I have a situation where I am trying to size the transformer based upon the load. Basically, I have two trailers one has two LTG Panels with 100A main breaker. The other trailer has one panel with a 100A breaker. I basically will utilize one single phase pole mounted transformer to feed into these two trailers. I was thinking 100 X 3 ==300A X 240V= 72kVA which equates to 80kVa transformer. Am I correct or wrong?
sizing transformer
- Thread starter brock1221
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- Location
- Massachusetts
But that is not what you seem to be doing. You seem to be sizing it by the breaker sizes. That will work but likely result in much larger transformer than really needed.
I think the standard transformer sizes in that range would be 25, 37.5, 50, 75 or 100 kVA.
Are these pre-manufactured buildings or something like that?
If so the calculated load is often on a tag near the connection point.
BTW, running two feeders to one structure could be an issue. See part II of article 225, specifically '225.30 Number of Supplies'.
- Occupation
- Licensed Electrician
You need to do a load calc for the trailers, al la Art. 220 or look for the tag that manufactured trailers have. If you do a load calc annex D is quite helpful. Just for comparison my whole house clacs out to 100A and pulls around 70A with every possible electric thing turned on.
kwired
Electron manager
- Location
- NE Nebraska
I don't know what my whole house calculation is, I do have electric water heater, 3 ton heat pump, 15 kW back up heat, electric range/dryer, well, plus shop (probably seldom exceeds 3-4000 VA, and when it does is for brief periods of time) - All that only supplied by 25 kVA transformer by the utility.
Just out of interest, can you get a 100kVA single ohase transformer as standard?
- Location
- Massachusetts
Thank you for your most informative comment.
kwired
Electron manager
- Location
- NE Nebraska
Bob's link showed some pretty big transformers for single phase as being standard.
You will occasionally see 167 kVA as a POCO owned service transformer for single phase, especially for rural services. If you need more then that I think they generally want you to subscribe to three phase service. Most of what is around this area if you do need more then that you typically have a lot of motor load and would want three phase anyway.
PVfarmer
Senior Member
- Location
- Newport County, Rhode Island, USA
what voltage?
what voltage?
You haven't said what voltage you are transforming from/to.
But- if starting from MV, these MV transformers are pretty pricey.
So yes, 100kVA is standard, but might be 2x too big for what you need.
100kVA is almost 50% more $$ than 50kVA.
http://www.temcoindustrialpower.com...search_bar=1&t1=12.47kv&submit_x=0&submit_y=0
what voltage?
You haven't said what voltage you are transforming from/to.
But- if starting from MV, these MV transformers are pretty pricey.
So yes, 100kVA is standard, but might be 2x too big for what you need.
100kVA is almost 50% more $$ than 50kVA.
http://www.temcoindustrialpower.com...search_bar=1&t1=12.47kv&submit_x=0&submit_y=0
- Location
- Massachusetts
It was off the OPs topic so ...start a new thread if you need more info
- Location
- Massachusetts
240 secondary but no mention of primary voltage.
No need to. You answered my question.
There are two trailers and so application of demand factor is required for calculating optimum size of transformer. So OP better visit other similar installations to find out optimum size transformer at work unless the needed demand factor is made available to him.
- Location
- Massachusetts
The OP needs to follow the NEC, not the advice of those untrained NEC.
PVfarmer
Senior Member
- Location
- Newport County, Rhode Island, USA
iwire and the others are 100% correct here below. Don't just use 300A.
You want to know the total load, meaning the electrical draw of both trailers when everything in them is turned on.
So like ActionDave said- for "around 70A", going by this chart in the link, kwired's 25kVA transformer is plenty for Dave's 70A.
And the price factor seems worse, if only going from 480V to 240V, the 100kVAs run nearly twice as much $$ as the 50kVAs and 3-4x the 25kVAs.
I can't think of any reason to pay 2-4x as much as necessary for....anything!
http://1y18vt40uea231dp7l2g1lsi.wpengine.netdna-cdn.com/wp-content/uploads/SizingTransformers.pdf
PVfarmer
Senior Member
- Location
- Newport County, Rhode Island, USA
I think everyone is simply telling the OP to go by the book and calculate the actual two trailer load. If you oversize the transformer when you don't have to, the xfmr isn't the only thing that becomes more expensive.
kwired
Electron manager
- Location
- NE Nebraska
If these "trailers" are mobile homes, see 550.18 and 550.31 to determine what the load possibly can be calculated at.
Very constructive suggestion.:thumbsup:
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