dryer

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Dennis Alwon

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Look at the example for ranges in annex D. It is a 3 phase load feeding single phase ranges. Art. 220.54 says

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.

That number is found by taking 40 and divide by 3-- we get 13 and a remainder of 1 so there are 14 ranges connected between any two phases-- this is assuming one is not trying to install them on the same phases.

Now we have 14 ranges but art. 220.54 states we double that number-- hence 28.

We are now working with 28 ranges not 40 except for the initial calculation of 40 * 5000 = 200 kw.

Now we take T. 250.54 for 28 ranges not 40. 35% of 200000= 70000. Now we must also minus .5% for each range over 23. Since we are working with 28 ranges we get 28-23= 5. 5* .5%= 2.5%. 70000 * 97.5%= 68, 250 or 68.25 kw
 

George Stolz

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Windsor, CO NEC: 2017
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Service Manager
I love it....:grin:

I thought you would. :cool:

The big problem is assuming a balanced load.

1. On paper, since they don't spell out a schedule, there becomes a heap of speculation.

2. In practice, keeping that straight would be cumbersome. There is no such thing as a typical service; do we have a main joined with 3-meter modules? 4-meter modules? Five? The jumpers are arranged in a fairly random pattern, it would be prohibitive to ensure that exactly 14 dryers are on the intended phases.

I vote KISS. :)
 

Little Bill

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Tennessee NEC:2017
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Semi-Retired Electrician
I thought you would. :cool:

The big problem is assuming a balanced load.

1. On paper, since they don't spell out a schedule, there becomes a heap of speculation.

2. In practice, keeping that straight would be cumbersome. There is no such thing as a typical service; do we have a main joined with 3-meter modules? 4-meter modules? Five? The jumpers are arranged in a fairly random pattern, it would be prohibitive to ensure that exactly 14 dryers are on the intended phases.

I vote KISS. :)

My question is, the dyers in 220.54 and the table are for "household dryers". Why would anyone have 40 household dryers, unless this is talking about a laundry room for an apartment building?
 
I thought you would. :cool:

The big problem is assuming a balanced load.

1. On paper, since they don't spell out a schedule, there becomes a heap of speculation.

2. In practice, keeping that straight would be cumbersome. There is no such thing as a typical service; do we have a main joined with 3-meter modules? 4-meter modules? Five? The jumpers are arranged in a fairly random pattern, it would be prohibitive to ensure that exactly 14 dryers are on the intended phases.

I vote KISS. :)
If you were in the feild installing this you would defenitly make sure a load like this is at least pretty darn close to balanced.
 

cadpoint

Senior Member
Location
Durham, NC
In general, wouldn't someone try and balance them according to the total amount to be connected?

If you were in the field installing this you would defenitly make sure a load like this is at least pretty darn close to balanced.

Well sure! But how does one handle the sum total of equipment being a 1/3 or 2/3 a greater draw from the equipment?

If it was a Laundry mat, how does one avoid the randomness? Really how does one control any random usuage of equipment? How could one control a total draw of power from one phase verses the other?

I think that they studied the math and have well considered the random usage of equipment at any one time, thus the tables. I beleive that there is a mathmatical study for this term and for these types of equations, but I could be well wrong to state one. :)

We as installers, install to the guide lines given in the Code. But it is required we address the service as it is called out. Were required to account for it by both usage of a circuit and the equipment. We add the other factors there.

Just remember that they don't mention or ask for 125% for the equipment used in the op, when they ask for the sum of units needing math, it's not even called out as a continual load...
 

George Stolz

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Windsor, CO NEC: 2017
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Service Manager
If you were in the feild installing this you would defenitly make sure a load like this is at least pretty darn close to balanced.

Really? You trace out jumpers in the service to see what phases different apartments land on? You would move dryer breakers up or down in a few of the panels and make differing panel schedules for those units?
 

iwire

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Location
Massachusetts
Really? You trace out jumpers in the service to see what phases different apartments land on? You would move dryer breakers up or down in a few of the panels and make differing panel schedules for those units?

