Large single-phase feeder from a three-phase panel?

[phil_edwards123]

We are trying to install a mobile office trailer and its electrical specification would be 100A at 120V single phase. However, our building electrical service is of three-phase system. Is it ok to run a single-phase 120V, 100A feeder from a three-phase 208Y/120V panel? If so, do we just install a single pole 100A breaker at the three-phase panel? Would it create unbalance issue at the three-phase panel?

 

[GoldDigger]

Depending on the actual load at the trailer, it may create imbalance at the panel, and upstream panels if that is not the main.
But simple imbalance is not a problem as long as the phase line you connect to does not end up overloaded.
To know what to do you know (or at least estimate) the trailer load and the existing load.
If the imbalance is too great you can pull two phases to the trailer and use a standard 120/240 three wire panel to split the 120V loads.
As long as you do not have any 240V loads, the fact that line to line loads would see 208V instead will not be an issue. Just remember to count the neutral as current carrying if the trailer has MWBCs.
If the trailer is prewired for 120 only on a single circuit feeder, then you will have more work to do.

 

[ActionDave]

We are trying to install a mobile office trailer and its electrical specification would be 100A at 120V single phase. However, our building electrical service is of three-phase system. Is it ok to run a single-phase 120V, 100A feeder from a three-phase 208Y/120V panel

Huh?

If so, do we just install a single pole 100A breaker at the three-phase panel?

100A single pole breaker???

I hate nick picking but I am lost.

Never in my life installed a 100A single pole breaker.

 

[ptonsparky]

Tell them they get 100 Amp @ 208v. Two hot, a neutral and an EG. How they plug things in is their problem.

 

[ron]

As mentioned, it will be tough to find a 100A-1P breaker for your 3 phase source panelboard. BTW, Many trailers are looking for 120/240V.

The imbalance on the upstream panel will be ok, as long as no single phase is overloaded in the source panel;board compared to its main breaker.

 

[ptonsparky]

As mentioned, it will be tough to find a 100A-1P breaker for your 3 phase source panelboard. BTW, Many trailers are looking for 120/240V.

The imbalance on the upstream panel will be ok, as long as no single phase is overloaded in the source panel;board compared to its main breaker.

On that note the last temporary office we hooked up had a note next to the thermostat that the control transformer in the A/C Heating unit required the voltage taps be changed as needed.

 

[phil_edwards123]

Thanks everyone for your responses.

Depending on the actual load at the trailer, it may create imbalance at the panel, and upstream panels if that is not the main.
But simple imbalance is not a problem as long as the phase line you connect to does not end up overloaded.
To know what to do you know (or at least estimate) the trailer load and the existing load.
If the imbalance is too great you can pull two phases to the trailer and use a standard 120/240 three wire panel to split the 120V loads.
As long as you do not have any 240V loads, the fact that line to line loads would see 208V instead will not be an issue. Just remember to count the neutral as current carrying if the trailer has MWBCs.
If the trailer is prewired for 120 only on a single circuit feeder, then you will have more work to do.

The trailer is equipped with an 120/240 single phase electrical panel and it specifies for an 100A feeder. I have the following 3 options (correct me if I'm wrong):

1) From upstream 3? 208Y/120V panelboard, pull single phase 120V 100A feeder to the trailer, and only connect to one leg of the 120/240V downstream panelboard.

2) From upstream 3? 208Y/120V panelboard, pull single phase 120V 100A feeder to the trailer, and use another centre-tap transformer to split the loads across two legs of downstream 120/240V panelboard.

3) From upstream 3? 208Y/120V panelboard, pull two phases each with 120V 50A feeder to the trailer, and each phase to each leg of the 120/240V downstream panelboard.

Correct me if I'm wrong, if I proceed with Option 3, I can reduce the feeder size to 50A and use a 2-pole breaker at the upstream 3? panelboard?

 

[kwired]

I'm just going to say if the thing needs 100 amps/120 volts, it is poorly designed.

30 amps is about the most one should design for in a 120 volt only supply. Even in somewhat rare cases if you need more then 30 amps capacity you are going to spend money either on getting 120/240 to the unit or spend money on oversized conductors, raceways, etc. as voltage drop will get you faster with 120 volts only.

 

[ron]

Thanks everyone for your responses.



The trailer is equipped with an 120/240 single phase electrical panel and it specifies for an 100A feeder. I have the following 3 options (correct me if I'm wrong):

1) From upstream 3? 208Y/120V panelboard, pull single phase 120V 100A feeder to the trailer, and only connect to one leg of the 120/240V downstream panelboard.

2) From upstream 3? 208Y/120V panelboard, pull single phase 120V 100A feeder to the trailer, and use another centre-tap transformer to split the loads across two legs of downstream 120/240V panelboard.

3) From upstream 3? 208Y/120V panelboard, pull two phases each with 120V 50A feeder to the trailer, and each phase to each leg of the 120/240V downstream panelboard.

Correct me if I'm wrong, if I proceed with Option 3, I can reduce the feeder size to 50A and use a 2-pole breaker at the upstream 3? panelboard?

None of the above. Close to #3. From upstream 3? 208Y/120V panelboard, pull two phases each with 120V 100A feeder to the trailer, and each phase to each leg of the 120/240V downstream panelboard.

 

[curt swartz]

I'm lost......
If the trailer has a 120/240 panel why would you even consider only running 120 volts to it? It clearly requires a 120/240 volt 3-wire feeder.

 

[zbang]

I'm lost......
If the trailer has a 120/240 panel why would you even consider only running 120 volts to it? It clearly requires a 120/240 volt 3-wire feeder.

Exactly, and unless you're running the heat full-blast, you won't even need 100a. Most of the ones I've connected are just fine on a 2-pole 60a.

Don't forget the ground rod.

 

[kwired]

Thanks everyone for your responses.



The trailer is equipped with an 120/240 single phase electrical panel and it specifies for an 100A feeder. I have the following 3 options (correct me if I'm wrong):

1) From upstream 3? 208Y/120V panelboard, pull single phase 120V 100A feeder to the trailer, and only connect to one leg of the 120/240V downstream panelboard.

2) From upstream 3? 208Y/120V panelboard, pull single phase 120V 100A feeder to the trailer, and use another centre-tap transformer to split the loads across two legs of downstream 120/240V panelboard.

3) From upstream 3? 208Y/120V panelboard, pull two phases each with 120V 50A feeder to the trailer, and each phase to each leg of the 120/240V downstream panelboard.

Correct me if I'm wrong, if I proceed with Option 3, I can reduce the feeder size to 50A and use a 2-pole breaker at the upstream 3? panelboard?

I thought you possibly had a trailer that required 120 volt only supply for some reason, but then you later said it has a 120/240 panel - so why are coming up with the want or need to supply it with 120 volts only? Is it a listed or other approved unit with a nameplate or a homemade unit?

 

[roger]

This his seems to be a DIYer question therefore I am closing the thread in accordance with the forum rules.

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Roger