While I was teaching Article 220 and particularly Parts III and IV and Sections 220.40 and 220.80, respectively, questions were raised about the differences in the two methods for service/feeder calculations. Why would theNational Electrical Codeseem to contradict itself by allowing two methods for these calculations? We will explore theCodeand see wherein the answer lies.
First, a brief look at Article 100 for the definition of a dwelling. Three definitions are found, ranging fromone-family,two-familyto amultifamily dwelling. The latter contains three or more dwelling units and will be used in our example. All three types of dwellings listed have the identical minimumNECrequirements. Section 220.12 of Part II requires using Table 220.12 for calculating minimum lighting loads based on a square footage calculation. Section 220.14(J) makes it clear that bathroom branch circuits, and outdoor outlets, basement, and garage loads, along with the lighting loads in 210.70 are included in the general lighting loads and no additional load calculations shall be required.
Standard or General Method
Section 220.52(A) and (B) provides instruction for calculating these circuits at 1500 VA each. Unlike otherCodesections that evaluate a specific volt-ampere for a circuit, 210.11(C)(3) remains silent about a volt-ampere value for bathroom branch circuits. This is where Part III of Article 220, General Method Calculation, also known as the standard method calculation, and Part IV, Optional Method Calculation begin to differentiate from one another.
What may seem like additional loads— such as recess lighting, outdoor flood lighting, track lighting, and multioutlet assembly— are already included in the 3 VA per square foot calculation. Keeping in mind that other branch circuits/calculations that are required for heating, air conditioning, electric cooking, clothes dryer and various other miscellaneous loads are not included in the 3 VA per square foot calculation and are to be independently calculated. For now, we will turn our focus to 220.54 for an electric clothes dryer and use the general method as an example for the calculation effects on the service/feeder and then compare that with the optional method found in 220.84.
With the standard method, the load for a household electric clothes dryer is 5000 watts or the nameplate rating, whichever is larger. In this example, the clothes dryer is single-phase. But what happens when single-phase clothes dryers are connected to a three-phase system? The third sentence of 220.54 states that the load is "calculated on the basis of twice the maximum number connected between any two phases.”
In our example, for multifamily dwellings, we will calculate 13 single-phase household electric clothes dryers and see what effect it has on a three-phase system of 120/208-volts. First, 13 dryers divided by 3 (for three phases, A, B, C) equals 4.333, or rounded to 5. That is to say, that there are 5 on A phase, 4 on B phase, and 4 on C phase. Thus, we have a maximum of 5 dryers connected between any two phases. Now, 5 dryers times 2 (twice per 220.54) equals 10 dryers connected between any two phases. Here is the advantage of a three-phase system as compared with a single-phase system. This is the starting point for determining the total load for the dryers in order to calculate the effect it has on the service/feeder. Rather than calculating 13 dryers, we can use the maximum connected between any two phases, which is 10 dryers.
Table 220.54 contains demand factors for household electric clothes dryers. Using 10 dryers (the maximum number of dryers connected between any two phases) and Section 220.54, which gives us a choice of VA load ratings per dryer (we will use 5000 watts or volt-amperes): 10 dryers times 5000 VA equals 50,000 per VA. Looking at Table 220.54, a demand factor for ten dryers is 50 percent. Thus, 5000 x 10 x 50% = 25,000 VA. Now, dividing this product by two is what will be connected between two phases of the three-phase system: 25,000 ÷ 2 = 12,500 VA per phase load.
Because the dryers are on a three-phase system, we must multiply this by three: 12,500 x 3 = 37,500 VA. Now we can determine the ampacity of our thirteen dryers. By obeying Ohm’s Law and dividing 37,500 by 208 volts, three-phase (208 x 1.732 = 360), 37,500 ÷ 360 = 104 amps dryer load for each ungrounded conductor of the service or feeder. This calculation is in accordance with Part III of Article 220 and Section 220.40. It is permitted by Section 220.61(B)(1) to reduce the neutral conductor by 70% of the phase conductors.
Now let’s look at Part IV of Article 220 and check out the minimum requirements versus Part III of this article. Service or feeder conductors must have an ampacity greater than 100 amps and there are specific voltages indicated. These conductors may serve loads found in 220.82(B), General Loads, and in 220.82(C), Heating and Air-Conditioning Loads. The first two items 220.82(B)(1) and (B)(2) are identical to the standard method in Part III regarding minimum lighting per square foot for the small appliance branch circuits and the laundry branch circuit. No mention of a bathroom branch circuits(s). Section 220.82(B)(3) eliminates demand factors for certain appliances such as ranges, ovens and cooktops, water heaters and electric clothes dryers. The nameplate rating of these appliances shall be used. Of the six items listed under 220.82(C), Heating and Air-Conditioning Loads, only the largest need be applied.
Now that we are in Part IV of Article 220, we must follow the course of action set forth for optional load calculations. Section 220.84(A) and Table 220.84 shall apply when determining the optional method calculation for multifamily dwellings. Connected loads for multifamily dwellings are somewhat identical to 220.82(B) for a dwelling unit.
In our example using 13 electric household dryers, we followed a strict formula to find the ampacity on the service/feeder conductors as described in Part III of Article 220 for the standard method. Now that we are in Part IV of Article 220, we must continue to follow the guidelines as set forth for the optional method. For our example of 13 dryers, Section 220.84(C)(3)c and Table 220.84 shall be used to determine the size of conductors to supply adequate ampacity to the dryers.
Using the nameplate rating of the dryers (5000 watts) times the number of units and the demand factor of Table 280.84 will produce our answer: 5000 VA x 13 x .41 = 26,650 VA. For amps: 26,650 ÷ 360 = 74 amps.
Looking back at our general or standard method calculation reveals 104 amps compared to the optional calculation of 74 amps. The neutral load may be determined by 220.61. There are demand factors for both methods that allow for reduced ampacity and the one with the lowest summation may be used. So, is there an inconsistency in theCode?Perish the thought! But why does theCodegive code users a choice? Let’s take a look at 220.80 and there we find the answer, "…shall be permitted…in accordance with Part IV.” Therefore, it is the electrician’s / designer’s choice to use the optional method in lieu of the standard method found in Part III of Article 220. Section 90.5(B), Permissive Rules, allows for an alternative solution when simple words such as "shall be permitted” are found in a particularCodesection.
In this article, we have looked at two different methods to determine the load calculation of one item (dryers) at a multifamily dwelling. Knowing these two methods can make a huge difference in bidding a job to the installer and in determining code-compliance to the AHJ.
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