I am not really following you here. :confused:

If I was to order modular meter stacks I damn well expect that they will be as balanced as possible.

30 meters would have 10 each A-B, A-C, B-C.

29 meters would have 10 A-B, 10 A-C, 9 B-C.

28 meters would have 10 A-B, 9 A-C, 9 B-C.

The code requires we balance the load, it would really suck if they sent me 30 meters all connected B-C. :grin:
 

Dennis Alwon

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Chapel Hill, NC
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Retired Electrical Contractor
I am not really following you here. :confused:

If I was to order modular meter stacks I damn well expect that they will be as balanced as possible.

Suppose the meters & panels in each apartment were 3 phase. How would you expect to balance that load?
 

iwire

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Staff member
Location
Massachusetts
Suppose the meters & panels in each apartment were 3 phase. How would you expect to balance that load?

For a 30 or 40 unit building I would expect that the engineer would already have done that and the panel schedules would be made accordingly.

Most of our prints arrive with the circuit loading noted.

If left up to me? I don't know, never had to worry about it at that scale. :)
 

George Stolz

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Location
Windsor, CO NEC: 2017
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Service Manager
I am not really following you here. :confused:

If I was to order modular meter stacks I damn well expect that they will be as balanced as possible.

30 meters would have 10 each A-B, A-C, B-C.

29 meters would have 10 A-B, 10 A-C, 9 B-C.

28 meters would have 10 A-B, 9 A-C, 9 B-C.

The code requires we balance the load, it would really suck if they sent me 30 meters all connected B-C. :grin:

I'm gonna have to think about that. Off the cuff, seems to me that all four-meter banks are gonna be either random, or A*B / B*C / C*A / A*B which would probably result in a theoretical higher load on A*B.

I'm not sure if they are random or all the same. I also think about semi-trailers full of service gear, for say a half dozen 40-plexes. I'd imagine they're all the same, rolling off the factory floor.

I don't think anybody would really care, if it was regarding apartment dwelling units, as there are no certainties about the actual load. :)
 

iwire

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Staff member
Location
Massachusetts
I'm not sure if they are random or all the same. I also think about semi-trailers full of service gear, for say a half dozen 40-plexes. I'd imagine they're all the same, rolling off the factory floor.

I don't think anybody would really care, if it was regarding apartment dwelling units, as there are no certainties about the actual load. :)

I would have to match the engineered drawing.


Here is info from Cutler Hammer Modular metering.


C. Moveable Phase Connector
Four-wire horizontal bus lets
you use any 3MM module
for single- or three-phase
incoming service. Shipped
with A and B phase
connection, phase balancing
is easily accomplished in the
field by moving one, front accessible
phase connector
post per module.

Here is GE

Phase connectors are factory installed on the A Phase &
B Phase bus. Remove appropriate phase connectors and
balance for Phase A-B, B-C, or A-C, as needed, across
several meter modules in a complete installation. Torque
all phase connectors to 60 lb-in (6.78 N-m). (See Fig. 4
above).
 
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david luchini

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Location
Connecticut
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Engineer
This is a textbook question. It has nothing to do with the real world. I stick by my answer...:grin:

I agree with Dennis. I don't think the code concerns itself with how many dryers are actually connected to each of the two phases. 40 dryers on a 240/120V system would have a 53kW demand - or a 26.5% demand factor. 40 dryers on a 208/120V system would have a 68.25kW demand - or a 34.1% demand factor.

It seems to me the higher demand factor on a 3 phase system builds in some possibility that the dryers may not be evenly balanced across the phases.
 
Well it seems no of us is correct since the answer is much higher. What does the textbook give as an answer???
Actually, the textbook was lower. It swithched the answer up and gave me va and not kva, admittedly a pretty big mistake on my part. Anyway, the book is still wrong with the answer given as 178,500va.

Has anyone recently passed a Thompson Prometric master exam? What study guide did you use?
 
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