Feeding 2 A/C units from a single home run

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ckelley3

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I am spec'ing a job right now to replace a 220v window unit that currently services the third floor of a ca. 1900 house with a pair of Mitsubishi mini splits. There is currently a single, 12-2 (romex) 20A circuit servicing the window unit that I'd like to reuse, as getting power to the area needed will be a PITA, but I'm not sure I could make it work for BOTH units. If reused, I'd have it feeding a separate FUSED disconnect for each unit fused at 15A. The nameplate data for EACH unit is:

MCA 12A
MOCP 15A
RLA 6.6A
LRA 8.2A
Outdoor fan 0.5A
Indoor fan (powered off the outdoor unit) 0.76A

If it weren't for the MCA being listed, the existing circuit would be sufficient. However, as I read 440.6(A), it looks as though the MCA must be used for sizing, though I'm not sure if it is the number used for sizing loads when multiple pieces of equipment are used. I'm thinking that since the MCA accounts for the 125% of the first motor load, I should be able to add the 12A MCA from the first motor to the 6.6, 0.5, and 0.76 of the second unit (total of 19.86). If I must use the sum of the MCA from both units, then I'm at 24A, which is over capacity for #12 NM-B. What am I missing?
 
ckelley3 said:
I am spec'ing a job right now to replace a 220v window unit that currently services the third floor of a ca. 1900 house with a pair of Mitsubishi mini splits. There is currently a single, 12-2 (romex) 20A circuit servicing the window unit that I'd like to reuse, as getting power to the area needed will be a PITA, but I'm not sure I could make it work for BOTH units. If reused, I'd have it feeding a separate FUSED disconnect for each unit fused at 15A. The nameplate data for EACH unit is:

MCA 12A
MOCP 15A
RLA 6.6A
LRA 8.2A
Outdoor fan 0.5A
Indoor fan (powered off the outdoor unit) 0.76A

If it weren't for the MCA being listed, the existing circuit would be sufficient. However, as I read 440.6(A), it looks as though the MCA must be used for sizing, though I'm not sure if it is the number used for sizing loads when multiple pieces of equipment are used. I'm thinking that since the MCA accounts for the 125% of the first motor load, I should be able to add the 12A MCA from the first motor to the 6.6, 0.5, and 0.76 of the second unit (total of 19.86). If I must use the sum of the MCA from both units, then I'm at 24A, which is over capacity for #12 NM-B. What am I missing?
Click to expand...

To answer the above in red:

your missing 240.4(D) which states that unless listed in E or G we must use the small conductor amperages listed in D, so your almost in luck because 440 is listed in table 240.4(G) which now allows us to use 440 to size our conductors, which points us to use table 310.16 which for NM-B we have to use the 60? column and for your #12 has a rating of 25 amps, but I'm not sure where it is used as a feeder because you still have to fuse each unit at its MOCP, doing this makes it a feeder, I don't see a danger because the odds of both units seeing a overload condition at the same time would be rare except in one case where the power went out and came right back on, this often leads to the unit tripping out on it's over load in the compressor, so this is one possible problem, but also if the run is long, say over 60' which can be even a bigger problem because it would add to the time the the compressor takes to get out of the LRA condition.

But after summing through 440 I don't see anything that will apply to a feeder so in my best judgment I would say that it's is a no-go because of the 15 amp MOCP requirement which makes the circuit home run now a feeder, and feeders are under the 240.4(D) requirement for small conductors, and I don't think 240.4(E) (tap rules) will allow a work around because I don't see an allowance for using 310.16 table over 240.4(D) requirements, and the 12/2 is not a tap it's a feeder.

Maybe someone else might know of a work around?

How about transformer to 480 then back down to 240? about the only way I can see to use the 12/2? or can you find a path up the outside that is not too visible at least then you would be looking at a 12 amp load?

maybe someone else will chime in.
 
I was concerned that this would be categorized as a feeder and therefore be subject to parts of the code that a/c equipment has exceptions to. That's really where I'm looking for a work around.

Btw, all motors on these units are inverter type and there is no large in-rush current. I'd be willing to bet they'd still be fine sizing the breaker for the "feeder" at 20A. I am a bit confused as to how mitsubishi came up with a MCA of 12, given the other numbers. It's almost like they used lra x 125% to calculate it.
 
A feeder is the wiring between a panel and the last overcurrent protective device. So when you install a panel at the end of the 12/2 you have made that wire a feeder. The wires to the unit are branch circuits.
 
hurk27 said:
But after summing through 440 I don't see anything that will apply to a feeder so in my best judgment I would say that it's is a no-go because of the 15 amp MOCP requirement which makes the circuit home run now a feeder, and feeders are under the 240.4(D) requirement for small conductors, and I don't think 240.4(E) (tap rules) will allow a work around because I don't see an allowance for using 310.16 table over 240.4(D) requirements, and the 12/2 is not a tap it's a feeder.
Click to expand...

