Calc Q

[Alwayslearningelec]

A few calculation questions.

1. Trying to determine the size of temp service distribution board I will need to feed all the trailer panels( it’s a large trailer complex with interconnected trailers). There are about 21 panels. Each one has a calculated load of between 77 and 80 amps. They are 1P 120/208v panels. with a 125A MCB. They give the panel schedule along with the calculated load in watts for each load, then they divide the total watts by the panel voltage for an amperage that is obviously less than the panel rating. Would I just need to make sure I have a breaker at my temp service for each panel that is rated for at least the panels calculated load in amps? The total size or rating of the entire distribution panel with these breakers would need to be at least the sum of all the panel calculated loads in amps for all the panels?
2. When trying to determine what size temporary generator is required in KW rating what formula would I used as it related to the loads it’s feeding? Thanks.

Thinking I need to add 125% somewhere as they are continuous loads.

 

[augie47]

You can size everything on calculated loads. It is very doubtful the loads would be continuous

 

[tortuga]

Trying to determine the size of temp service distribution board I will need to feed all the trailer panels( it’s a large trailer complex with interconnected trailers). There are about 21 panels. Each one has a calculated load of between 77 and 80 amps. They are 1P 120/208v panels. with a 125A MCB.

What are the trailers ? Dwelling units or ?

They give the panel schedule along with the calculated load in watts for each load, then they divide the total watts by the panel voltage for an amperage that is obviously less than the panel rating. Would I just need to make sure I have a breaker at my temp service for each panel that is rated for at least the panels calculated load in amps?

Based on the data in the 21 panel schedules you could aggregate the loads and do a giant service load calc.
Also for this large of a setup I consider using staggered 120/240 transformers with smaller feeders, not one large 208 as it appears you have no 208 3 ph loads.

 

[Tyler C]

Also for this large of a setup I consider using staggered 120/240 transformers with smaller feeders, not one large 208 as it appears you have no 208 3 ph loads.

I would be careful doing this without checking the loads first, especially if there are electric water heaters or similar loads. Any of the following could exist:

1) There are 208V only loads connected.
2) There are buck-boost transformers connected to feed 240V only loads.
3) There is equipment connected that is 208 / 240, but must be wired different depending on voltage.
4) There are resistive loads rated for 208 / 240, but their OCPDs were sized for 208V, so the increased current at 240V will push them past 80%.

 

[tortuga]

I would be careful doing this without checking the loads first, especially if there are electric water heaters or similar loads. Any of the following could exist:

1) There are 208V only loads connected.
2) There are buck-boost transformers connected to feed 240V only loads.
3) There is equipment connected that is 208 / 240, but must be wired different depending on voltage.
4) There are resistive loads rated for 208 / 240, but their OCPDs were sized for 208V, so the increased current at 240V will push them past 80%.

Thats an excellent point thank you and thinking about it more he could do a 3ph feeder and simply balance the 1 ph load.

 

[Tyler C]

could do a 3ph feeder and simply balance the 1 ph load.

That is the typical way, especially with a number like 21 that is evenly divisible. You could use a few transformers as you suggested and spread them out if needed to alleviate voltage drop, although could still stick with 3 phase since there is enough kVA here to make it worthwhile to use several smaller 3 phase transformers.

Not sure that there are even any transformers involved here though, looks like from the original post that that the intention is to just use a 208/120 generator.