Circuit for Replacement HVAC System

[david luchini]

Based on data in OP I don't see why MCA should be any more then 153 amps - and that is with all the single phase blower motors connected to same two lines. Balance them across all three lines and it shouldn't be more then about 144.

I agree. The data in the OP doesn't add up to a unit FLA of 142, either. It looks to me like this is a rooftop A/C unit with 45kw of electric heat. That would give a unit FLA of 142 and MCA of 177, with the heat and supply fan.

 

[kwired]

I agree. The data in the OP doesn't add up to a unit FLA of 142, either. It looks to me like this is a rooftop A/C unit with 45kw of electric heat. That would give a unit FLA of 142 and MCA of 177, with the heat and supply fan.

That would work out - presuming compressors and electric heat never run at same time.

They missed the heaters on the OP's spec sheet if that is what he has though.

 

[MyCleveland]

ALL - you really need to download link doc and start going through

kwired - see doc 36.13(a)(3).....1/.64 = 1.56

topgone - THANK YOU !
Three items from your calc....
(1) From your doc
you are using 36.14 (c) ? for minimum cct amps
why not 36.14 (b) ?

(2) In your FLA calc for each compressor
where does 74% originate from....your doc ?

(3) Math for the 3 single phase fans....7a each
A 208-1P load at 7a, with three balanced across phases
Total amps reflected back are 7 x sqrt 3..... = 12.11
compare with 3 1P loads, added kVA, and converted to 3P ampacity.
agree or disagree

 

[david luchini]

ALL - you really need to download link doc and start going through

kwired - see doc 36.13(a)(3).....1/.64 = 1.56

topgone - THANK YOU !
Three items from your calc....
(1) From your doc
you are using 36.14 (c) ? for minimum cct amps
why not 36.14 (b) ?

(2) In your FLA calc for each compressor
where does 74% originate from....your doc ?

(3) Math for the 3 single phase fans....7a each
A 208-1P load at 7a, with three balanced across phases
Total amps reflected back are 7 x sqrt 3..... = 12.11
compare with 3 1P loads, added kVA, and converted to 3P ampacity.
agree or disagree

With the information given in the original post, the MCA of the unit should be 153, per the UL document, as kwired mentioned (using 36.14(a)).

 

[jumper]

So some equipment lists a Minimum Circuit Ampacity and other equipment has a Maximum Continuous Amperes?
Same abbreviation, different quantity?

Sent from my XT1585 using Tapatalk

I have not ever seen MCA on a nameplate to mean Maximum Continuous Amperes.

440.4(B) Multimotor and Combination-Load Equipment.

Multimotor and combination-load equipment shall be provided
with a visible nameplate marked with the maker’s name,
the rating in volts, frequency and number of phases,
minimum supply circuit conductor ampacity, the maximum rat-
ing of the branch-circuit short-circuit and ground-fault pro-
tective device, and the short-circuit current rating of the
motor controllers or industrial control panel. The ampacity
shall be calculated by using Part IV and counting all the
motors and other loads that will be operated at the same
time. The branch-circuit short-circuit and ground-fault pro-
tective device rating shall not exceed the value calculated
by using Part III. Multimotor or combination-load equip-
ment for use on two or more circuits shall be marked with
the above information for each circuit.

 

[MyCleveland]

Cannot use 36.14(a), equipment has compressor loads
He was NOT saying MCA on nameplate is the MAX CCT AMPS, this relates ONLY to each compressor

You guys are getting 153 MCA from
MAX x 1.25 + others
except you are adding the three single phase loads x 3, which is not correct.

 

[Npstewart]

Oh wow. For some reason, the forum wasnt emailing me replies to this chain like it normally does. I was think the forum went silent and then I come back to 5 pages lol.

FYI, there is a 45.1 kW electric heater in this machine.

In order to figure out the rating of the conductors, im going to have to open up the panel. Maybe they are 90c. The conductors are being routed through an Air Conditioned space through bar joists and then they poke directly through the roof, and into a disconnect mount on the side of the unit.

Also, when I asked the manufacturer to cut the heat back, it had no effect on the MCA / MOCP. Presumebly because the heat is smaller then cooling.

 

[jumper]

For 2 amps I would do what iwire said and ask AHJ/inspector if you can fly with that.

If not, hopefully you have 90C wires and can do the splice thing.

Worst case is trannies like Goldy suggested or new wire.

 

[david luchini]

Cannot use 36.14(a), equipment has compressor loads

Would HAVE to use 36.14 (a) with a compressor, but no heater.

You guys are getting 153 MCA from
MAX x 1.25 + others
except you are adding the three single phase loads x 3, which is not correct.

Why is that not correct?

