Balancing the load on a 208->120V transformer

[Anonymous user]

We have the following unit:
APC Smart-UPS RT 5000VA RM 208V w/ 208V to 120V 2U Step-Down Transformer

- a 208V base unit with a 208V to 120 V step down transformer.

We understand that the input to the transformer is 208 V across two phases. On the back of the transformer, the twelve 120V outlets are 'color coded' with black or silver backgrounds, and this suggests that perhaps six of the outlets are on one phase, and six of the outlets are on the other phase.

Is there any requirement or 'good practice' to try to load this unit equally across the two phases - or could we plug everything we have into all the black outlets, and nothing into the 'silver' outlets, without any consequence? (using an extreme example to illustrate the point)?

The reason this question arises is, we bought an APC power strip so that all our servers, etc, can plug into the power strip rather than into the transformer directly. We are wondering if we should buy a second power strip, so that we can balance our load across two outlets on the transformer, rather than one.

Thanks!

[TheNotoriousKMP_apc]

What do you mean by Power Strip? Do you mean Power Distribution Unit or an actual Surge Protector? Usually each color coded group is designated to a circuit, so you could balance it off that way. However, if you get Switched or Metered PDU's, you will get told the total load in amps on an LCD display.

[Anonymous user]

Sorry, yes, I meant a Power Distribution Unit - a very expensive power strip 🙂 - we bought this one:

APC Rack PDU Switched Zero U 20A 120V

So my question is this - we have one PDU with 24 outlets. We plug this into the transformer. Obviously, all power we draw through this PDU comes out of the one socket on the transformer. The PDU has a maximum rating of 20A and the transformer has a "Maximum Total Current Draw per Phase: 24A" (from it's spec).

Let's assume we plug enough load into this PDU to draw 20A. Is it perfectly 'OK' to draw all 20A from ONE transformer phase, or would it be better to get two PDAs, and draw 10A from each phase?

Obviously, running 20A through one PDU - it's limit - is not a good thing, but that's not my question here - I'm talking about the loading on the Transformer phases - is there a 'best practice' to draw evenly from both phases or is it irrelevent?

Thanks

[TheNotoriousKMP_apc]

Chris,

I wish more of our customers were concerned with this.


You can draw a maximum of 20A per outlet, not to exceed 24A on the XFMR. As for your question, it would be at your discretion.

If you're going to draw all 20A from one PDU to one outlet on the XFMR, then you do risk a potential overload on the PDU first, as I doubt the XFMR will ever trip. The PDU circuit breaker would have to trip and that'll stop the flow of power from the XFMR to the equipment. So if you're going to monitor it, and leave some overhead for other equipment, it may be best to split the load between two current monitoring PDU's, and throw them on two different circuits.

[Anonymous user]

So other than your concerns about the PDU, there is no need to worry about transformer loading being 'balanced'? This question really is not related to exceeding maximum ratings, it's all about 'balance across phases'. I was using current values as an illustration.

I was wondering why they went to the trouble of color-coding the outlets on the back of the transformer - half black, half silver.

With a real 'rotating coil' generator, you do need to try to balance the load across all three phases; at least, the electric company goes to some lengths to ensure that. I don't know what damage (if any) a rotating coil generator would suffer if all the load came off of one phase, but I think it would be a bad thing. So I was wondering if the same 'balance across phases' is recommended on a two-phase UPS and/or a two-phase transformer. I understand how three phase power is generated by generators, then distributed and consumed. I don't understand how a two-phase battery-powered UPS handles phases, and whether the same concerns/isues apply.

Thanks!

[TheNotoriousKMP_apc]

Chris,

Since all we guarantee is 208V phase to phase on the output side of the UPS/Input side of the XFMR, I wouldn't be concerned at all with load balancing. Single phase load balancing (as we don't really think of it as second phase) isn't all that complicated unless you're dealing with the Symmetra LX as that right now, is APC's only true "split phase" UPS.

Sorry if I failed to go over the half black - half silver outlets, I thought I did in my original reply...they're just circuit groups. Usually the breaker is within that colored group as well.

[Anonymous user · Accepted solution]

Commercial buildings are wired 208Y120 with 120 between ground and any phase and 208 between any two phases.

The RM5000 is a single phase 208v device, so there's no such thing as balancing the load.

Balancing the load is for the power source side (upstream of UPS)

There's A, B and C and if you were connecting all 208v loads, you'd want to put one set between A,B; another between B,C and the third set between A,C. Does this make sense?
Downstream of the UPS, it's like any other single phase circuit. The transformer converts single phase 208v to single phase 120v.

On the output side, you just need to make sure you don't exceed 16A per outlet and not to exceed more than 24A per transformer. 16A is 80% of 20A and UL code says the load is not to exceed more than 80% for continuous loads.

The transformer does a little more than change 208v to 120v. There is no neutral on 208v side. Both sides are hot. The transformer drops the voltage, then makes 120v with one pin being 0v, and other pin being 120v, with respect to chassis, which is grounded.

With many of today's full-range 96-264v IT equiment, you can run it striaght off 208v, but it would be a code violation.