Calculate the electrical dryers load in 208v three phase

[binwork91]

Question: 40 apartment (208v 1phase) feed from a meter bank (208v 3phase). each apartment has electrical dyers with 5000KW. Use standard method to calculate the total load for this 40 electrical dryer in meter bank.

220.54 Electric Clothes Dryers — Dwelling Unit(s). The
load for household electric clothes dryers in a dwelling
unit(s) shall be either 5000 watts (volt-amperes) or the
nameplate rating, whichever is larger, for each dryer
served. The use of the demand factors in Table 220.54 shall
be permitted. Where two or more single-phase dryers are
supplied by a 3-phase, 4-wire feeder or service, the total
load shall be calculated on the basis of twice the maximum
number connected between any two phases. Kilovoltamperes
(kVA) shall be considered equivalent to kilowatts
(kW) for loads calculated in this section. (2008 NEC)


When I try to comply with code above (red text). I get confuse. see below. which one is correct?
1.
5000*40*(0.35-(40-23)*0.005)=53000

2.
40/3=13.3
(13+1)*2=28
5000* 28*(0.35-(28-23)*0.005) /2*3=68250

3.
40/3=13.3
(13+1)*2=28
5000* 40*(0.35-(28-23)*0.005)/2*3=97500

 

[ggunn]

Exam question?

 

[HEYDOG]

It has been a while since I have done this calculation. So I would get it verified by someone else! This would only apply for the service! Using 220.54. 14 Dryers Between A-B. 13 Dryers between B-C. 13 Dryers between C-A
maximum between any two phases is 14 between A-B. 14x2= 28


Table 220.54 states that from 24. - 42 Dryers start with 35% and minus .5% for each Dryer over 23.
28-23 = 5 x .5% = 2.5% 35% - 2.5% = 32.5%

28 Dryers x 5,000 x .325 = 45,500

As I stated……You need to have this verified by a Licensed Electrical Engineer!

 

[electrofelon]

As I stated……You need to have this verified by a Licensed Electrical Engineer!

We need an EE to do a dryer calculation?

 

[david luchini]

Your #2 is correct.

 

[HEYDOG]

We need an EE to do a dryer calculation?

I’m glad that you agree

 

[wwhitney]

Your #2 is correct.

So why the extra 3/2 at the end of the computation? Taken at face value, the red sentence in the OP means you just ignore the other dryers. Which doesn't make a huge amount of sense, but that's what it says.

I would agree with the 3/2 if the red sentence said "the demand factor shall be selected . . ." instead of the "the total load shall be calculated . . ."

Cheers, Wayne

 

[HEYDOG]

You are not ignoring the other Dryers. There is 40 single phase Dryers that are being installed using a three phase system. Keeping the load as equally balanced as possible across all three phases.

 

[wwhitney]

You are not ignoring the other Dryers. There is 40 single phase Dryers that are being installed using a three phase system. Keeping the load as equally balanced as possible across all three phases.

The question is what does "on the basis of twice the maximum number connected between any two phases" mean?

(1) Say you just had 28 dryers all installed on phase A-B. Then the load would definitely be 5000 * 28 * .325 =45.5 kVA, right?

(2) Now you have 14 on A-B, 13 B-C and 13 C-A. The red sentence could be interpreted as "just treat this the same as case 1". 45.5 kVA.

(3) Or it could be interpreted as "calculate the load on one phase conductor for case 1, then apply that load to each phase conductor." Which means multiply by 3/2. 68.3 kVA.

Certainly (3) is a rational method. But (2) is the more straightforward implementation of "on the basis of twice the maximum number connected between any two phases."

Comparing these to the case of 40 dryers on a 2-wire single phase supply, the demand factor is 0.265, so you would end up with 53 kVA. So engineering wise, should changing from single phase to 3 phase increase the dryer demand or decrease the dryer demand?

Cheers, Wayne

 

[jim dungar]

I read the Red sentence as use 2X any single phase connection. Basically your #2 equation.

Based on the OP there are 14 dryers connected single phase, say between A and B. There are also 13 dryers on each of the other two phase connections, say AC and BC.
So you have a total of 27 dryers on any one phase conductor, which is close to the 2X factor in the code

This is easier than having to go through the complex math to combine the single phase values, while being conservative.

