220.54 & 55: 3-phase dryers

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CodeQuandary

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I'm trying to bone up on the NEC 2011, and I can't quite figure out what this means: "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."

Are they saying double the size of the larger dryer? Or something else? Sorry, it might be obvious, I just can't quite put it together.
 
CodeQuandary said:
I'm trying to bone up on the NEC 2011, and I can't quite figure out what this means: "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."

Are they saying double the size of the larger dryer? Or something else? Sorry, it might be obvious, I just can't quite put it together.
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Take a look at example D5(a) in Appendix D. It shows an example of how to calculate ranges using the "where two or more single-phase ranges are supplied by a 3-phase, 4-wire feeder or service." It is similar for dryers.
 
Thanks, I didn't think to look at the examples. I'm working my way through them, but I haven't gotten that far. Wow, I wouldn't have figured that out easily. Thanks for the pointer.
 
mbrooke said:
Im confused too.
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Say you have four ranges or dryers.

If you connected them all A-N-B on a 208/120 feeder or service, you have to calculate the load as if 8 were connected to the feeder or service.

If you balanced them out as best possible you'd have a max of two connected between two phases... so you calculate the load as if 4 are connected, which just happens to be the number connected.

However, if you had six and balanced, you'd only have to calculate the load as if 4 were connected.

It sort of forces one to make sure they are connected as balanced as possible.
 
I am not quite sure where the factor of two comes from, but at least in part it reflects the fact that, before diversity calculations, the load on the common line conductor of 12 ranges on one phase and six on the other will be greater than the current on the same line with 9 on each phase, even though the total load is 18 in both cases.
(Vector current cancelation effect)
 
GoldDigger said:
Not after applying diversity factors though....
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Sorry, but I wish people would quit using that word?diversity. It is essentially archaic, Code-wise. There are only a couple places in the enforceable part of the NEC where a diversity factor is implementable... and certainly not in Article 220, which deals with demand factors.

Anyway, what does demand factoring have to do with it...???
 
Smart $ said:
Say you have four ranges or dryers.

If you connected them all A-N-B on a 208/120 feeder or service, you have to calculate the load as if 8 were connected to the feeder or service.

If you balanced them out as best possible you'd have a max of two connected between two phases... so you calculate the load as if 4 are connected, which just happens to be the number connected.

However, if you had six and balanced, you'd only have to calculate the load as if 4 were connected.

It sort of forces one to make sure they are connected as balanced as possible.
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But if you had 6 balanced, why on calc as 4? Is it because 4 vs 6 will yield the same feeder size in all cases? I guess its more code than a real bases?
 
mbrooke said:
But if you had 6 balanced, why on calc as 4? Is it because 4 vs 6 will yield the same feeder size in all cases? I guess its more code than a real bases?
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4 (best balancing) vs 6 (balanced) will not yield the same feeder or service line current (actual, not referring to calculation).

Beyond that, IDK is the best answer I can give.
 
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Smart $ said:
4 (best balancing) vs 6 (balanced) will not yield the same feeder or service line current (actual, not referring to calculation).

Beyond that, IDK is the best answer I can give.
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Calculations always tend to be hyped. Ie if your load is 20 amps phase 1, 30 amps phase 2 60 amp phase 3, you size everything on that 60 amps. I think your right in that they want you to balance this out.
 
mbrooke said:
But if you had 6 balanced, why on calc as 4? Is it because 4 vs 6 will yield the same feeder size in all cases? I guess its more code than a real bases?
Click to expand...

If you have 6 balanced, then you have 4 connected to any line...A, B or C. So the demand factor is calculated as 4.
 
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