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i am a mechanical engineer, and i want to know the electrical data related to the HVAC system. and how to reply to the clients questions
Mini split unit (outdoor + indoor)
outdoor unit is always3 phase
indoor unit may be single or three phase
the outdoor unit required high kw (compressor and fan) so if you have indoor unit in each floor and all the out door unit on the roof you will need high KW cable on the roof (up to130kw for a vila )and small kw cable for each floor
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other type the package unit ( no indoor unit ) every thing on the roof ( higher kw on roof)
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ex : the total area of the building ( all floors ) is800 m2
you will need kw =2000 m2 X60 w/m2 =120 kw
in one sentence go through the load requirements and find suitable cable sizes ,fuse ratings,power outlets.
Even if you work with stamped drawings, you'll eventually need to do commercial load calculations in the field or on a licensing exam. The NEC covers commercial calculations in Art.220, but other articles also apply. For example, you must know the definitions in Art.100, be familiar with what Art.210 says about continuous loads, and understand the overcurrent protection requirements set forth in Art.240.
Two items associated with this type of calculation repeatedly need clarification:
The voltage to use for your calculations depends on the system design voltage. Thus when you calculate branch-circuit, feeder, and service loads, you must use a nominal system voltage of120V,120/240V,208Y/120V,240V,347V,480Y/277V,480V,600Y/347V, or600V unless otherwise specified (220.2) (Fig.1 below).
Refer to200.2(B) to end the rounding mystery. When the ampere calculation exceeds a whole number by0.5 or more, round up to the next whole number. If the extra is0.49 or less, round down to the next whole number. For, example, round29.5A up to30A, but round29.45A down to29A.
Specific loads. Art.220 doesn't cover all specific loads. For example, you'll find motors in Art.430 and air conditioners in Art.440. To know if you should look in another Article, use the NEC index.
<b>Fig.1.</b> Don’t make the mistake of using actual field measurements of system voltage in your calculations. Unless specified otherwise, loads shall be computed using the nominal system voltage such as120V,120/240V,208Y/120V,240V,347V,480Y/277V,480V,600Y/347V or600V.
Art.220 has specific requirements for most loads, including the following:
Dryers. Size the branch-circuit conductors and overcurrent protection device for commercial dryers to the appliance nameplate rating. Calculate the feeder demand load for dryers at100% of the appliance rating. If the dryers run continuously, you must size the conductor and protection device at125% of the load [210.19(A),215.3, and230.42]. Table220.18 demand factors don't apply to commercial dryers.
Let's apply what we've just learned. What size branch-circuit conductor and overcurrent protection does the NEC require for a7kW dryer rated240V when the dryer is in a multi-family dwelling laundry room (Fig.2)?
I=P÷E
7,000W÷240V=29A
The ampacity of the conductor and overcurrent device must be at least29A (240.4). Per Table310.16, a10 AWG conductor at60°C is rated30A. Therefore, you must use a30A breaker with a10 AWG conductor.
<b>Fig.2.</b> When determining proper branch-circuit protection and conductor size for a commercial clothes dryer, you must use a demand load of100%. The reduced demand factors for multiple dryers (Table220.18) don’t apply in a commercial setting.
Electric heat [424.3(B)]. Size branch-circuit conductors and the overcurrent protection device for electric heating to not less than125% of the total heating load, including blower motors. Calculate the feeder/service demand load for electric heating equipment at100% of the total heating load.
Kitchen equipment. Size branch-circuit conductors and overcurrent protection for commercial kitchen equipment per the appliance nameplate rating.
To determine the service demand load for commercial kitchen equipment that has thermostatic control or intermittent use, apply the demand factors from Table220.20 to the total connected kitchen equipment load. The feeder or service demand load can't be less than the sum of the two largest appliance loads. The demand factors of Table220.20 don't apply to space-heating, ventilating, or air-conditioning equipment.
Laundry equipment. Size these circuits to the appliance nameplate rating. You can assume a laundry circuit isn't a continuous load and that commercial laundry circuits are rated1,500VA — unless noted otherwise in the project drawings or exam question.
Lighting. The NEC requires a minimum load per square foot for general lighting, depending on the type of occupancy [Table220.3(A)]. For the guestrooms of hotels, motels, hospitals, and storage warehouses, you can apply the general lighting demand factors of Table220.11 to the general lighting load.
Assume the general lighting load for commercial occupancies other than guestrooms of motels, hotels, hospitals, and storage warehouses is continuous. Calculate it at125% of the general lighting load listed in Table220.3(A).
