VD equation
- Thread starter guschash
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hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
If you have a 240/120 volt feed to to this building using the 240 volts in the equation is the best method as for the most part even if all you have in this building is 120 volt loads they will still balance out over the 240 volt instead of the 120 volts because of how a center tap transformer works unless you severely install all the loads on just one leg (hot) in the panel at the building which I very doubt you will, but even if not balanced the unbalanced loads will use the neutral so this slight unbalancing will not affect the VD very much if at all, the only time you would use the 120 volt figure is if that is all you ran to the building like in just a single hot and a neutral.
I cheat and use this easy to use calculator here: http://www.electrician2.com/calculators/vd_calculator.html
It has 4 calculators for different methods, but for a job like yours when I want to know what size conductor I need at what percentage of VD I need to keep within I use the third one down.
It can make life a littler easier
At least you have a nice distance to figure for as 2x500' is 1k' which is what chapter 9 gives you the resistance for of the wire, bad thing is at this distance it's going to be costly for the wire depending on how much load you have to figure for.
Using the above caculator for a 100 amp load @ a 3% VD it give me a 4/0 copper or 300 kcmil Aluminum conductor,
going to a 5% VD gives me a 2/0 copper or 4/0 Aluminum.
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
sounds like a couple transformers might be a lot cheaper than running that big wire.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
Step up to 240 volts? did you mean 480 volts? not sure your going to find the transformer route cost effective with todays transformer prices with the fact you need two, what amperage are you shooting for? as a 100 amp load at the building will require two 25kva transformers.
another route is if you have utilities close to this building it might be cost effective for your customer to just put another service on this building if your power company will allow two services to one property? here they do and still bill it as residential.
Also if you can use 230.40 exception 3 that allows you to install a meter with double lugs on its load side, so that you can run right back into the ground and treat these conductors as service entrance conductors so you don't need an EGC, just the two hots and neutral, at the out building you treat it just like a service without a meter since the meter is already at the house, but make sure your utility and inspector will allow it as some don't, even though it is allowed by the NEC, Here we do this all the time to save the customer money, also you don't even need a disconnect at the house as they don't enter a building or structure until they reach the other building where you just install a main breaker panel and do the bonding just like it is a service, which because it is.
Just a few ideas that offer some flexability on cost savings
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
It takes a lot of 4 al to equal 2 - 25 KVA transformers
Without even doing any figuring I bet you would be fine with VD with 1/0 or 2/0 aluminum and still cost less than having 2 - 25 KVA transformers.
Plus the transformers will have core losses adding to the energy bill.
Without even doing any figuring I bet you would be fine with VD with 1/0 or 2/0 aluminum and still cost less than having 2 - 25 KVA transformers.
Plus the transformers will have core losses adding to the energy bill.
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