Dryer Calculation

[volt101]

A landlord of a thirty unit apartment complex trying to increase his profits decides to add ten coin-op washers and ten coin-op dryers in a laundry room. The dryer nameplates indicate that they are 4500 volt-amperes each. What will be the total calculated load, in amperes, that these ten dryers will contribute to the ?house? panel?s calculation if the service is 120/208 3P 4w?

Jim

 

[charlie b]

That is not the question to which you need an answer. I?ll tell you why in a moment.

The NEC requires you to use 5000 VA for a dryer, regardless of its nameplate rating. But it allows a 50% demand factor for 10 dryers. So if these will all go on the same feeder, then 10 times 5000 times 50% divided by 208 and divided again by 1.732 gives you about 70 amps.

The NEC requires at least one laundry circuit of 1500 VA under some circumstances. But that would not apply here. 220.53 says you can take the nameplate rating of the 10 washers and multiply by a demand factor of 75%. But you didn?t give us that nameplate rating. So you will have to do the math yourself.

But don?t bother with the math, or I should say don?t bother with the demand factors. I will guarantee you that these 10 washers and 10 dryers will all be running at the same time for a significant portion of their useful lifetimes. Using the NEC demand factors will give you a minimum installation, but not an acceptable installation. You need to go beyond the minimums, or else the owner will be talking to you about ?fixing? the problem of tripping breakers at your own expense. You might think you have the right side of the law on this one, but do you really want to undergo that discussion? Talk with the owner now, and make sure the owner knows why you need to go beyond the NEC minimums.

 

[bob]

That is not the question to which you need an answer. I’ll tell you why in a moment.

The NEC requires you to use 5000 VA for a dryer, regardless of its nameplate rating. But it allows a 50% demand factor for 10 dryers. So if these will all go on the same feeder, then 10 times 5000 times 50% divided by 208 and divided again by 1.732 gives you about 70 amps.

Charlie
I believe that the 50% reduction in demand is applied to 10 apartments with washers and dryers. You will have a reduced demand. I agree with you that when all this equipment is in one building and everyone in the complex is there on Saturday morning, you will have all the the equipment running at the same time.

 

[charlie b]

I don?t think a ?household dryer? ceases to be a ?household dryer,? simply by virtue of having a coin operator box installed. I can?t see any language that forbids the use of demand factors for the house loads of an apartment building. I think they can be used, but I also think they shouldn?t be used.

 

[steve66]

I think this sounds like a test question. And if the landlord installs these where they are accessible to more than one tenant, I don't think they are considered "in" a dwelling unit. So the demand factors would not apply. It looks to me like the definition of "dwelling unit" is the deciding thing here.

 

[iwire]

I don?t think a ?household dryer? ceases to be a ?household dryer,? simply by virtue of having a coin operator box installed.

I agree the coin operator may mean little.

What does matter IMO is how the unit is listed by the NTL.

I think there is a strong possibility that these units will be listed as some sort of commercial unit.

I doubt highly that a household unit would last long at all doing the this type of duty.

 

[qcroanoke]

But don?t bother with the math, or I should say don?t bother with the demand factors. I will guarantee you that these 10 washers and 10 dryers will all be running at the same time for a significant portion of their useful lifetimes. Using the NEC demand factors will give you a minimum installation, BUT NOT AN ACCEPTABLE INSTALLATION. You need to go beyond the minimums, or else the owner will be talking to you about ?fixing? the problem of tripping breakers at your own expense. You might think you have the right side of the law on this one, but do you really want to undergo that discussion? Talk with the owner now, and make sure the owner knows why you need to go beyond the NEC minimums.

Yeah, what he said. You can't always go buy what the code says is good.

 

[gndrod]

qcroanoke, I agree. Going beyond minimums avoids future 'upgrade' corrections or modifications that get installed without professional work or inspection.

 

[volt102]

Yes, this is a test question. I wrote it for our "Granite State (IAEI) Code Tease" in our newsletter. It sparked up some very interesting debates at our monthly meeting.

The first - whether or not you have to carry 5000 VA and whether or not you can use the demand tables.

The second - how to apply the 3 phase calculation language in 22.54 ( divide by 2 and multiply by 3 OR multiply by the square-root of three) see example in code book (D)(4)(a) and whether or not the language in that section even applies to the original question.