transformer sizing

Status
Not open for further replies.
J

jojo

Member
Location
Philippines
Occupation
Electrical Engineer
hello! i just wnt 2 know if its okey to use a single phase 75kVA distribution transformer if the estimated load is 72 000 VA..the supply is single phase. actually, the "spare" is included in the calculation. so the actual load is about 65 000 VA.. is it ok to use the transformer or is it better to use a single phase 100 kVA distibution transformer? thnks..
 
Re: transformer sizing

thnks.. but what if i choose the 100 kVA transformer? does it have an advantage over the 75 kVA transformer considering that 72 000 VA is close to 75kVA and the transformer might be overloaded after some years? is there a derating factors for a given loads (lighting loads, receptacle loads, ACU and motors) in selecting a transformer?
 
Re: transformer sizing

Is your 72,000 VA calculated the connected load, or demand? Did that number come from an NEC Article 220 load calc? If it is connected load, or calculation per Article 220, a 75 kVA transformer should be ok. If it is demand load, I would use a 100 kVA.
 
Re: transformer sizing

Transformers are rated for 100% loading.

Personally, I design for 75-80% loading to allow for future additions. At one time I did a transformer watts loss analysis for commercial offices and found this sizing of standard H150 transformers was one of the "best" energy savings (this was before the new NEAM TP-1 units) designs.

The real answer depends on what your loading profile is going to be (how much load for how many hours). Heat is what kills a transformer so cool down is very important. An office building might run at 100% load for 10 hours, 50% for 4 hours, and 20% for 14 hrs that is a whole different case than an industrial machine running 100% for 24hrs.
 
Re: transformer sizing

according to other electrical practitioners, in transformer sizing the total load of the circuit is computed and after which, the demand factor is introduced: 80% for all lighting loads, 90% for all continuous motors (including ACU loads)... but others prefer to use 100% demand factor especially if you dont know the actual nature of loads..
by the way, i am from the Philippines.
 
Re: transformer sizing

My inclination is to look at what the likely situation will be down the road. if it is likely that additional load will be added, I'd be inclined to go with a larger transformer.

But, if budget was tight, I might go with a smaller one.

This kind of decision is not really something that is typically decided based on a calculation so much as a gut feel, or a budgetary constraint.
 
Re: transformer sizing

If there is a large single motor of 20 - 30 KW, it will have a hard time to start and will cause a voltage deep for the entire system. I would go 100 KVA (future use and better performance). The price difference is not so high.
 
Re: transformer sizing

gud pm! i just want to thnk everybody for giving suggestions regarding transforming sizing.. it really helped me a lot especially for a new electrical engineer like me. i will not hesitate to ask questions pertaining to electrical engineering in this code forum. i hope that we, as engineers will continue to share each others ideas regarding electrical engineering knowledge.
 
Re: transformer sizing

Gotta agree, I had a few blow up on me. However, that's because the load was non-linear. If linear, 75KVA, non-liner go higher if that's the demand load. :)
 
Re: transformer sizing

Do they make 100KVA, I thought the next size is 112 KVA?
Click to expand...
lady, after 75KVA single phase , the next step is 100 KVA. After 75KVA three phase , the next step is 112.5 KVA
 
Status
Not open for further replies.
Top