11 gas dryers feeder
- Thread starter nickelec
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wwhitney
Senior Member
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- Berkeley, CA
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- Retired
Gas dryers are not electric dryers. What does the nameplate on the gas dryer say?
Cheers, Wayne
Cheers, Wayne
Maybe the “gas dryer” is an electrically-powered machine that removes moisture from gas in a pipeline?
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
I don't believe those have a "5 kW minimum per". That strongly suggest clothes dryers.
Cheers, Wayne
Joe Villani
Senior Member
- Location
- Westchester County NY
Is this in each individual unit of a multifamily dwelling, a common laundry room, or commercial laundry mat?
hbiss
EC, New York NEC: 2017
- Location
- Little Falls, New York NEC: 2017
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- EC
No you don't. Nameplate rating plus or minus what they are used for. Need to know.
-Hal
I was only being half serious when I posted that. Since gas was mentioned….
- Location
- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Yeah, yet... it turned out that the "gas" part is what was wrong?
Anyway, since we are now talking electric dryers, 220.54 applies. Note that it says "... either 5000 watts (volt-amperes) or the nameplate rating,
whichever is larger, for each dryer served."
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
You can't balance 11 dryers on 3 phases, so I'd do the calculation for 12:
12*5000 * 0.46 / 208 / 1.732 = 77A
Jon
12*5000 * 0.46 / 208 / 1.732 = 77A
Jon
wwhitney
Senior Member
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- Berkeley, CA
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- Retired
First, this assumes the clothes dryers are in dwelling units, as 220.54 refers to "household electric clothes dryers in a dwelling unit(s)". Assuming that's true, if you put all 11 dryers on the same pair of ungrounded conductors, then the load is 47% * 11 * 5000 = 25,580 VA, or 124A on just those two legs.
If you arrange the 11 dryers as balanced as possible, then 220.54 tells you that "the total load shall be calculated on the basis of twice the maximum number connected between any two phases." I take that to mean we calculate the load for 8 dryers, and then multiply by 1.5 (implicit). So the Table 220.54 demand factor is 60%, not 46%. That gives a load of 100A on each of the 3 legs.
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
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- Electric motor research
Serves me right for not actually re-reading the code section 
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
For those who may be confused, Wayne has shortened the process as to not go thru each step individually so look at Example D4(b) in annex D. The 1.5 threw me for a minute until I realized you need to multiply by 3 and divide by 2. 3/2 = 1.5
Something I learned is whenever you have multiple single-phase loads, as in the op's example, on 3 phases then you take the number of units (11) and divide by 3. If there is a remainder then go to the next number. 11/3 is 3 and a remainder of 2 so you use 4 dryers then multiply by 2 = 8 dryers. Use 8 dryers from the table.
This is the example for ranges but it is similar for dryers.
wwhitney
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- Berkeley, CA
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- Retired
Those sound like they are not in dwelling units. So ignore 220.54. No demand factor, but you get to use the nameplate rating, no 5 kW minimum.
At least that's my understanding.
Cheers, Wayne
- Location
- Chapel Hill, NC
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- Retired Electrical Contractor
I am not sure that I agree. It really doesn't say that the dryer must be inside the dwelling unit.
wwhitney
Senior Member
- Location
- Berkeley, CA
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- Retired
The start of 220.54:
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.
And the title of Table 220.54 is "Table 220.54 Demand Factors for Household Electric Clothes Dryers".
Now the first sentence of 220.54 clearly says "in a dwelling unit(s)". So if the electric clothes dryer is not in a dwelling unit, there's no 5 kW minimum.
But the second sentence on demand factors does not reiterate the "in a dwelling unit(s)" requirement. The title of Table 220.54 says the demand factors are for "household" electric clothes dryers, rather than repeating the "in a dwelling unit(s)" language.
So are you suggesting that the arrangement in the OP (one clothes dryer per floor, shared among several dwelling units) counts as "household" electric clothes dryers, and the second sentence of 220.54 still applies? That interpretation is at least plausible.
But I think the intent is pretty clearly that each clothes dryer serves one "household." And once you start sharing electric clothes dryers among dwelling units, the utilization rate of the dryer will go up, so demand factors based on only a single household using each dryer would become non-conservative.
Cheers, Wayne
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
I think Wayne has it.
Consider 40 families with 40 separate dryers. What is the likely maximum number of dryers in use at any one time? This is answered by the load diversity calculation.
Now consider 40 families sharing 10 separate dryers. I'd expect a similar maximum. Same number of people, same amount of laundry to do. So IMHO the load diversity calculation can't be similarly applied.
-Jon
Consider 40 families with 40 separate dryers. What is the likely maximum number of dryers in use at any one time? This is answered by the load diversity calculation.
Now consider 40 families sharing 10 separate dryers. I'd expect a similar maximum. Same number of people, same amount of laundry to do. So IMHO the load diversity calculation can't be similarly applied.
-Jon
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I think you have to look at the likelihood of all the dryers being on at the same time. Obviously, there is more of a chance for that to happen when you have so many families using one dryer.
So suppose there is an 11-story building with one unit per floor but the laundry is out in a common area and not inside the unit. Unlikely scenario perhaps but where does that land us?
Given all that Wayne & Winne have said I will have to agree that the load diversity can't be used here. In a dwelling clears it up but one could consider the entire structure a dwelling but I think that may be pushing it..lol
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