480 or 240 3 phase for new service

[electronclem]

Other than the obvious difference of wire size for different applications, is there a significan difference in energy consumption when comparing 480 3phase to 240 3phase?
Thanks

 

[dpc]

The 480V service would be slightly more efficient, but not a really big difference. 240V three-phase is not typically used - 208V three-phase is much more common.

When you are in the grey area, the deciding factor is often the large loads that you have to serve. If you have a big motor, 480V will be better. But if all loads are small, you can go with 208V and eliminate the double transformation.

 

[busbar]

Very roughly, an economic breakpoint for 208- (240- may be discouraged for new service) versus 480-volt service for motor load is 15-30hp. Future growth favors the higher voltage.

 

[electronclem]

I appreciate the feedback,but without creating alot of hassle for you guys, why does this make a difference. Does it have to do with a difference with starting loads? Once up and running wouldn't the P=IxE cover the bulk of it?
Thanks again.

 

[dpc]

I'm not sure what your question is. As load increases, use of 208V becomes more difficult due to the higher current required. This increases the voltage drop, and reduces the circuit lengths that are reasonable. Because current levels would generally lower at 480V, the copper losses would be somewhat lower than at 208V, and cost of the conductors will be significantly less at 480V.

The only advantage of 208V is simplicity. If the majority of your loads are small 120V loads, I'd go with 208/120V. If you have a lot of larger loads (heating, water heaters, air conditioners, motors, etc), 480V probably makes more sense. Many utilities will limit the size of 208V service they will provide. Also, 480V motors are more readily available than 208V three-phase.

 

[busbar]

A “for instance” comparison of magnetic starters for a NEMA size 1 rating: 200/230V 3ø — 7.5 HP and 460/575V 3ø — 10 HP. The higher voltage offers copper savings in motor circuits.

 

[skiier]

electronclem;

Go with the 480v system. Smaller conductor sizes, easier to work with, cheaper to install, easy to modify. A 20hp 480v motor will probably cost double to install at 208V. I have also found by experience there are greater heat losses and a higher incidence of bad connections with larger conductors. For example; do you think you can tighten a connection for say a #3 conductor inside a tiny disconnect with an allen key or flat screw driver sufficiently? Or do you think it would be easier to tighten a #10 with a flat? The guy connecting the #3 conductor 9 times out 10 will not tighten the conductor down properly but he will with a #10.
Think about it, all that current heating and expanding connections leads to bad connections and voltage dip and lights fading and whatever else.

 

[alehman]

I agree with dpc's first post. Conductors for either voltage must be sized such that they don't overheat, so I doubt there would be much difference in I^2*R losses. I can believe there may be some difference in motor efficiency, but it's probably not a lot either. I think the choice really boils down to cost of materials and availability of equipment at the selected voltage. 240 3-phase is rare for new systems, but was very common in the U.S. 50+ years ago. 480 has become nearly standard practice in the U.S. for larger commercial and some industrial facilities that don't justify higher voltage. 208 is very common for small commercial and retail where incandescent lighting is common.

 

[twosockets]

electronclem,

25-35 years ago 480/277 volt, 4-wire wye was rare in U.S. small commercial applications. That is not at all true today. Even rather small strip malls and convenience stores are going to the higher voltages as the loads are predominately lighting, air conditioning, and refrigeration. Typically, most of the lighting is 277 volt in these applications with the three phase loads being 480 volt. The reduced wire size and wide availability and low cost of 480/277 volt equipment offsets the cost of step down transformers for minimal 120 volt loads.

Frankly, I am surprised that computer work stations and other office equipment are not widely available in 277 volt versions for the U.S. market.

Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com

 

[stevenal]

A utility I've heard of charges power factor penalties for services of 480 and above. Upgrade the service to 480 to use those nice efficient motors, then the bill arrives!! Check the rate structures.

