help...480/277 transformer ???

[nickforn]

i have a 3 phase 480/277 volt service coming into my new building. (600 amp)this power was brought in to power a large mahine we bought and had wired for that voltage.
my electrician has now told me i need a tranformer to step down to 120v for the light switches, fans etc, in the building. he is offering a 30kva transformer with a 100amp panel to power 32 receptacles, 2 bathrooms and 25 large 250 watt ballust lights like you see at home depot . there will be alot of hand tools and things plugged in simaltaneusly. i don't know anything about eletrical and feel i am not getting enough info from him on the limitations of the 30kva/ 100 amp panel. can some one please help?

thanks,nick

 

[itsmoked]

nick: there are others here who can probably say YEA/NAY on the sizing.

What I would do, is sketch your facility on a large piece of paper then mentally walk through it and put in a mark for every light and every bathroom fixture then make a list of all the expected tools.

Next jot down the watts each item will use. There are lots of web sites that will tell you the power used by all your tools.

Once you've put numbers on everything, add them all up.

Decide if the future holds any major possibilies like a bunch more tools or lots of computers etc.

Based on the numbers you have come up with and the chances of increased load the other guys on this site will quickly tell you the rule of thumb to use and you will have the transformer size you should use.

Compare that with the electrician's suggesting.

 

[mpparent]

Typically, anytime you change/add to your existing electrical infrastructure, it must be engineered by a licensed professional and then submitted to an AHJ for permit. I would not install anything without a load study being done by an engineer.

Mike

 

[itsmoked]

AHJ ?

 

[wareagle]

Nick
I would have the electrician use 250 watt lights that can be connected to the 277/480 volt service. The ballast come rated for several voltages. Then the 30kva transformer would be more that adequate. 30 kva at 208 3 phase is 83 amps.

 

[amazingmg]

Just a couple of quick comments.

First, I don't know of any Industrial or Commercial customer that calls for an inspection and permitting everytime they add something or change something in their plant. Are you in the USA? Are you on this planet? Major new construction (like a whole new section of the plant) sometimes gets permitted (sometimes not). Oh, and a load study? by a professional Engineer? for adding a Transformer and a lighting panel? LOL! <Shrug> Just my experience.

Second, 480/277V lighting is fine, unless your the poor SOB working on it! I would much rather work on 120V lighting than 480V! Sure, sure, the power should be off, but you never know what you are going to get into. Many, many maintenance departments require 120V lighting for safety reasons.

480/277V is a very common for plant distribution. So is transforming down to either a 120/240V single phase or 120/208V three phase panel for lighting, receptacles, etc. So, basically he is telling you right. Electricians are notorious for undersizing things though, so I understand your concern about the 30KVA Xfmr.

Three phase will get you more power so that sounds good. The only problem I've had with that is if you have a load that will be affected by the 208V rather than 240V, like heaters for instance. Other than that, I think 120/208V is great.

A 30KVA 120/208V Transformer will be a 45A Primary with a 100A Secondary. Ok, sounds good. 30KVA is good for 100A.

Loads:
32 receptacles x 1.5A = 48A
25 Lights (250W add 20% for ballast) x 3A = 63A
2 Bathroom Fans? X 3A = 6A
Bathroom Lights? Maybe another 10Amps?

127Amps

Now that is over the 100A but with some diversity of the loads you could still be ok. A judgement call. In other words, all 32 receptacles will never be used at once, will they? How many drills and small tools could be used at once? If a lot then I would go to a 45KVA Xfmr (I probably would anyway).

Hope that helps.

 

[alehman]

32 receptacles at 180VA each = 5760 VA
25 lights @ 300 VA each = 7500 VA
Need to know the load for the fans, but for here say 360VA each = 720VA

Total = 14 kVA.

If the transf. is 3-phase 208/120 volts that would be 39 amps. If it is 1-phase 240/120 volts, 59 amps.

NEC says you can load a panel up to 80% of it's rating (for continuous loads, which lights and recepacles are).

