Incoming Service 3

[Reesh14]

I am helping with the electrical design for a renovation of a building in Elkton, Maryland. It will require an electrical upgrade to about 1000-1200A. The existing MDP's have 120/240V and the electrical panels in the building are also 120/240V or 115/230V. There is a pole mounted transformer outside. The power company has informed me that they would much rather bring in 120/208V to the building. I would like to know if this takes place, if all the electrical panels would have to be replaced because of this change in incoming service (MDP's are already being replace). I understand that there will be some de-rating of the motors. The panels are for lighting, receptacles, hvac equipment, etc. We would like to avoid this from happening due to costs. Please share any knowledge on this matter. Thank you very much.

 

[dpc]

I think you need to hire a local electrical engineer to sort this out for you.

If the existing service is single-phase, you might be better off bringing in a new 480V service for the new loads and sub-feeding the existing panels through some transformers.

I'd be surprised if any utility is going to provide a 1200A 120/240 service.

 

[jraef]

You need to determine if any of the downstream panels have 3 phase loads on them, or only 1 phase 120/240V. If they are 3 phase you may need to replace them depending on how and where the 1 phase loads are connected. You may need only rearrange loads for better balancing as well. If everything is 1 phase, no need to replace or rearrange anything.

Then you also need to determine if any 240V equipment connected is going to tolerate 208V. Most will, but it is not for sure.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[itsmoked]

This will often be fatal to HVAC compressors. Older ones rated at 240 were utterly fried by 208. Newer ones rated 208/240 are still fried by 208. This is because the lower rating of 208 was seen as the bottom of the 10% variation a 240 might normally be exposed to. However a person's 208 could easily hit 200, (here we see 197 often), this, by the time the power hits the compressor with all the losses tossed in.

In which case again the compressors fry. Recently they have started coming out with 208 compressors. These work fine on 208.

So I would say that probably all your HVAC compressors would definitely be put at risk if you change to 208.

We always try very hard to get customers to let us put in boost transformers when we are faced with the miserable 240/208 compressors being fed with 208.

So you could still switch the plant to 208, you will just need to boost all the comps. Not a big deal cuz you're just transforming the boost with a wall mount tranny. As the comps are replaced over time you can just use 208 versions. Like I say not a big ordeal .. But an effort.

 

[jraef]

"...wall mount tranny."
Go-go cage dancer at a San Francisco bar?

LOL. Gotta be careful with terms like that around here, eh?!

Sorry, couldn't resist. RF it once you've read it itsmoked.


Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[dpc]

I would recommend against trying to run equipment originally designed for 240V at 208V. Motors are the main concern, but there can be other problems.

You can get away with it in a residential setting, where things like water heaters and electric resistance will work OK. But any 240V motors will suffer.

 

[itsmoked]

hahaha jraef!

Hey I don't need to RF it since only you or me understand this local environ.

 

[EEJaime]

When we do conversions such as this we identify the loads which require 240v to operate properly. Generally speaking on commercial work, this is limited to the HVAC and other motor loads which may or may not tolerate the lowered voltage. You didn't mention whether this was a 120/240v, 1ph,3w or 120/240v, 3ph, 4w, center tapped delta service, but the approach is similar. The simplest way is to identify the loads which require 240v. Segregate them on a branch panel and backfeed that panel from your new 208y/120v/3ph/4w. service with a buck/boost transformer that will provide the necessary voltage. As this equipment ages and is replaced, it can be phased out until all the equipment is at the service voltage. Just be certain to balance the loads on your service to account for the increased single phase 240v loads where they occur. This approach will provide the owner with a long term solution, assist the utility company in updating their distribution system and minimize initial construction costs.

 

[rob46]

Do not forget that the existing panels may be not rated for the new available fault current.