three phase vs. single phase light load calculations.

[HighPotter]

Had a bit of a "conversation" with an engineer today, regarding his sizing of OCP and wire for a lighting project.

Project involves(2) 100' light poles, with (12) 1000w hps light fixtures on each tower. These are the " Halophane" drop ring style tower and lowering device. The distance is 250' from the 480 3 phase panelboard to the first tower, then 500' more to the last tower. At each tower base, is a backboard with a 60 amp 3 pole 480 v fusible disconnect tapped from the (3) 2/0 copper wire that is run from the main panelboard to the first tower and then on to the 2nd tower.

He calculates the connected load per tower at 25 amps " single phase", because he says that even though the lighting is balanced over 3 phases, the lights themselves are single phase. Or 31.25 at 125%

I calculate the load per tower at roughly 14 amps -3 phase. Becaus the feeder IS 3 phase. Or 17.5 at 125%

As you can see, the difference in these calculations effect everything from wire size, to ocp to disconect size.

Have I been wrong all these years?


HP



HP

 

[HighPotter]

EEJaime,

Thank you. Finally someone that understands this project. It isn't more complicated then it sounds. It IS a very simple, very common installation. I just wish I could convince him of that.

HP

ps. BJC....the 2/0 was spec'd in conduit. Also, I agree that some projects take a turn, that wasn't to be expected. I am trying not to come off as wanting to " take the legs out from under" this engineer. To the contrary. I am only trying to suggest changes that I feel are in error based on his design. I am not in any way wanting to change any forward thinking, or room for expansion.

Advidiana..... I guess our "experiences" differ.

 

[Jensgisla]

Mr. Potter:

If I were you I would draw a multiline diagram including load connections, load sizes, voltages and currents. Send him the diagram, ask him to correct the miscalculations or connections and send it back.

 

[HighPotter]

Jensgisla

That is a very good idea. I will take your advice. Thank you

HP

 

[advidana]

It may be a good idea to check with the lighting mfg rep what was spec on the light for balance phases on each tower. They may need to be changed to comply with what you are proposing.

Good luck I know these spec battles can be messy because of bid cost versis what has designed and called out on the specifications and drawings.

 

[HighPotter]

advidana,

The light "whips" are pre-wired by the factory, and already balanced across all 3-phases, as much as is possible.
In other words, at the top of the pole, you have 9 prewired cords to a prewired, raintight junction box. All that is needed to finish the wiring is plugging the end of these 9 "whips" into the mounted light fixtures, and completing the connnections in the fixture.

But yes, these instances do become messy.

Thank you

HP

 

[waross]

An engineer sent me to install a smoke detector in a duct. When I arived at the site I found two ducts. I phoned him to ask him which one he wanted the detector installed in. (Fire code required detectors in both ducts.) He said there was only one duct. I said I counted two. I asked if he had ever visited the site. He said no, but there was only one duct. I asked him to visit the job site. He said he didn't need to, there was only one duct. I told him that I did sometimes make mistakes, and I might be making a mistake this time, but I was confident of my ability to bleeping count all the way to two with out making a serious mistake and there were two bleeping ducts. I thought he had retired, but I guess he just moved.

 

[alehman]

OT: Good point waross. Engineers do need to get in the field more. We sometimes fall in the trap of thinking about things the way the "should" be rather than looking at what really exists. The problem is driven by several factors -

1. Lack of time
2. Cost. We are often asked to base the design on "as-built" drawings. Our clients don't want to pay us to make many trips or we are on a fixed fee and didn't plan to make many trips.
3. Bad information. When we get to the field, we find that while there are two ducts, they are actually two branches from a common supply duct, where the one detector should be located.

 

[advidana]

If you new these answers to your question why did you make this post. But it was interesting to see all the different discussion.

Yes engineers do make some designs that could be done differently to save cost but I don't see a code discripancy.

 

[waross]

A good point also, alehman. Right on, but in reverse. The original design was for the ducts from two areas to be combined before entering the walk-in plenum. This was changed during construction and not noted on the as builts. There was no common point that could be served by one detector.
Point is, an engineer with a reluctance to consider new information may not be well thought of.