Receptacles. You don't do all receptacle load calculations the same way. The NEC has separate requirements, depending on the application.
Multi-outlet receptacle assembly. For service calculations, consider every5 feet (or less) of multi-outlet receptacle assembly to be180VA. When you can reasonably expect a multi-outlet receptacle assembly to power several appliances simultaneously, consider each foot (or less) as180VA for service calculations. Normally, a multi-outlet receptacle assembly isn't a continuous load [220.3(B)(8)].
Receptacle VA load. The minimum load for each commercial or industrial general-use receptacle outlet is180VA per strap [220.3(B)(9)]. Normally, receptacles aren't continuous loads.
Number of receptacles permitted on a circuit. The maximum number of receptacle outlets permitted on a commercial or industrial circuit depends on the circuit ampacity. To calculate that number, divide the VA rating of the circuit by180VA for each receptacle strap.
<b>Fig.3.</b> The minimum load for each commercial general-use receptacle outlet is180VA per strap. In this example, the15A,120V breaker could accommodate1,800VA of load (120V x15A =1,800VA). Therefore, you could install a total of10 receptacles on this circuit.
Let's work a sample problem. How many receptacle outlets are permitted on a15A,120V circuit (Fig.3)?
Total circuit VA load for a15A circuit:120V×15A=1,800VA Number of receptacles per circuit:1,800VA÷180VA=10 receptacles
Receptacle sizing. The NEC permits15A circuits in commercial and industrial occupancies, but some local codes require a minimum20A rating (310.5).
Receptacle service demand load. In other than dwelling units, you can add — to the lighting loads — receptacle loads computed at not more than180VA per outlet per220.3(B)(9). You can also add fixed multi-outlet assemblies computed per220.3(B)(8). Both of these must adhere to the demand factors given in Table220.11 or in Table220.13.
Bank and office general lighting and receptacles. Calculate the receptacle demand load at180VA for each receptacle strap [220.3(B)(9)] if the number of receptacles is known, or1VA for each square foot if the number of receptacles is unknown [Table220.3(A) Note b].
Signs. The NEC requires each commercial occupancy that's accessible to pedestrians to have at least one20A branch circuit for a sign [600.5(A)]. The load for the required exterior signs or outline lighting must be at least1,200VA [220.3(B)(6)]. A sign outlet is a continuous load. You must size the feeder load at125% of the continuous load [215.2(A)(1) and230.42].
The following question will allow you to practice what we've just covered. What's the demand load for one electric sign?
1,200VA×1.25=1,500VA
Neutral calculations. The neutral load is the maximum unbalanced demand load between the grounded (neutral) conductor and any one ungrounded (hot) conductor — as determined by the calculations in Art.220, Part B. This means you don't consider line-to-line loads when sizing the grounded (neutral) conductor. What about load reduction? That depends on certain factors, which we'll look at next.
Reduction over200A. You can reduce the feeder/service net computed load for3-wire, single-phase or4-wire,3-phase systems that supply linear loads for that portion of the unbalanced load over200A, by a multiplier of70%.
To see how this would work for an actual installation, determine the neutral demand load for a balanced400A,3-wire,120/240V feeder.
Total neutral load for400A service: First200A at100%:200A×1.00=200A Remainder at70%:200A×0.70=140A Total demand load:200A×140A=340A
Reduction not permitted. You can't reduce the neutral demand load for3-wire, single-phase,208Y/120V or480Y/277V circuits that consist of two line wires and the common conductor (neutral) of a4-wire,3-phase wye system. This is because the common (neutral) conductor of a3-wire circuit connected to a4-wire,3-phase wye system carries about the same current as the phase conductors [310.15(B)(4)(b)].
<b>Fig.4.</b> Sizing the grounded (neutral) conductor can be tricky. Just remember that you can’t reduce the neutral demand load for3-wire, single-phase,208Y/120V or480Y/277V circuits that consist of two line wires and the common conductor (neutral) of a4-wire,3-phase system.
As proof of this theory, see the example in Fig.4.
In addition, you can't reduce the neutral demand load for nonlinear loads supplied from a3-phase,4-wire, wye-connected system, because they produce triplen harmonic currents that add on the neutral conductor. This situation can require the neutral conductor to be larger than the ungrounded conductor load (220.22 FPN2).
Knowing the correct way to do commercial load calculations makes you more valuable because you can play a key role in the field design, inspection, and implementation process. It's one more skill that helps you do the job right the first time.
