75 kva transformer

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sparkync

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Could someone help me find the load a 75 kva 3 phase transformer on 120/208 voltage can hold? It's been a while since I've had to do the calculation.
Thanks
 
Thanks for the quick reply. The reason I'm asking is that I've got a potential customer who already has a business on one side of their building, and wants to put a tanning bed business in the other side. They already have a 175 amp 3 phase panel in the building with no empty spaces, and wants me to wire 5 outlets for tanning beds. Each bed pulls 37 actual amps according to the specs they gave me. If I take each bed at $ 100% that is 185 amps. I'm not sure how long the beds run, but if they go over 3 hrs. I would have to figure for continuous duty, correct? Or is that only for the branch circuit? Can't remember now. But any way, the transformer feeding the business is 75 kva. According to your calculations, it would look like the transformer would have to be changed, and I would have to change the service from 200 amps to 400 amps. I'm assuming the service is 200 amp 3 phase right now. I have to go for another look today sometime. Any input on this would be appreciated.
Thanks
 
sparkync said:
Thanks for the quick reply. The reason I'm asking is that I've got a potential customer who already has a business on one side of their building, and wants to put a tanning bed business in the other side. They already have a 175 amp 3 phase panel in the building with no empty spaces, and wants me to wire 5 outlets for tanning beds. Each bed pulls 37 actual amps according to the specs they gave me. If I take each bed at $ 100% that is 185 amps. I'm not sure how long the beds run, but if they go over 3 hrs. I would have to figure for continuous duty, correct? Or is that only for the branch circuit? Can't remember now. But any way, the transformer feeding the business is 75 kva. According to your calculations, it would look like the transformer would have to be changed, and I would have to change the service from 200 amps to 400 amps. I'm assuming the service is 200 amp 3 phase right now. I have to go for another look today sometime. Any input on this would be appreciated.
Thanks
Click to expand...

I can't imagine anyone would be in a tanning bed for 3 hours, so I don't see how it can be considered continuous.

But they all could be on at the same time.
 
sparkync said:
Thanks for the quick reply. The reason I'm asking is that I've got a potential customer who already has a business on one side of their building, and wants to put a tanning bed business in the other side. They already have a 175 amp 3 phase panel in the building with no empty spaces, and wants me to wire 5 outlets for tanning beds. Each bed pulls 37 actual amps according to the specs they gave me. If I take each bed at $ 100% that is 185 amps. I'm not sure how long the beds run, but if they go over 3 hrs. I would have to figure for continuous duty, correct? Or is that only for the branch circuit? Can't remember now. But any way, the transformer feeding the business is 75 kva. According to your calculations, it would look like the transformer would have to be changed, and I would have to change the service from 200 amps to 400 amps. I'm assuming the service is 200 amp 3 phase right now. I have to go for another look today sometime. Any input on this would be appreciated.
Thanks
Click to expand...

Presumably you would spread the new load out among the three phases so it would be just 74 Amps per phase. I think you will need to do a load calculation to see what the actual calculated load will be before you proceed any further. Just because you have a 175 A main CB does not mean you have a 175 A service. It might be a 200 A service that the load calcs allowed them to use smaller wire on if they went with a 175 A CB. Run the numbers first and see what you end up with. That is where you have to start.
 
Are the beds fed by single phase circuits, or three phase circuits? It makes a big difference on the total load.
 
infinity said:
Would you use 125% to determine the maximum load?
Click to expand...

yes and most engineers here size using 125 %

Notes:
1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the next higher standard rating or setting shall be permitted.

Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
 
david said:
yes and most engineers here size using 125 %

Notes:
1. Where the required fuse rating or circuit breaker setting does not correspond to a standard rating or setting, a higher rating or setting that does not exceed the next higher standard rating or setting shall be permitted.

Notes:
1. Where 125 percent of this current does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted.
Click to expand...
125% of what? OP asked about the load of a 75 kVA transformer- which is 75 kVA, breaks down to the already mentioned 208 amps per phase. Secondary protection typically can be 125%, most cases secondary conductor needs to be 125% if continuous load, but the designed rating of the transformer is still 208 amps and not an additional 25%, nor is it 80% of 208.
 
I use short math for Xfmr amperage

KVA x 1.2 for 480v
KVA x 2.77 for 208v
KVA x 2.4 for 240v

Ex. 75 x 2.77 = 207.75 or 208 amps secondary
75 x 1.2 = 90 amps primary

And then kick it around from there if it needs to be.


JAP>
 
Main disconnect

Main disconnect

Maybe just one more question, kinda related to this post. Didn't want to start another one since I already had this one. I don't do much heavy commercial so I'm sorta confused on the idea of not having a main outside on a commercial service. The business I have talked about, has a CT can outside and a meter, but I see no kind of disconnect. Am I missing something here? I've seen other buildings like this. The sub panel ( or in this case would be the Main Panel) is about 10 or 12ft. away at least.
Thanks
 
kwired said:
but the designed rating of the transformer is still 208 amps and not an additional 25%, nor is it 80% of 208.
Click to expand...

Right. The "80%/125% rule" is for OCPD's and conductors. It does not apply to transformers. Personally, I see no need to add an extra 25% to an NEC load calc to size a transformer. If it was ACTUAL load, I might build in a bit of headroom however.
 
sparkync said:
Maybe just one more question, kinda related to this post. Didn't want to start another one since I already had this one. I don't do much heavy commercial so I'm sorta confused on the idea of not having a main outside on a commercial service. The business I have talked about, has a CT can outside and a meter, but I see no kind of disconnect. Am I missing something here? I've seen other buildings like this. The sub panel ( or in this case would be the Main Panel) is about 10 or 12ft. away at least.
Thanks
Click to expand...
Commercial/residential isn't a factor in NEC, rules of location of service equipment is general in nature and applies to all types of installations. You may have local rules that differ though. All NEC says is the service disconnecting means needs to be either outside or nearest the point of entry if inside, though that last part is interpreted in many different ways around the country.
 
sparkync said:
Thanks for the quick reply. The reason I'm asking is that I've got a potential customer who already has a business on one side of their building, and wants to put a tanning bed business in the other side. They already have a 175 amp 3 phase panel in the building with no empty spaces, and wants me to wire 5 outlets for tanning beds. Each bed pulls 37 actual amps according to the specs they gave me. If I take each bed at $ 100% that is 185 amps. I'm not sure how long the beds run, but if they go over 3 hrs. I would have to figure for continuous duty, correct? Or is that only for the branch circuit? Can't remember now. But any way, the transformer feeding the business is 75 kva. According to your calculations, it would look like the transformer would have to be changed, and I would have to change the service from 200 amps to 400 amps. I'm assuming the service is 200 amp 3 phase right now. I have to go for another look today sometime. Any input on this would be appreciated.
Thanks
Click to expand...

You only need to worry about continuous duty if the tanning beds have an integral motor, which i doubt they do. They should be a linear load like a heating element or a light bulb and should consume power according to the temperature setting. So at max "tanning" the machine will draw 37A.

I would recommend that you meter the existing panel for 30 days in accordance with the NEC to determine the actual load. If you have enough capacity for the tanning beds + 125% then its all good. If not, you would need to upsize the service or bring in a second service and panel.
 
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