apartment complex - cloths dryer load calculation?

[Ragin Cajun]

220.54 says the load for each dryer shall be 5000w or the nameplate rating, whichever is greater.

That's what I get from trying to look at the issue as an engineer and logically analyze it. Problem is the NEC is not always logical. But - don't get me started on that!


Does any one on the forum work for a POCO? They use actual historical demand data to size services and the data have been proven over the years. Would love to see the numbers. I'm sure you have noticed the difference between NEC service sizes and the POCO's. Yes, I know, they have the ability to change a service out should they run into issues, but I personally haven't run into this. But, that's another thread all by itself.


RC

 

[JDBrown]

IMO the Code as written doesn't make sense. First, we have no empirical data upon which to base any demand factors for a single phase system let alone determine different demand factoring where the same number of identical single-phase dryers are supplied by a 3-phase, 4-wire feeder or service.

Without supporting evidence, the demand for multiple single phase dryers on a two-conductor system should be the same as on a three-conductor system. The only difference that should actually be accounted for is the issue brought up by Ragin Cajun (i.e. kW at 240V vs. 208V).

To say the demand factor for the same number of identical dryers on a three phase system is higher than on a single phase system is ludicrous from the perspective of statistical analysis.

Let's take just the bold part above. While we may not have empirical data to support the demand factors, we do know that they work out in real life. Have you ever heard of an apartment building's main breaker tripping because too many tenants ran their dryers at the same time?

That's anecdotal, not empirical, but now I'm curious if anyone has ever run into that situation (it seems like the sort of thing somebody would have heard about if it were at all commonplace). The NEC is intentionally written to be conservative, and that includes the demand factors. In the absence of hard empirical data, choosing to just go with what has worked reliably in the field for years seems like a pretty good decision to me.

 

[david luchini]

IMO the Code as written doesn't make sense. First, we have no empirical data upon which to base any demand factors for a single phase system let alone determine different demand factoring where the same number of identical single-phase dryers are supplied by a 3-phase, 4-wire feeder or service.

I disagree. I think the Code makes good sense with these demand factors. I don't need to know what the empirical data that the Code has based their demand factors on. It's not my problem. I just need to use the demand factors that the Code has provided. And their demand factors appear reasonable. Certainly, the more dryer units that are on the same feeder, the less likely they are to be used at the same time.

Without supporting evidence, the demand for multiple single phase dryers on a two-conductor system should be the same as on a three-conductor system. The only difference that should actually be accounted for is the issue brought up by Ragin Cajun (i.e. kW at 240V vs. 208V).

Again, I disagree. Consider 33 dryers on the same single phase feeder. 220.54 tells us to calculate that 10 may be used at the same time (33 * 0.3 = 9.9.) Now consider the same 33 dryers on a three phase feeder. I think the code considers that the dryers will be installed in a balanced way, ie, 11 dryers each from A-B, B-C and C-A. Now consider the same demand, or 10 dryers being used at the same time. How do we know that 10 would be used in a balanced way...4 on A-B, 3 on B-C, 3 on C-A? What if 5 are being used on A-B, 3 on B-C, and 2 on C-A. Or 6 on A-B, 2 on B-C and 2 on C-A?

Where it is known on the single phase system that all of the dryers will be used on the same phase conductors, it is not clear that the dryer use will be balanced on the three phase feeder. The Code addresses this in 220.54, and creates a higher demand because the load between any two phase conductors may be higher than any other two phase conductors.

To say the demand factor for the same number of identical dryers on a three phase system is higher than on a single phase system is ludicrous from the perspective of statistical analysis.

It doesn't seem ludicrous to me, it seems quite logical.

 

[GoldDigger]

I disagree. I think the Code makes good sense with these demand factors. I don't need to know what the empirical data that the Code has based their demand factors on. It's not my problem. I just need to use the demand factors that the Code has provided. And their demand factors appear reasonable. Certainly, the more dryer units that are on the same feeder, the less likely they are to be used at the same time.



