Optional Method

[SonofAdam]

Good evening,
What's the rule for doing the optional method for a duplex...? I had notes on it somewhere but i can't seem to find them..

I remember something along the line that you treat it as 3? Any tips appreciated, taking my masters soon

 

[tortuga]

How about an example question to chew on:
A homeowner is adding a 900 SQFT mother-in-law unit to a 1600 SQFT house,
this will be an attached additional dwelling unit (ADU), the structure is now a duplex.
A common service and feeder will be used to supply both units.

A survey of nameplates in the main house show
a 11.7 kw range,
10a 120V dishwasher.
The Main house 240 volt HVAC nameplates are;
A Ductless Heatpump 18.25 RLA, Cadet Wall heater 1.2kw.
There is a 120V 500 Watt bathroom in tile floor warmer.

The ADU loads are:
11.3kw range,
8A 120V dishwasher.
ADU HVAC load is a 240V Ductless Heatpump 13 RLA.

Each unit will have a 240V 4500W electric water heater and full laundry hookups with a 30A 240V electric dryer receptacle.
For the purpose of this calculation the dryer load may be determined by 220.54.
what is the calculated load in VA per 220.85?

 

[wwhitney]

A Ductless Heatpump 18.25 RLA

Surely every heat pump has a fan motor in addition to a compressor, so you need to know more than the compressor RLA. In the simplest case of one fan motor and one compressor motor, no other loads, with compressor RLA > fan FLA, then MCA = 1.25*RLA + FLA. Which means knowing any two of those numbers would suffice. But there are certainly nameplates which list only MCA, RLA, and FLA, and for which MCA - 1.25*RLA - FLA is still positive, so presumably that represents some additional load. Safest to note all the numbers.

Which raises the question of what 220.82(B)(3) means by the "nameplate rating" of such an appliance, as the nameplate has multiple ratings. Should one use the MCA, or just add up all the loads, which would give you in effect MCA - 0.25 * RLA, assuming the RLA is the largest motor.

Note that Part 220.82 has no reference to 220.18 or Article 430, so I do not see any requirement to include an extra 25% of the largest motor. Note also that 220.82(4) in reference to motors not in an appliance has no discussion of including an extra 25% of the largest motor, and also references the nameplate, rather than the Article 430 tables.

Cheers, Wayne

 

[tortuga]

Surely every heat pump has a fan motor in addition to a compressor, so you need to know more than the compressor RLA. In the simplest case of one fan motor and one compressor motor, no other loads, with compressor RLA > fan FLA, then MCA = 1.25*RLA + FLA. Which means knowing any two of those numbers would suffice. But there are certainly nameplates which list only MCA, RLA, and FLA, and for which MCA - 1.25*RLA - FLA is still positive, so presumably that represents some additional load. Safest to note all the numbers.

Which raises the question of what 220.82(B)(3) means by the "nameplate rating" of such an appliance, as the nameplate has multiple ratings. Should one use the MCA, or just add up all the loads, which would give you in effect MCA - 0.25 * RLA, assuming the RLA is the largest motor.

Note that Part 220.82 has no reference to 220.18 or Article 430, so I do not see any requirement to include an extra 25% of the largest motor. Note also that 220.82(4) in reference to motors not in an appliance has no discussion of including an extra 25% of the largest motor, and also references the nameplate, rather than the Article 430 tables.

Cheers, Wayne

Excellent point, I thought about that and I figured rated load current or Rated Load Amps (RLA) to be the nameplate rating for the purposes of load calculations 220.82 / 220.84 / 220.85 like how 440.6 (A) says to use 'rated load current' , as you pointed Minimum circuit Ampacity (MCA) and Maximum Over Current Protection (MOCP) are technically for the electrician size conductors & OCPD etc, not an engineer or master/supervising electrician calculating loads.
Therefore since one can mix and match the outdoor and indoor units a well worded question should include the quantity and nameplate of the indoor units nameplate.
I thought about that also with dryer receptacles, a plan reviewer could say I can't use the 5000W for a dryer receptacle in a 220.84 calc because of the 'firewall' between part III and part IV (the optional calcs), I'd be forced to come up with a with a nameplate (or I could have used the full breaker rating) but 220.84 does not reference 220.54.

 

[wwhitney]

Excellent point, I thought about that and I figured rated load current or Rated Load Amps (RLA) to be the nameplate rating for the purposes of load calculations

Disagree, RLA covers only the compressor, not the whole unit. So it is the compressor nameplate rating, but not the nameplate rating of the entire appliance (outdoor unit).

Cheers, Wayne

 

[tortuga]

Full-Load-Amperage (FLA) to me refers to the motor's rated-current at the motor manufacturers rated-load for that motor and rated-voltage for that motor, however when doing load calculations I assume (and I get your saying this could be a bad assumption) the appliance manufacturer has a known load attached to that motor so RLA in my mind should be the actual shaft at the rated operating load, not what that motor is capable of under another load.

 

[wwhitney]

the appliance manufacturer has a known load attached to that motor so RLA in my mind should be the actual shaft at the rated operating load

That may be, but the outdoor unit has other loads in it beside the compressor, so the compressor RLA is not the nameplate rating of the entire unit.

Cheers, Wayne

 

[tortuga]

That may be, but the outdoor unit has other loads in it beside the compressor, so the compressor RLA is not the nameplate rating of the entire unit.

Cheers, Wayne

Okay how would you suggest wording the question? How about Like this :
A homeowner is adding a 900 SQFT mother-in-law unit to a 1600 SQFT house,
this will be an attached additional dwelling unit (ADU), the structure is now a duplex.
A common service and feeder will be used to supply both units.

A survey of nameplates in the main house show
a 11.7 kw range,
10a 120V dishwasher.
The Main house 240 volt HVAC nameplates are;
A Ductless Heatpump 16.25 RLA,
and 1 FLA fan motor.
the indoor blower unit has nameplate of 1 Amp.

Cadet Wall heater 1.2kw.
There is a 120V 500 Watt bathroom in tile floor warmer.

The ADU loads are:
11.3kw range,
8A 120V dishwasher.

ADU HVAC load is a 240V Ductless Heatpump 11 RLA compressor.
and 1 FLA fan motor.
The indoor blower unit has nameplate of 1 Amp.

Each unit will have a 240V 4500W electric water heater and full laundry hookups with a 30A 240V electric dryer receptacle.
For the purpose of this calculation the dryer load may be determined by 220.54.
what is the calculated load in VA per 220.85?

 

[ggunn]

That may be, but the outdoor unit has other loads in it beside the compressor, so the compressor RLA is not the nameplate rating of the entire unit.

Cheers, Wayne

But doesn't the compressor loading dwarf any other loads in the enclosure by an order of magnitude or more?

 

[tortuga]

So what do you guys get ?

Spoiler: I got

196 Amps Note that the general calc mighe be a lower number