(2023) 220.54 and 220.55 Example Calculations

[Abraham V]

I'm looking at the example calculations in the NEC (2023) Handbook.

Here is the example question in 220.54:

Assuming a load of 10 single-phase dryers, each with a nameplate rating of 5500 W, is connected as evenly as possible to the 3-phase system (3 dryers connected between phases A and B, 3 dryers connected between phases B and C, and 4 dryers connected between phases A and C), what is the minimum calculated load?

Solution

Although no VA-rating is provided, the 5500 W may be treated as a VA-rating, according to 220.54 (kVA shall be considered equivalent to kW for loads calculated in this section). Since there is more than one dryer present on this 3-phase, 4-wire system, the total load shall be calculated on the basis of twice the maximum number connected between any two phases, which in this case is four.

4 dryers x 2 = 8 dryers

Refer to Table 220.54 using the calculated 8 dryers; the demand factor is 60%.

5500 VA/dryer x 8 dryers x 0.06 = 26400 VA

The dryers are connected between two phases; find the load on a single phase.

26400 VA / 2 phase loads = 13200 VA/phase load

A three phase system carries this load; find the load on the entire service.

13200 VA/phase load x 3 phase loads = 39600 VA

Find the actual load on each of the feeder conductors.

39600 VA / 208 V x sqrt (3) = 110 A

Where does the 208 V in that last line of calculations come from?

Here is the example question in 220.55:

Calculate the load for a single branch circuit that supplies the following cooking appliances:

• One counter-mounted cooking unit with rating of 8 kW
• One wall-mounted oven with rating of 7 kW
• A second wall-mounted oven with rating of 6 kW

Solution

Table 220.55 applies. Since there is one counter-mounted cooking unit and two wall-mounted ovens, Note 6 applies: add the nameplate rating of the individual appliances and treat this total as equivalent to one range.

8 kW + 7 kW + 6 kW = 21 kW

Now, use Note 1. Determine the amount of rating above 12 kW.

21 kW - 12 kW = 9 kW

According to Note 1, the maximum demand in Column C shall be increased 5 percent for each additional kilowatt of rating or major fraction thereof by which the rating of the individual ranges exceeds 12 kW.

5 %/kW x 9 kW = 45 %

One range corresponds to a maximum demand on Column C of 8 kW. The maximum demand is increased by the percentage found above.

8 kW x 145 % = 11.6 kW ~= 11.6 VA

Calculate the branch circuit load.

11,600 VA / 240 A = 48.3 A = 48 A

Where does the 240 A come from?

Additionally, and maybe I should post this in another part of this form, I think that there are some mistakes in this calculation example (220.55). I think that they need to write 11.6 kVA, rather than just VA. Also, I think that the 240 A should be 240 V.

Thank you in advance for any help. I would have formatted this more nicely, but I couldn't figure out how to include LaTeX on this post. Also, for some reason, when I pasted this text here from the LaTeX document I was working in, I got an error that took me over an hour to figure out. Turns out that I needed to cut the word 'system' out of the first question and paste it back in. Only then was I able to post this. Strange...

 

[wwhitney]

Where does the 208 V in that last line of calculations come from?

An unstated but realistic assumption that the supply voltage system is 208Y/120V.

Additionally, and maybe I should post this in another part of this form, I think that there are some mistakes in this calculation example (220.55). I think that they need to write 11.6 kVA, rather than just VA. Also, I think that the 240 A should be 240 V.

Yes on both counts.

Cheers, Wayne

 

[HEYDOG]

Looks like they didn’t give you the voltages. On the first one it would be a 3 phase 4 wire. 120/208. So phase to phase voltage would be 208.

on the second one the voltage would be single phase 120/240 3 wire. Line to line voltage would be 240 volts.

 

[Abraham V]

An unstated but realistic assumption that the supply voltage system is 208Y/120V.


Yes on both counts.

Cheers, Wayne

Thank you, Wayne. What part of the question statement should tell me that the supply system is 208Y/120V? A three phase supply system could also be 480Y/277, right? I understand that we have to make assumptions but I want to make sure I make them correctly.

 

[Abraham V]

Looks like they didn’t give you the voltages. On the first one it would be a 3 phase 4 wire. 120/208. So phase to phase voltage would be 208.

on the second one the voltage would be single phase 120/240 3 wire. Line to line voltage would be 240 volts.

Thank you very much for taking the time to answer. I would ask you the same questions that I asked in my reply to Wayne about the first question. Additionally, concerning the single phase voltage, what tells me that I need to use 240 V and not 120 V in the calculation? Is it simply knowing that these types of appliances are always connected between the two ungrounded lines? Also, what tells me that this is a single phase circuit at that particular voltage? How do I know that this is the most practical assumption? I'm not doubting your answer, I just want to keep digging until I know why I should make any particular assumption.

 

[HEYDOG]

The two tables 220.54 and 220.55 are for Dwelling units. You well not see 277/480 three phase 4 wire used in Dwelling units.
Electric Ranges in Dwelling units typically operate off of 120/240 single phase some maybe only on 240 volts Single phase. You could also have a large Dwelling unit complex where the utility supplies a 120/208 3 phase 4 wire where the Dryers and the ranges would operate from two phase conductors (208 volt) and the grounded conductor. You would also need to run equipment grounding conductor no matter which voltage is used.

 

[Abraham V]

The two tables 220.54 and 220.55 are for Dwelling units. You well not see 277/480 three phase 4 wire used in Dwelling units.
Electric Ranges in Dwelling units typically operate off of 120/240 single phase some maybe only on 240 volts Single phase. You could also have a large Dwelling unit complex where the utility supplies a 120/208 3 phase 4 wire where the Dryers and the ranges would operate from two phase conductors (208 volt) and the grounded conductor. You would also need to run equipment grounding conductor no matter which voltage is used.

That makes sense. Thank you very much.