Again, I disagree. Consider 33 dryers on the same single phase feeder. 220.54 tells us to calculate that 10 may be used at the same time (33 * 0.3 = 9.9.) Now consider the same 33 dryers on a three phase feeder. I think the code considers that the dryers will be installed in a balanced way, ie, 11 dryers each from A-B, B-C and C-A. Now consider the same demand, or 10 dryers being used at the same time. How do we know that 10 would be used in a balanced way...4 on A-B, 3 on B-C, 3 on C-A? What if 5 are being used on A-B, 3 on B-C, and 2 on C-A. Or 6 on A-B, 2 on B-C and 2 on C-A?

Where it is known on the single phase system that all of the dryers will be used on the same phase conductors, it is not clear that the dryer use will be balanced on the three phase feeder. The Code addresses this in 220.54, and creates a higher demand because the load between any two phase conductors may be higher than any other two phase conductors.



It doesn't seem ludicrous to me, it seems quite logical.

I think that the one logical constraint that we can apply is that for very large numbers of dryers on a three phase supply the value calculated for a three-phase loads should approach the value for the same number of single-phase loads. That is simple statistics. And Smart $'s method meets that test.
The question of single phase dryers on single phase or split phase versus three phase can use some more analysis.

 

[david luchini]

I think that the one logical constraint that we can apply is that for very large numbers of dryers on a three phase supply the value calculated for a three-phase loads should approach the value for the same number of single-phase loads. That is simple statistics. And Smart $'s method meets that test.
The question of single phase dryers on single phase or split phase versus three phase can use some more analysis.

I don't follow. What you suggest (calculated value for 3 phase approaching the calculated value for single phase) is a feature of the method I suggest.

For instance, with 33 dryers the single phase demand is 49.5kW and the 3 phase demand is 59.4kW. (Compare to 110.0kW for the two times the largest with no demand)

With 60 dryers, the single phase demand is 75.0kW and the 3 phase demand is 79.5kW. (Compare to 200.0kW)

With 90 dryers, the single phase demand is 112.5kW and the 3 phase demand is 112.5kW. (Compare to 300.0kW)

As you can see, as more dryers get added, the three phase value gets closer to the single phase value, until eventually, for a large enough dryer load they are the same.

Smart$'s method suggest that the calculated load will be the SAME for three phase or single phase. I think this method ignores the possible unbalance between phases that could exist when using the same demand factor on a 3 phase feeder.

Consider 33 dryers on the same single phase feeder. 220.54 tells us to calculate that 10 may be used at the same time (33 * 0.3 = 9.9.) Now consider the same 33 dryers on a three phase feeder. I think the code considers that the dryers will be installed in a balanced way, ie, 11 dryers each from A-B, B-C and C-A. Now consider the same demand, or 10 dryers being used at the same time. How do we know that 10 would be used in a balanced way...4 on A-B, 3 on B-C, 3 on C-A? What if 5 are being used on A-B, 3 on B-C, and 2 on C-A. Or 6 on A-B, 2 on B-C and 2 on C-A?

 

[Smart $]

I disagree. ...

It wasn't my intention to challenge the concept of demand factoring, even lacking any empirical data to support such. I agree with your assessment, so we are on the same page in that sense.

It doesn't seem ludicrous to me, it seems quite logical.

I only meant the total demand being different for 1? and 3? as ludicrous. The method involved for 3? is not really a change in total demand, but an additional factoring for diversity. I condone the intent and the result. Regardless of all that, I'd at the very least like the wording to actually state the method required rather than vaguely implying it is to be implemented in the same manner as ranges in the Annex D example.

 

[Smart $]

..

Smart$'s method suggest that the calculated load will be the SAME for three phase or single phase. I think this method ignores the possible unbalance between phases that could exist when using the same demand factor on a 3 phase feeder.

Whoa, there!!!

While demand considerations do change the total calculated load, I was not suggesting we ignore the potential of an unbalanced condition on a 3? 4W system. I just wish the "figuring" more readily made it apparent it is not true demand but rather an adjustment for diversity in the load.