220.54 Need some help
- Thread starter wawireguy
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- Lockport, IL
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- Semi-Retired Electrical Engineer
The requirement is not all that tricky, though the wording might be a bit tricky. This will only come into play in a large multi-family dwelling unit, such as an apartment building that allows each resident to have laundry equipment. It is talking about the building calculation, not about the calculation for an individual unit. The building has a 3-phase service, but the dryers are each served by a single phase, 208 volt branch circuit. So if one dryer in one unit is connected between phase A and phase B, that particular dryer does not add any load to phase C.
So how do you count the load on dryers, when some are connected A-B, and others are connected A-C, and the rest are connected B-C? The process is that you find out which of those three has the highest dryer load. Are more dryers connected A-B, or are more connected A-C, or are more connected B-C? Suppose that A-B has the highest dryer load. You double that number, as a way of accounting for the load on phase C. Then you go into Table 220.54 to get the demand factor.
So how do you count the load on dryers, when some are connected A-B, and others are connected A-C, and the rest are connected B-C? The process is that you find out which of those three has the highest dryer load. Are more dryers connected A-B, or are more connected A-C, or are more connected B-C? Suppose that A-B has the highest dryer load. You double that number, as a way of accounting for the load on phase C. Then you go into Table 220.54 to get the demand factor.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I think you have the process right.
But as to the "watts is watts" conjecture, they certainly is. So if you wanted to calculate the total load that would be present if all were running at the same time, you take the total number of dryers times 5000, and then divide by 360 to get amps. But the NEC allows us to use smaller numbers on the theory that not all will be running at the same time. Using your numbers, if you used 20 dryers to get the demand factor, but used 20 times 5000 to get the load, you would be overstating the likely demand. I don't know how the CMP came up with this calculation process. But it is evident that they are being generous with the allowances, letting us use smaller numbers for the calculated load.
But as to the "watts is watts" conjecture, they certainly is. So if you wanted to calculate the total load that would be present if all were running at the same time, you take the total number of dryers times 5000, and then divide by 360 to get amps. But the NEC allows us to use smaller numbers on the theory that not all will be running at the same time. Using your numbers, if you used 20 dryers to get the demand factor, but used 20 times 5000 to get the load, you would be overstating the likely demand. I don't know how the CMP came up with this calculation process. But it is evident that they are being generous with the allowances, letting us use smaller numbers for the calculated load.
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