Regarding wire size vs system voltage

[CelectricB]

I have a very general question that won't have a definitive answer, but I'm looking for some sort of rule of thumb. At what size does a project become cheaper if you go from either 208V or 240V to 480V? In other words, when does upgrading the system voltage noticeably impact the cost of wire?

 

[LarryFine]

There is no rule-of-thumb because of how physically different every installation is.

You have to price it both ways and compare, including the cost of idling currents.

 

[GeorgeB]

At what size does a project become cheaper if you go from either 208V or 240V to 480V? In other words, when does upgrading the system voltage noticeably impact the cost of wire?

Larry gave excellent info, but didn't answer your question. Smaller wire is less expensive than larger; current will be 50% lower at 480 vs. 240, 43% lower at 480 vs. 208. I know of no industrial power wire (THWN, XHHW) rated lower than 600V. So your WIRE costs should be lower.

 

[LarryFine]

Larry gave excellent info, but didn't answer your question.

Well, I think I really did, as the base question was:

At what size does a project become cheaper . . .

That requires taking much more into account than just wire size/cost.

 

[ptonsparky]

Wire costs may be less, but the panels and circuit breakers will be more, so Larry is correct. The billing structure will be different for each system as well.

 

[LarryFine]

Wire costs may be less, but the panels and circuit breakers will be more, so Larry is correct. The billing structure will be different for each system as well.

Let us not forget the transformers!

 

[ptonsparky]

Let us not forget the transformers!

Those guys running the conduit to put the 480 in are more expensive than ropers, too.

 

[GeorgeB]

Well, I think I really did, as the base question was:

That requires taking much more into account than just wire size/cost.

You are correct. I read it as when does wire cost ...

 

[Fred B]

Also electrical cost savings from use of 3ph utilization equipment might be insufficient to reasonably recover the cost to upgrade where 3ph not currently available. From my limited experience in making change from1ph to 3ph, everyone that I've been asked to look at to change, the cost of conversion equipment far exceeded the potential recovery when 1ph equipment was available to perform tasks the owner wanted the equipment for and just the equipment costs was enough for the owner to back out. The only time I've had a change work out is when the needed equipment was not available in 1ph, then more or less if the owner wanted to use that equipment the upgrade was necessary regardless of cost of changing to 3ph.

 

[mbrooke]

Squaring of the voltage is very attractive.

Double the voltage for the same current and you can go twice the distance.

Halve the current and you can go twice the distance.

Double the voltage which also halves the current for the same load- you can now go four times the distance for the same VD.

Automatic disconnection of supply may skew this a bit, but it still holds economical overall.

 

[paulengr]

I have a very general question that won't have a definitive answer, but I'm looking for some sort of rule of thumb. At what size does a project become cheaper if you go from either 208V or 240V to 480V? In other words, when does upgrading the system voltage noticeably impact the cost of wire?

Upgrading is rarely worth doing.

First a shout out to Canadians. Hands down 600 V is the way to go there when available over 480. But in Canada 600 V rated equipment is usually stock where it’s special order in the US.

First consider your loads. If it’s an “industrial” plant with mostly motor loads there is no question 480 is preferred, going to 4160 if there are a few loads 500-1000 HP or always over 1000 HP. If however the vast majority of loads are 120 V and line lengths are reasonable it costs a lot more to pay for all those 480/120 transformers. So in general it’s a load based decision. Going into the question of 208/120 vs 240/120 single phase vs 240/120 high leg delta it depends on the cost from the utility and the cost of the HVAC. Usually the differences are not very much regardless especially when high SEER HVAC is all drives anyways..,the power source doesn’t matter a lot.

Physically in a residential area if the utility only has single phase available 3 phase service cost will be quite high (pulling a third line) but you will be the only customer so it’s more reliable. If three phase is available delta high leg requires just two transformers so it’s a cheap way to run a small amount of three phase load. Once you get into serious load (say over 50 kVA) then a full blown true 3 phase transformer (208 or 480) is necessary.

Don’t get snookered into delta. Business loss insurance carriers heavily frown on it (higher premiums). If you have the staffing high resistance grounded wye 480 V (not NEC legal on 208) is the cheapest distribution system since grounding can be very light duty and long term maintenance costs are reduced (damage is minimal). You get all the advantages of an ungrounded delta with none of the disadvantages (transients, ground fault locating).