The transformer must have overcurrent protection per the NEC. The basic rule for secondary protection is a maximum of 125% of the transformer rated secondary current. That can be exceeded if the transformer primary is protected at less than 125% of the primary rating. The transformer secondary X0 must be grounded as well. The NEC specifies the method and conductor size. It's basic stuff, but the person doing it must know how.

That's why you need a PE and a permit. Many electricians have PE's on staff who can handle basic design such as this and obtain the required permit.

 

[mpparent]

AHJ = Authority Having Jurisdiction


Mike

 

[mpparent]

amazingmg,

Yes...I'm on this planet. Yes...I'm in the US. As alehman points out, additions/changes made to ANY existing system must be engineered, stamped by a PE, etc. to obtain permit. That's just my experience...I've only been doing this stuff for 15 yrs. Does that qualify?

Mike

 

[amazingmg]

Aleman,

Obviously I was thinking all 120V 1ph. Sorry to confuse the issue. My only excuse is too many hours and not enough sleep. Major goof.

mpparent,

Sorry to rub you the wrong way. I think that additions and changes being engineered, stamped by a PE, etc. are the exception, not the rule. Maybe that's not the way it should be, but....

<slinks off stage right to get some sleep...>

 

[mpparent]

amazingmg,

No problem. I've never been in a municipality that didn't require a stamped set of professionally designed drawings to obtain an electrical permit for any type of alteration, etc of an existing system or the construction of a new one. Period. It may be the case in some areas (the midwest where I'm originally from), that an engineered set of drawings isn't required. But, I would think from a liability perspective, any owner of a building, etc. would want that to ensure no "consequences" later.

Mike

 

[mpparent]

To add another final point,

I don't know of many electrician's (I do know a couple that could), that would be able to calculate the fault current at any point in any given electrical system and then have the knowledge of applying X/R values at that point to come up with an equipment rating. I think many people think that slapping in a panel here and there is no big deal, but at some point, that thinking is dangerous and could get someone seriously hurt, not to mention it is a part of the NEC to ensure proper bracing. This is just one example. That's why I think careful consideration should be given when installing electrical equipment.

Mike

 

[itsmoked]

mpparent: Thanks for the definition.

Sorta like "no controllng authority".

I would have to agree with amazingmg. Of all the "additions" I have ever seen in a large tannery, perhaps 30 over 100KW I have NEVER seen a hint of any drawings and certainly no PE stamps.

Most companies I've seen would rather "take a risk" to avoid yet more taxation.

Just my experience.

 

[alehman]

mpparent has a good point about fault current. 480V systems of this size could easily exceed the capacity of a typical cheap 14kAIC 480V breaker. That won't be a problem until the breaker has to open a fault. Then there's a good chance of an explosion.

Yes, you can do it the "cheap" way. Maybe I've seen too many burn-downs, but it would scare me. Even if you don't want to pay for a permit, at least get a qualified engineer to review what you're doing.

 

[itsmoked]

alehman: Very good point. I'm sure it never crossed their minds.

 

[dpc]

mpparent,

Actually, in some states (like Oregon), electricians are permitted to design systems 600V and below up to a certain ampere rating. No PE stamp is required.

Engineers? We don't need no stinking engineers!

 

[itsmoked]

"Engineers? We don't need no stinking engineers!"

BAZZZZUUUUUUUUUMMPHKAAAPOW!

 

[c12sc16]

Let’s compare apples to rotten tomatoes.
If a request is for the utility to change/add to the service feeding the customer, then a PE is required in most municipals.
If the customer wants to change/add anything on the load side of the point-of-service (generally the billing meter), then “What is an engineer?”

 

[mpparent]

dpc,

I'll remember not to buy any commercial property in Oregeon.




Mike

 

[amazingmg]

Well, in all fairness, we don't do any municipal work and very little commercial. Our customers are all industrial type facilities.

I'm certain municipalities required permits which in turn required a stamp if it's of any significant size.