The feeder would fall under Art 430.
 
430.24 Several Motors or a Motor(s) and Other Load(s).
Conductors supplying several motors, or a motor(s) and other load(s), shall have an ampacity not less than the sum of each of the following:
(1) 125 percent of the full-load current rating of the highest rated motor, as determined by 430.6(A)
(2) Sum of the full-load current ratings of all the other motors in the group, as determined by 430.6(A)
(3) 100 percent of the noncontinuous non-motor load
(4) 125 percent of the continuous non-motor load.
Informational Note: See Informative Annex D, Example
No. D8.
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Here is exmple D8 in Annex D... Conductor ampacity

Example D8
Motor Circuit Conductors, Overload Protection, and Short-Circuit and Ground-Fault Protection (see 240.6, 430.6, 430.22, 430.23, 430.24, 430.32, 430.52, and 430.62, Table 430.52, and Table 430.250)
Determine the minimum required conductor ampacity, the motor overload protection, the branch-circuit short-circuit and ground-fault protection, and
the feeder protection, for three induction-type motors on a 480-V, 3-phase feeder, as follows:
(a) One 25-hp, 460-V, 3-phase, squirrel-cage motor, nameplate fullload current 32 A, Design B, Service Factor 1.15
(b) Two 30-hp, 460-V, 3-phase, wound-rotor motors, nameplate primary full-load current 38 A, nameplate secondary full-load current 65 A,
40?C rise.


Conductor Ampacity
The full-load current value used to determine the minimum required conductor ampacity is obtained from Table 430.250 [see 430.6(A)] for the
squirrel-cage motor and the primary of the wound-rotor motors. To obtain the minimum required conductor ampacity, the full-load current is multiplied
by 1.25 [see 430.22 and 430.23(A)].


For the 25-hp motor,
34 A ? 1.25 = 43 A
For the 30-horsepower motors,
40 A ? 1.25 = 50 A
65 A ? 1.25 = 81 A


Motor Overload Protection
Where protected by a separate overload device, the motors are required to have overload protection rated or set to trip at not more than 125% of the
nameplate full-load current [see 430.6(A) and 430.32(A)(1)].
For the 25-hp motor,
32 A ? 1.25 = 40.0 A
For the 30-hp motors,
38 A ? 1.25 = 48 A


Where the separate overload device is an overload relay (not a fuse or circuit breaker), and the overload device selected at 125% is not sufficient
to start the motor or carry the load, the trip setting is permitted to be increased in accordance with 430.32(C).

Branch-Circuit Short-Circuit and Ground-Fault Protection
The selection of the rating of the protective device depends on the type of protective device selected, in accordance with 430.52 and Table 430.52.
The following is for the 25-hp motor.
(a) Nontime-Delay Fuse: The fuse rating is 300% ? 34 A = 102 A. The next larger standard fuse is 110 A [see 240.6 and 430.52(C)(1), Exception
No. 1]. If the motor will not start with a 110-A nontime-delay fuse, the fuse rating is permitted to be increased to 125 A because this rating does
not exceed 400% [see 430.52(C)(1), Exception No. 2(a)].
(b) Time-Delay Fuse: The fuse rating is 175% ? 34 A = 59.5 A. The next larger standard fuse is 60 A [see 240.6 and 430.52(C)(1), Exception
No. 1]. If the motor will not start with a 60-A time-delay fuse, the fuse rating is permitted to be increased to 70 A because this rating does notexceed 225% [see 430.52(C)(1), Exception No. 2(b)].


Feeder Short-Circuit and Ground-Fault Protection
The rating of the feeder protective device is based on the sum of the largest branch-circuit protective device (example is 110 A) plus the sum of the fullload
currents of the other motors, or 110 A + 40 A + 40 A = 190 A. The nearest standard fuse that does not exceed this value is 175 A [see 240.6 and
430.62(A)].
Click to expand...
 
david luchini said:
The feeder would fall under Art 430.
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Never though about 430 covering the feeder for a motor load, this is the work around that would work:

so 6.6+0.5+0.76= 7.86x1.25=9.825a for the first unit and 7.86a for the second unit, for a 17.685a calculated load on the feeder, so the 12/2 is well within the table 310.16 60? rating for #12 NM of 25 amps for the feeder part even if you have to add the 125% to both motors which would give you 19.65 amps it still will work with the 20 amp OCPD for the feeder, lucky that these units have VFD's for the compressors as if they didn't the LRA would be a problem if power cycled because the head pressure would lock the compressors on a short cycle a regular compressor would have a LRA more likely around 25-35 amps which would have tripped out the 20 amp breaker.

