unit and heater. I dont get it do you? Max fuse same way


ac377b8f199f41144b9ed62104121d62.jpg

44a15c5f2d3829582dfbcc6f81c650d1.jpg
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
So red rectangle two attachments below I see heater amps column and has dual circuit L1,L2,L3,L4 they dont seem to be one circuit for heater and one for unit? Seems like all four wires seem to share loads between unit and heater. I dont get it do you? Max fuse same way


ac377b8f199f41144b9ed62104121d62.jpg

44a15c5f2d3829582dfbcc6f81c650d1.jpg
The two numbers are 208/240v the L1/2 is the heater circuit and L3/4 is probably motors, controls, etc.

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
And yes it looks like max fuse is the same situation. 208/240v.

Odd it says 208/230v which is motor rating not standard circuit voltage. Definitely wouldn't apply to the heaters.

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hhsting

Senior Member
The two numbers are 208/240v the L1/2 is the heater circuit and L3/4 is probably motors, controls, etc.

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But it doesnt add up for example the rectangle red mark has 11.3kw hearer at 208V single phase which has FLA of 54.3A.

If I look at L1 heater amps its only 36.2A??? How?

However if I add up L1,L2,L3,L4 amps which is 36.2A+18.1A = 54.3A which is exactly FLA of heater. Seems like shared between all four lines

I dont know what is going on?

Also if you look the second column Min ampacity has one asterisk which at bottom says it takes into account motor blower largest fan used with heater and that too has L1,L2,L3,L4. I dont know largest blower motor size is but if you know then one can cerify that too Seems to share all four lines.

???

It looks like both heater and unit require one dual circuit L1,L2,L3,L4 but then how would disco work if that is true?

I dont know what is going on their at all?
 
Last edited:

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
Actually for the disconnect of the heater I'm not sure that needs to be in sight. Not sure if that falls under art. 424. I know thermostats for baseboards have to have an off position that disconnects all ungrounded conductors.

For safety reasons though it would be bad practice to place a disconnect at that air handler that doesn't disconnect all power sources to the device. I suppose you could post a sign on it saying: warning this device is served by multiple power sources. And perhaps put the circuit numbers too.

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Last edited:

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
But it doesnt add up for example the rectangle red mark has 11.3kw hearer at 208V single phase which has FLA of 54.3A.

If I look at L1 heater amps its only 36.2A??? How?

However if I add up L1,L2,L3,L4 amps which is 36.2A+18.1A = 54.3A which is exactly FLA of heater. Seems like shared between all four lines

I dont know what is going on?

Also if you look the second column Min ampacity has one asterisk which at bottom says it takes into account motor blower largest fan used with heater and that too has L1,L2,L3,L4. I dont know largest blower motor size is but if you know then one can cerify that too Seems to share all four lines.

???

It looks like both heater and unit require one dual circuit L1,L2,L3,L4 but then how would disco work if that is true?

I dont know what is going on their at all?
Interesting. Yeah that is odd. I suppose it's possible. I think maybe they're trying to balance the circuits more maybe.

I do know that circuits larger than 60 amps have to be subdivided. They do instantaneous water heaters that way. Sometimes they take 2 or 3 60 amp circuits. It's to limit fault current should a heating element short out.

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
I asked Super05 from the DIY chat to help out here. He's the foremost HVAC guy there it seems.

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DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
I figured it out. So the heater is rated 11 point something kW right? So that's like 54a at 208. So it's more than likely 2 or more heating elements. So the max circuit size is 60 amps and we can't exceed 80% of that which is 48 amps, right? So they put some of the heaters on the other circuit.

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hhsting

Senior Member
I figured it out. So the heater is rated 11 point something kW right? So that's like 54a at 208. So it's more than likely 2 or more heating elements. So the max circuit size is 60 amps and we can't exceed 80% of that which is 48 amps, right? So they put some of the heaters on the other circuit.

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It looks like they also divided the blowers as well since mini circuit amps column also has L1,L2,L3,L4?? Not sure? How would heater make sense they have single circuit with 11 point something kW as well? They don't divide that?
 

DrSparks

The Everlasting Know-it-all!
Location
Madison, WI, USA
Occupation
Master Electrician and General Contractor
It looks like they also divided the blowers as well since mini circuit amps column also has L1,L2,L3,L4?? Not sure? How would heater make sense they have single circuit with 11 point something kW as well? They don't divide that?
I think because the single circuit module probably has overcurrent/short circuit protection built in? So basically it would be like a feeder to a control panel.

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hhsting

Senior Member
I think because the single circuit module probably has overcurrent/short circuit protection built in? So basically it would be like a feeder to a control panel.

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I don't know. I dont even know what is single circuit module? But whats more confusing is should their be two separate disconnects for dual circuits which is four wires and two EGC at the unit according to Section 440, 422 would be required or not? And which section?
 
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