 

[HEYDOG]

It has been a while since I have done this calculation. So I would get it verified by someone else! This would only apply for the service! Using 220.54. 14 Dryers Between A-B. 13 Dryers between B-C. 13 Dryers between C-A
maximum between any two phases is 14 between A-B. 14x2= 28


Table 220.54 states that from 24. - 42 Dryers start with 35% and minus .5% for each Dryer over 23.
28-23 = 5 x .5% = 2.5% 35% - 2.5% = 32.5%

28 Dryers x 5,000 x .325 = 45,500

As I stated……You need to have this verified by a Licensed Electrical Engineer!

I need to amend the above. I think that it is correct to divide the 45,500 by 2 to get watts per phase and then take that times 3.
45,500 /2 = 22,750 * 3 = 68,250. I believe that is one of the answers the op had!

 

[wwhitney]

A related question: why is the Red sentence referring only to 3 phase 4 wire supplies? Does that imply it would apply only apply to 3-wire dryers connected L-N-L, or would it also apply to 2-wire dryers connected L-L? If the former, why is it written that way, what difference in the electrical loading justifies the different treatment?

Cheers, Wayne

 

[JoeStillman]

This is why I almost always use the 220.84 method. Even by pretending to have electric ranges when you don't, you come out ahead of all the 220 Part III gobbledy-gook.

 

[binwork91]

This is why I almost always use the 220.84 method. Even by pretending to have electric ranges when you don't, you come out ahead of all the 220 Part III gobbledy-gook.

Optional method 220.84 required to have AC/heater and ranger. it does not work on some buildings.
I like optional method.

 

[Dennis Alwon]

Annex D has an example for ranges which is the same

For 208Y/120-V, 3-phase, 4-wire system,
Ranges:
Maximum number between any two phase legs = 7
2 × 7 = 14.
Table 220.55 demand = 29,000 VA
Per phase demand = 29,000 VA ÷ 2 = 14,500 VA
Equivalent 3-phase load = 43,500 VA
Net Calculated Load (total):
69,150 VA + 43,500 VA = 112,650 Va

112,650 VA ÷ (208 V)(1.732) = 313 A

Don't forget this

 

[wwhitney]

Annex D has an example for ranges which is the same

Thanks! I actually checked Annex D for examples on dryers, but didn't realize 220.55 on ranges has the same text as 220.54 on dryers.

So that makes it clear what it is intended by the Red Sentence in the OP. Basically round up the dryers to a balanced configuration, then base the demand factor on 2/3 of the total, which is the number connected to any given phase conductor.

However, there's still the question of why it specifies a 3 phase 4 wire supply. Seems like the procedure would make just as much sense for 2-wire dryers in a delta configuration on a 3 phase 3 wire supply.

Cheers, Wayne

 

[Dennis Alwon]

Thanks! I actually checked Annex D for examples on dryers, but didn't realize 220.55 on ranges has the same text as 220.54 on dryers.

So that makes it clear what it is intended by the Red Sentence in the OP. Basically round up the dryers to a balanced configuration, then base the demand factor on 2/3 of the total, which is the number connected to any given phase conductor.

However, there's still the question of why it specifies a 3 phase 4 wire supply. Seems like the procedure would make just as much sense for 2-wire dryers in a delta configuration on a 3 phase 3 wire supply.

Cheers, Wayne

Yes but this is for household ranges and dryers. You are probably not going to find those configurations in a dwelling.

 

[Do You Like Ham]

I agree w/ Heydog that this is 68,250

The key for going from the 45,500 to the 68,250 is burried in the bit Dennis Alwon posted from the Annex D5 example:

"Per phase demand = 29,000 VA ÷ 2 = 14,500 VA
Equivalent 3-phase load = 43,500 VA"

*note.. 43,500 = 14,500 * 2... or, 29,000 / 2 * 3.

the load is "based on" the max per phase... but then you need to consider the total load across all three.

 

[JoeStillman]

Optional method 220.84 required to have AC/heater and ranger. it does not work on some buildings.
I like optional method.

I ran a bunch of calcs to see where 220.84 becomes favorable. After about 30-40 units, 220.84 wins the race. it doesn't matter much what kind of cooking or dryer you have. But, as you say, I had to assume all units have air conditioning. YMMV. I ran it for various combinations of gas appliances (All with A/C) and the break point was always just north of 30 units. Here's an example.

 

[binwork91]

Question: Why we use 28 dryer demand factor? total is 40 dryer. The table show "Number of Dryers".
I understand how we get 28, but it kinda confuse me why we use 28 demand factor. no mater how we convert into 3 phase system, it still 40 dryers.