 

[pablo02]

Take a good look at your loads -- you'll probably be limited to 1000 kVa or smaller for (secondary) voltages less than 480/277 3 phase services... [but at that level you would probably need to consider the 480v option anyway]

Consider the comments above regarding you loads, cable sizes, etc... if your considering servicing a load, also approach your local utility, they'll provide what options, services, and voltages levels required and/or recommended...

for a small customer, I don't think the power factor issue will arise; if you are a large customer, I don't think the voltage level will be an issue --

 

[tulum]

Hello,

Does anyone have any experience with Green Credits? The term has been thrown around alot when dealing with new projects.

From my understanding, companies can increase there environmental standings by purchasing green credits...

Is there a financial payoff as well?

Thanks

 

[tulum]

Ooops,

Sorry... not where I wanted to put my post.

Forgive me.

 

[stevenal]

pablo,

The PF issue only arose after the bill arrived.

Consider a service to a school athletic field during the summer. PF is calculated from kWh and kvarh. Schools out, so the main load is the no-load losses from one or more dry transformers > very low PF. Every percent below 95 is used to adjust the demand charge upward by a percent. Although energy use was low, the field lights were on long enough to set a normal demand. School paid maximum possible for the smallest energy use. They would have been ahead paying for the extra losses of 240 equipment and avoiding the PF penalty. This customer is not the only one. When getting the utility recommendation, make sure you understand the different rate structures.

 

[electronclem]

Okay folks, some good feedback here. It sounds like reduced copper use and perhaps ease of installation--- potentially cheaper motors may be a good reason to go to 480. But a synopsis of an age old argument that 480 from a power consumption standpoint is cheaper to operate than 208 is really not accurate. I totally agree with the higher copper costs, motor costs, and tougher install and proper torques due to bigger wire size being required for the 208. Thanks to all who replied to this---everyone made good points. What a great forum!
ElectronClem

 

[alehman]

In response to twosockets, 480/277V wiring for general occupant use in offices, etc. scares me. I don't have a problem using 277 for permanently installed lighting in offices, but I wouldn't want 480/277 in pre-wired office furniture or in the computer under my desk. The electrocution hazard is too high and available fault energy is commonly much higher at the branch circuits than in 208/120 systems.

 

[twosockets]

Stevenal,

You have described a worst case scenario for PF penalty billing. A very poor load factor combined with an average PF calculated from Kwh and KVarh is very unfair (in this case)and has little relationship to what it costs the utility to serve this load.

Rather than investigating going to a lower voltage to avoid the PF penalty, you should simply add a PF correction capacitor to just offset the lagging KVars required by the idle transformer. Case closed.

Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com

 

[stevenal]

Greg,

As far as I know, no one is suggesting changing the service. The time to investigate is before the system is designed. Yeah, caps would do it. So would (a) switch(es) to turn off idle transformer(s). Unbudgeted for expenses like high electric bills and electrical work can always be handled by chopping another day off the too short school year. Anyway, I wasn't writing to ask for advice, but to give it. Sounds Clem might be at a better point to investigate rate structures than the school is. I'm just suggesting he do so.

 

[dsteves]

Unless your intended loads are entirely 120/208V, you have no equipment capable of being converted to 277/480V, and your expected total demand is so low that you can't justify $1500 for a 480/208 D-Y transformer, the logical choice is 480V. If by "new service" you mean that you have an empty building, and the utility can give you either or, consider the purpose of the electric power and choose the voltage that can provide the anticipated kVA requirement without the need for parallel conductors and high-current panels. Of course, if you're wiring a factory, consider a customer-owned transformer and bring it in at 4.16kV or higher; but if you're just talking about a machine shop or something like that, you can get about 65kVA out of a standard 120/208V service, and about 150 kVA from a 277/480V service, assuming a standard 225A main disconnect and proper derating for NEC compliance.

A friend who owns a machine shop took up residence in a building with 120/208V service until his second CNC machine showed up. The second machine put him over the top of the 65 kVA available. Now he's got 150 kVA available, and runs his 120/208 loads from a small 480/208 D-Y transformer and panelboard.

I apologize if a facsimile of this post shows up as well. My first post, and I think the last try went to the infamous /dev/null, but it may still be floating around somewhere.

 

[busbar]

There is a lot to consider along with energy consumption when choosing service voltage. Chapter 3 in IEEE Std 141 {red book} may be worth review.