Currently have a 400 Amp service that feeds multiple motors plus a small lighting load. The user is adding another 50Hp and 20Hp motor (240 3ph) and I already know that the current 400 Amp won't cut it. I'm thinking of going with a 600A service due to limitations from the POCO. So I want to accurately calculate what the true load is going to be. This is for a fertilzer dealership, and they probably have in excess of 25 motors total!! Just need some advice on accurately doing a load calc! Thanks!
Multiple Motor Load Calculation
- Thread starter gk351
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ptonsparky
Tom
- Occupation
- EC - retired
Yup, find out which motors actually need to run at the same time. With a little control work you may be able to save them some POCO demand charges as well
Well I have a slight idea of what would be running at the same time. I know a combination of a 50, 20, 10, 10, 7.5, 7.5 Hp would be. Not continuously all day everyday, but the potential for more than 3 hours. When they are in different peak season, a combination of 2- 10hp, 6- 7.5 Hp, 6-5 Hp motors could be running a the same time.
kwired
Electron manager
- Location
- NE Nebraska
If you decide you need more capacity figure out what it will take to add it at 240 volt, and then figure out what it will take to change it to 480 volt. If enough is being added 480 may be the way to go. Same size conductors will handle twice the kVA. You will need transformer for 120/240 items but those items are usually limited loads in that type of place. Some of your lighting may easily convert to 277 volt.
I sat down with the owner today and discussed some options. Since we are going to be limited by the POCO with the current 240V Delta Wild Leg System ( They won't install larger than 600 Amp, and will not add a new service or move the xfmr bank from the pole) I suggested that we install a new 480V service. I feel this woud suit the larger motor loads much better than 240V. I just have to find a 480V-240V Delta (wild leg) step down dry pack xfmr. I can just feed my smaller existing load with that, and everything new can go 480V.
Be careful on your transformer. Most, if not all, 480/240-120 3 phase transformer have a 5% or less loading limit on the neutral.
I have found on many of these "changeovers" that I have needed a 208Y/120 transformer to pick up a majority of my 120v load and, on occasion, a 480/240 on those loads which I could not convert to 208.
I have found on many of these "changeovers" that I have needed a 208Y/120 transformer to pick up a majority of my 120v load and, on occasion, a 480/240 on those loads which I could not convert to 208.
kwired
Electron manager
- Location
- NE Nebraska
Why not feed all the three phase loads with 480 volt then all you need is transformer for any single phase load. All you need is to reconnect motors for high volts, change/adjust overloads. You will likely be installing 480 volt panelboard so get some extra breakers. Conductors to motors will already be plenty large enough.
They have made the simple (5%) more complicated
from the catalog:
Three-Phase—480 V Primary
240 V Delta Secondary w/120 Center Tap 60 Hz;
cULus Listed
240 V Delta Secondary w/120 Center Tap 60 Hz;
cULus Listed
240 Delta with 120 center taps have historically been limited to 5% capacity on
the center tap. The new units from Schneider Electric offer greater limits on
120 V center tap. Limits are determined by the total transformer loading and the
following formula used to size new 120 V center tap units:
(240 V balanced loads) + 2.5 x (120 V loads) = kVA requiredthe center tap. The new units from Schneider Electric offer greater limits on
120 V center tap. Limits are determined by the total transformer loading and the
following formula used to size new 120 V center tap units:
kwired
Electron manager
- Location
- NE Nebraska
5% of what? You should try to balance neutral between A and C phase anyway but if it is 5% of rated current per phase you could not even run a 10 amp 120 volt load and nothing else if the rated secondary was 100 amps. If it is 5% of total rated KVA of transformer then the same 100 amp @ 240 volt transformer would not be allowed more than about 17 amps of neutral current. 2 10 amp loads connected to same phase with nothing else operating would be more than what is allowed.
I can see where a max of about 1/3 of the total kVa rating of the transformer of neutral current may be about all that the transformer can handle, with 2/3 rating max for total KVa between A phase and C phase. That is if it is a 3 coil transformer with all three the same size, which a dry transformer likely will be, when POCO builds these services with multiple single phase transformers they often put a larger pot on the portion with the single phase load connected to it - open or full delta.
I can see where a max of about 1/3 of the total kVa rating of the transformer of neutral current may be about all that the transformer can handle, with 2/3 rating max for total KVa between A phase and C phase. That is if it is a 3 coil transformer with all three the same size, which a dry transformer likely will be, when POCO builds these services with multiple single phase transformers they often put a larger pot on the portion with the single phase load connected to it - open or full delta.
We are talking about 25 motors all in different facilities. A lot of equipment would have to be changed becuase it is only rated for 230V. If it was that easy, I would be trenching already. I guess I just need some clarification on the 5% rule. Does that mean, that my 120V load may not exceed 5% total rating of the xfmr? This plant is 30 years in the making, and just keeps getting pieced together.
There are folks on the forum who can explain the technicalities to us and I do not know what changes SquareD has made, but as there catalog notes, historically three phase transformers with 240/120 secondary have had a limit on the neutral to 5% of the kva rating.
I can only say I have seen failure when this is ignored.
POCOs arrangement using 3 single phase pots does not have this same limitation so it is often overlooked on changeovers.
kwired
Electron manager
- Location
- NE Nebraska
I would still suggest anything over 7.5 hp - maybe even 5 hp be converted to 480 volt. Maybe you can put in 208/120 3 ph Y transformer for small motors and other single phase load. Some will cry that the motors are not for 208, but many are rated both 208 and 240. If limited to small general purpose motors they will last long time anyway and are not that expensive to replace. You said this was fertilizer plant - the environment will get these motors before the load does if they are not stainless steel.
kingpb
Senior Member
- Location
- SE USA as far as you can go
- Occupation
- Engineer, Registered
May want to consider a 480-240D (3ph/3W) transformer only for the 3 phase loads, and install another 480-208Y/120V for the other loads. Would give you more flexibility. Plus that gets single phase loads, such as lighting and what-not off the panel that feeds three phase motors.
Just make sure the transformer is big enough to handle loading/starting requirements.
Just make sure the transformer is big enough to handle loading/starting requirements.
I want to thank everyone for their input. I decided that I will just install a new 208V system, and just make adjustments to the current system by changing wire, heaters, starters, etc. I think in the long run it will be the easiest way to go. Somehow, some way...it will be easier!! Thanks again...GK
kwired
Electron manager
- Location
- NE Nebraska
Changing overloads may not be necessary unless you have a motor operating near 100% of its rating. You will have overload protection set at lower rating than the 208 volt full load, and if not a motor that is loaded hard will be just fine. Nothing says you cant set overload protection lower than what code says is maximum that is just a personal choice or design.
weressl
Esteemed Member
Sounds like 480V service from the POCO would be a way. Renegotiate the Service Contract and for the higher voltage they may even have a lower rate.
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