Guess I should not try using my mind at 2:00am

so the OP should be good to go as he planed as long as he can get an inspector to understand all of this:roll:.
 
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But if he uses the appropriate breaker and has both units turned on when he flips the breaker, I would not expect it to hold during startup.
 
GoldDigger said:
But if he uses the appropriate breaker and has both units turned on when he flips the breaker, I would not expect it to hold during startup.
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With the LRA of each unit is only 8.2 amps (VFD driven) so at 16.4 amps+the fan loads of 1.26 amps I don't see a problem, if it wasn't for the soft start then yes the normal LRA would most definitely take out the 20 amp breaker for the feeder.
 
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hurk27 said:
Never though about 430 covering the feeder for a motor load, this is the work around that would work:

so 6.6+0.5+0.76= 7.86x1.25=9.825a for the first unit and 7.86a for the second unit, for a 17.685a calculated load on the feeder, so the 12/2 is well within the table 310.16 60? rating for #12 NM of 25 amps for the feeder part even if you have to add the 125% to both motors which would give you 19.65 amps it still will work with the 20 amp OCPD for the feeder, lucky that these units have VFD's for the compressors as if they didn't the LRA would be a problem if power cycled because the head pressure would lock the compressors on a short cycle a regular compressor would have a LRA more likely around 25-35 amps which would have tripped out the 20 amp breaker.
Click to expand...

You would only need 25% of the largest motor, so...

6.6+0.5+0.76 = 7.86

7.86 * 2 units + 6.6(0.25) = 17.37a.

BUT, I don't see how they get an MCA of 12 for a unit with those motor loads. That would be 2.5A additional load per unit. If that needed to be included then the load current would be 22.37A. I think the 60? rating for #12 has changed to 20A.
 
david luchini said:
BUT, I don't see how they get an MCA of 12 for a unit with those motor loads. That would be 2.5A additional load per unit. If that needed to be included then the load current would be 22.37A.
Click to expand...

I am beginning to wonder myself. I was told in the factory install class from Mitsubishi that the units would run above 100% capacity for a few minutes to bring a room to temperature quickly. I'm wondering if the 12 MCA comes from 125% of the l.r.a. plus the fan loads, as the compressor may be able to run for a short period of time (<30 min) at a level above the "rated" capacity, which I'm guessing is at the r.l.a. In that case, I'd be at 8.2*125%+8.2+0.5+0.5+0.76+0.76=20.97 with a breaker size of 25.

In reality, however, since there is no real surge at startup, the l.r.a. is the most amperage the compressor is capable of. So 8.2+0.5+0.76=9.46. Could the manufacurer have just labeled the unit with an MCA of 12 just to satisfy the listing agency by using the 125% multiplier?

I'm thinking on going to go with the 12-2, breaker at 20, and roll with it.
 
I always assumed the discrepancy in MCA -vs- all the RLA's they give you is internal electronics. The electronics needs some amount of power, and seems to typically be in the 2 to 4 amp range.

What you want to do will most likely work. Is it legal, I don't think so (but perhaps it would have been under the 2011 code when 60C #12 wire had an ampacity of 25A). It sure would be nice if HVAC vendors gave a FLA value too in addition to MCA. You need that when you have multiple units on a feeder or branch circuit and I've never seen one provide it. Perhaps the manufacturer can provide the answer.
 
suemarkp said:
I always assumed the discrepancy in MCA -vs- all the RLA's they give you is internal electronics. The electronics needs some amount of power, and seems to typically be in the 2 to 4 amp range.

What you want to do will most likely work. Is it legal, I don't think so (but perhaps it would have been under the 2011 code when 60C #12 wire had an ampacity of 25A). It sure would be nice if HVAC vendors gave a FLA value too in addition to MCA. You need that when you have multiple units on a feeder or branch circuit and I've never seen one provide it. Perhaps the manufacturer can provide the answer.
Click to expand...

Well, I finished the install today. Fired both units up to full capacity and took a reading at the panel. 9.2a was the total draw for both units. Then, it hit me. Mitsubishi makes a drain pan heater (resistance) as an option for the outdoor unit and I'm betting that's the reason for the MCA being higher than 125% of the compressor plus fan motors. Without the heaters installed, I definitely can defend the #12 wire and 20a OCP. Thank you to everyone who chimed in on this thread. These are the situations that increase everyone's knowledge.
 
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