Peak demand, double check my math - Electrician Talk - Professional Electrical Contractors Forum

**MDShunk** · 10-24-2007, 05:27 PM

The job is at a church. They'll be adding a new 15-ton RTU. The RTU draws 90 amps at 208 (max breaker size listed is 100, oddly). In any event, they have a 400 amp overhead 208 wye service that visually looks pretty "loaded". Got peak demand data from the utility, and the highest peak was one August a couple years ago, and it was 64 kva. Seems like 64 kva at 208 volts is 307 amps. Add my new load of 90 amps, and we can have potential peak demands of 397 amps on this 400 amp main. There will be several "window shaker" style air conditioners that will get retired when the RTU goes in that will eliminate some load right there. This one is so borderline, I'm open to any comments on the matter. It is an I-line panel, so the main could be changed if it wasn't for the fact that the panel rating is only 400 amps. Seems like this one is right on the brink of them spending a couple grand for an air conditioner circuit, or 10's of thousands on a service upgrade.

**Speedy Petey** · 10-24-2007, 05:32 PM

I personally think losing the window units will give you enough "cushion" to allow it.

Are you sure it's an actual draw of 90 amps? I agree, it just doesn't seem right.

**MDShunk** · 10-24-2007, 05:37 PM

Quote:

Originally Posted by Speedy Petey

I personally think losing the window units will give you enough "cushion" to allow it.

Are you sure it's an actual draw of 90 amps? I agree, it just doesn't seem right.

Well, I don't know. The cut sheet shows minimum circuit ampacity 90, max breaker size 100. I'm reasonably sure that the measured draw will be less, but that's all I have to go by. The Trane cut sheets are really nice, because they have a copy of the equipment dataplate right on the cut sheet. 90 sounds high to me for only 15 ton with no heat package. I generally figure about 1 horsepower per ton, about a horsepower for the condensing fan motor, and 2 or 3 for the supply fan motor. That works out to about 19 horsepower using my rules of thumb, which sure isn't no 90 amps on 208, three phase.

**Speedy Petey** · 10-24-2007, 06:39 PM

Yeah, a 90 MCA would take into account the 125%. That would be roughly a 72A draw no?

micromind · 10-25-2007, 10:11 PM

I'm not an engineer, but if this is a 208 3PH service then 64KVA comes to 178 amps. If it's single phase then it's 307.

POCOs are known for small overhead wire, hook it up anyway, fire and brimstone!! LOL.

sckeeth · 10-26-2007, 06:11 PM

I have that same delema right now. This machine shop has a 3 phase 200 amp service feeding two panels on wednesday it was 40 amps and thursday afternoon it was 80 amps. Well he just got a machine that is 36 kva at 240 so it puts it around the 90 amp marker. So what do I do. He wants to try and hook it up with out getting a new service and see what the amp draw is and if he needs to put in a service after then we do it.
The only problem is it has a 125 disconnect on the machine and the panels only accept 100 max because they are stab breakers. I would have to go and put a 400 amp panel and feed it out of there, and if the load is two big than the panel is ready for a new service. Any thoughts? I hope I made it clear. thanks

micromind · 10-26-2007, 09:27 PM

Without actually seeing it, here's what I'd do.

Since the owner is willing to 'experiment', he's not likely to blame you if this fails. I'd go ahead and install the 100 amp breaker, and see what happens. I'd also leave (I really hate doing this) enough slack in the panel to A), allow for replacement in a different location when the 100A stab-in breaker burns up the bus extensions and B), allow for moving the circuit to a fused disconnect (or breaker enclosure) if a 100 doesn't hold.

**MDShunk** · 10-26-2007, 09:29 PM

Quote:

Originally Posted by sckeeth

The only problem is it has a 125 disconnect on the machine and the panels only accept 100 max because they are stab breakers.

What brand? You can get a QO, for instance, in up to 200 amp plug on breaker. Bigger than 100 amp (125-200), and they take up four spaces.

**frenchelectrican** · 10-28-2007, 12:21 AM

Quote:

Originally Posted by MDShunk

What brand? You can get a QO, for instance, in up to 200 amp plug on breaker. Bigger than 100 amp (125-200), and they take up four spaces.

Marc :

just want to give you a head up if you run into those large QO series breakers [ 150-200 amp verison ] just becarefull with those on 3 Ø boxes if you want to run 1 Ø off from 3 Ø box ya can but not easy with it otherwise use the 3 Ø verison and run 1 Ø off from it [ btw the 3 Ø breakers are not cheap ugh ]

Merci , Marc

JohnJ0906 · 10-31-2007, 05:33 AM

Quote:

Originally Posted by Speedy Petey

I personally think losing the window units will give you enough "cushion" to allow it.

I agree.

Single phase or 3 phase service?

**MDShunk** · 10-31-2007, 04:44 PM

Quote:

Originally Posted by JohnJ0906

I agree.

Single phase or 3 phase service?

Three phase, but that peak demand the utility gave me was the highest peak demand of any of the three phases. The 1.732 doesn't really play into the calculation when that's the case. I'm going to go ahead and do this, and the utility is going to change out their 75 kva poletop kettles with 100's, but they need to set a second pole and put them on an H frame. Aparently, 75's are the biggest they're willing to hang (3 of them) on a single pole. Bigger than that, and they want to set a second pole and put a platform in between to set them on.