New Space Electrical Power - 208v vs. 240v

As part of our lease for the new space, the property owners are to provide a new 50 circuit 240v single phase power distribution panel for our space.

The electrical contractor the new space’s owners have hired is proposing that our space be wired with a 480v 3-phase panel that then is broken out to provide our single phase main panel with two of the 3 phase legs. This approach is cheaper, as the 480v 3-phase cable could be smaller, and it would provide us with access to 3-phase power more conveniently.

However, there is one major concern with this approach, instead of true 240v single phase power (120v each phase to ground), we would have two legs of the three phase distribution, which is each phase 120v to ground, but out of phase by 120 degrees, for an effective phase to phase voltage of 208v.

While this is a common approach in industrial buildings, it can cause some problems for equipment design for 220/230/240v single phase power (most machines are designed for 230v so they can work either on 220v or 240v). For things like resistance heaters, lighting, and other resistive loads, there is no difference. But anything that has a power supply with smart power monitoring features (battery chargers, high end power supplies) may trip faults due to the power being out of spec. Also, some motors designed for 230v will have reduced starting torque and run at slightly lower speeds/higher slip, which can affect the motor cooling and impact it’s lifespan when run on 208v.

We must determine if we can or should accept this change in the power we are being provided. I personally, and none of us who have discussed this on slack, do not have the expertise in this area to say if this trade-off will be harmful to us, or if it is acceptable. Do any Hive13 mailing list followers have significant experience with this? If so, we could really use the input.


Kevin M.
Hive13 Secretary

After reviewing this video, I believe we may be chasing our tail and will discuss with the contractor.

As I said on slack, that video doesn’t really cover anything except that if all you need is line to neutral 120v single phase, that you don’t need to worry about it, which we already knew.

It does not address what to consider when you need the line to line 240v.

We have many more tools which require 240V than tools which can, or will, use 3 phase. We can always use a VFD for those tools and focus on correct 240V. Or run 3 phase in the future.

Ok, I took the time to consult some experts and do a little math to convince myself.

The electrician can still run 3-phase to the elevator shaft to save on conductor cost.

Assuming that 3-phase is 240v (120v line to ground) two legs of that 3-phase will be 208v single phase. Likewise, if it's 480v 3-phase (240v line to ground), two legs would give us 416v single phase.

All that is required to get us the proper 240v single phase we need is a transformer. These are commodity transformers that are easy to get, the primary side is wound for either 208v or 416v single phase, and the secondary is wound with a neutral center tap to provide two oposing 120v lines to ground, and 240v line to line.

For our 200amp main panel, we would need a transformer sized for 48kva or more. Pricing these out indicates they are around $2k. That price should be low enough that it is not the greatest expense the building owners are paying for getting our electricity set up.

I will note that doing a single transformer would put our entire load on two phases of the 3 phase mains, which is called an unbalanced load and typically avoided. What we may consider is offering to accept 3 150 amp panels instead of a single 200 amp, so that they can use 3 smaller transformers and keep the load balanced (power company will charge them extra for unbalanced loads).

If the transformer was only used for 240V devices and 120V (everything else) is run directly from the 3-phase, would that reduce the load requirements on the transformer enough to use smaller, and presumably cheaper, transformer?

Your general assessment is helpful.
However, to balance circuit loading, we actually need to be using all
three of the panels/phases at the same time.

More discussion with the contractor is needed.

Dave Velzy

If they're bringing in 480V 3-phase we'd be needing a transformer for most of our 3-phase equipment anyway.

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Scott, theoretically, yes. However: our highest power loads are all 240v, so in practice, the transformer would still need to be almost as large to be able to power those bigger tools when we need them. Our biggest loads are the welders, and they would drive a need for a transformer of ~24 kva by themselves.

Dave, if they gave us three panels on the elevator each powered on separate phase combos, we could approximately balance the load between the phases by strategically placing the high power stuff on separate panels, and then randomly assigning the lower power circuits to the panels.

Jeff, we have no 3-phase equipment, we could plan to buy only equipment that can run on 480v, or get a 480 to 240v transformer. Most 3-phase equipment have the motors wound for both 480v and 240v 3-phase.

Placing three panels at the same place puts the cost of running lines to the other end of the floor on HIVE13. Not a good solution for us.

I am not suggesting they would be appropriate for when we expand. A similar arrangement would need to be placed in each expansion.