de-rating receptacles

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badgers

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If I have outlets for in sink disposals, microwaves, refridgerators ect, I am not allowed to take 50% of the load above 10,000 and I have to count each receptacle at the VA of the equipment.

Is there any de-rating factor I can use when figuring the load on the transformer? We did a project where we calculated the above loads as 100% and then did the derating on the general convenience receptacles.
We had a data logger watching the load on the transformer for a week(EE type transformer) We are peaking at about 40% of the calculated load.
The peak came at noon when everyone was using the microwaves. EE transformers are designed for 35% load on a 24 hour average, and the night time load was only around 4-6% depending on the day. Most of the day the load was running about 20% to maybe 26%.

That makes me think that either everybody ends up with oversized transformers, or there is a derating factor I am missing.
 
I don't think you are missing out on an available derating factor. It is common to have a measured load well below the NEC calculated load. It is common knowledge that the Article 220 calculation methods are very conservative. The utilities are empowered (essentially because the NEC does not apply to them) to take advantage of this conservatism. That is why they will provide a service transformer with a rating no more than 50% of the calculated load.

We, however, with the ?we? meaning engineers, designers, electricians, contractors, and inspectors, are not empowered to take advantage of the conservatism built into the calculation process. The NEC method gives us a minimum standard for a safe installation. If the book does not give us a specific demand factor, then we cannot invent our own by simply saying that our measurements on similar projects prove that the load is not as high as the calculation says.
 
Thank you. I was not looking to apply my own demand factor. I was looking to see if I was missing some section of the code that accounted for the diversity of a bunch or refrigerators, ect....
thanks
 
badgers said:
If I have outlets for in sink disposals, microwaves, refridgerators ect, I am not allowed to take 50% of the load above 10,000 and I have to count each receptacle at the VA of the equipment.

Is there any de-rating factor I can use when figuring the load on the transformer? We did a project where we calculated the above loads as 100% and then did the derating on the general convenience receptacles.
We had a data logger watching the load on the transformer for a week(EE type transformer) We are peaking at about 40% of the calculated load.
The peak came at noon when everyone was using the microwaves. EE transformers are designed for 35% load on a 24 hour average, and the night time load was only around 4-6% depending on the day. Most of the day the load was running about 20% to maybe 26%.

That makes me think that either everybody ends up with oversized transformers, or there is a derating factor I am missing.
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Any fixed appliances using receptacles will be count 100% load in NEC. The load calculation on NEC is very consevative and is good from safety stand point. The service transformer sized by utility company is not covered by NEC. They will take full responsibility for it. IEC is an international electrical code used outside United States. It does allow electrical engineers to apply aggressive diversity to size the service cables based on the levels of power distribution system.
 
charlie b said:
I don't think you are missing out on an available derating factor. It is common to have a measured load well below the NEC calculated load. It is common knowledge that the Article 220 calculation methods are very conservative. The utilities are empowered (essentially because the NEC does not apply to them) to take advantage of this conservatism. That is why they will provide a service transformer with a rating no more than 50% of the calculated load.


I always wondered what percentage the utilities used. I ve always noticed that it seem that the transformer they have on the pole was smaller than our calculation (200 amp) for residential services. I usually see a transformer feeding 2 or 3 houses and it looks like its on a 15k..
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