 

[david luchini]

FYI, there is a 45.1 kW electric heater in this machine.

Figured as much, as that would make the unit FLA and MCA information you provided make sense.

Also, when I asked the manufacturer to cut the heat back, it had no effect on the MCA / MOCP. Presumebly because the heat is smaller then cooling.

That doesn't make sense. Without electric heat the MCA would be 153. Knocking off some kW from the electric heat should lower the MCA.

 

[Ingenieur]

Clg load (2 comp, 3 cond fans, 1 main fan)
47.4 kva
132 A
125% 165 A

htg load (heater, 1 main fan)
51.1 kva
142A
125% 177 A

if the heater could be trimmed to 40 kw
46.1 kva
128 A
160 A

so mca would be determined by clg 165 A

 

[david luchini]

Clg load (2 comp, 3 cond fans, 1 main fan)
47.4 kva
132 A
125% 165 A

htg load (heater, 1 main fan)
51.1 kva
142A
125% 177 A

if the heater could be trimmed to 40 kw
46.1 kva
128 A
160 A

so mca would be determined by clg 165 A

Cooling MCA is 153A

 

[Ingenieur]

Cooling MCA is 153A

how was this arrived at?

kva based
2 x sqrt3 x 208 x 51.3 = 37 kva
1 x sqrt3 x 208 x 16.7 = 6 kva
3 x 208 x 7 = 4.4 kva
total 47.4 kva or 132A
1.25 x 132 = 165

Current based
2 x 51.3 = 102.6
16.7
21 (common phase) or 12 (3 ph distributed)

(141 or 132) x 1.25 = 176 or 165

they must be using the 176 for clg mca
that is why they are saying lowering htg won't help
probably ran 2 legs common to 3 staged contactors rather than a phase to each (cheaper)

 

[david luchini]

how was this arrived at?

kva based
2 x sqrt3 x 208 x 51.3 = 37 kva
1 x sqrt3 x 208 x 16.7 = 6 kva
3 x 208 x 7 = 4.4 kva
total 47.4 kva or 132A
1.25 x 132 = 165

Current based
2 x 51.3 = 102.6
16.7
21 (common phase) or 12 (3 ph distributed)

(141 or 132) x 1.25 = 176 or 165

they must be using the 176 for clg mca
that is why they are saying lowering htg won't help

2.25*51.3 + 16.7 + 21

 

[Ingenieur]

2.25*51.3 + 16.7 + 21

then the mfg is lying to him
htg load > clg load
removing 1 kw would get <175

I'm not sure how they calc clg load
I agree 1.25 x largest plus sum of balance = 153 for nec multiple motor branch, not sure the same is used for rating the unit

I'll look at some units to see

 

[charlie b]

Try this: Show the owner the cost of replacing the conductors, and include the risk factor that existing conditions might result in an even higher cost. Then ask the owner what it would cost to replace the new unit with a different new unit.

 

[Ingenieur]

2.25*51.3 + 16.7 + 21

pg 83 http://www.trane.com/download/equipmentpdfs/rtprc010en.pdf

you are correct
1.25 x largest plus sum of balance

 

[topgone]

Nothing in 36.12 says that the RLA is maximum continuous amps divided by 1.56:?

My bad! Sorry for the typo. What I really meant is 36.13, in particular (a)(3) --> 64 percent of max continuous current. 1/0.64 = 1.56

 

[Mgraw]

So some equipment lists a Minimum Circuit Ampacity and other equipment has a Maximum Continuous Amperes?
Same abbreviation, different quantity?

Sent from my XT1585 using Tapatalk

They are two different things. Minimum Circuit Amps = MCA

Maximum Continuous Amps should be Maximum Continuous Current = MCC. MCC is the maximum continuous current a compressor can run at without tripping the overload.

 

[topgone]

2.25*51.3 + 16.7 + 21

I think there's something amiss there in that calculation. Here's how I see the distribution (USING RLA = FLA):

EQUIPMENT                 PHASE      FLA               PHASE "A"       PHASE "B"         PHASE "C"
COMPRESSOR 1               3          51.3              51.3             51.3             51.3
COMPRESSOR 2               3          51.3              51.3             51.3             51.3
CONDENSER FAN 1            1           7.0              7.0               -                -
CONDENSER FAN 2            1           7.0               -                7.0              -
CONDENSER FAN 3            1           7.0               -                -                7.0
SUPPLY FAN 3               3           16.7             16.7             16.7             16.7
                                          LINE AMPS =  126.3A          126.3A            126.3A

Given those figures above, the Minimum circuit amps:
MCA = 1.25 x 51.3 + (51.3 + 7.0 + 16.7) = 139.12A