3 phase question from an electrician who rarely does 3 phase

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Stevenfyeager

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Location
United States, Indiana
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electrical contractor
I am always doing residential single phase work. Sometimes I will run a new circuit for a store with 3 phase. That is as much as I have done. But this customer wants an estimate for a new 3 phase service in addition to their single phase service because they are adding two machines, both 60 amp, 208 v. The customer said that converters are too expensive for the machines to use the existing single phase service.
I've priced a 3 phase underground feed meter, 3 phase 200 amp panel, and breakers and disconnects. I still need to price the permit, the last time I worked for a store in this town, the permit was $400. I will be meeting with POCO Friday. Is there anything I need to know about my first new 3 phase install? Grounding, for example? (this is a pole barn type building) Any advice would be appreciated. Thank you very much!
 
The things that come to mind are will you need a neutral from your transformer (probably so) or not, and with motor loads making sure your phase rotation is correct. That and making sure that 3-phase is available from the Poco.
 
POCO will probably offer a grounded three phase wye (208Y/120). If so you need to bond the incomong POCO neutral to ground at (or near) the service disconnect. You will use the same GES for both services.
In the unlikely event that POCO provides an ungrounded delta you will have to determine whether the customer can handle a corner ground. It might be difficult to justify using it ungrounded.

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180508-2418 EDT

Stevenfyeager:

What kind of machines? Do they really run at 60 A? Are they really only 208 machines?

If these are CNC machines, then they will probably work on a wild-leg delta requiring only one added transformer, and probably have internal taps for 240. Further, if CNC, then unlikely to run continuously at 60 A.

In our area we have two transformer wild-leg deltas all over including my neighborhood.

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gar said:
180508-2418 EDT
What kind of machines? Do they really run at 60 A? Are they really only 208 machines?
If these are CNC machines, then they will probably work on a wild-leg delta requiring only one added transformer, and probably have internal taps for 240. Further, if CNC, then unlikely to run continuously at 60 A. In our area we have two transformer wild-leg deltas all over including my neighborhood.
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Point well made, do not trust the information from the Owner, get the machine info first and make sure you are doing right.

I would also find out how much the "converters' would cost. I assume he is talking about using a VFD. If your cost for the new 3 phase service is going to be more than the cost of the 2 VFD's then your wasting your time bidding it.
 
One of the things that people often miss, and discover when it's too late, is that a 3 phase service drop may come with an unexpected surprise; a Demand Meter and subsequent Peak Demand charges. It doesn't affect your installation costs, but it will be a rude awakening for the owner and he will look to you to explain it to him. I suggest finding out from the PoCo if this is going to be the case, it may trigger a re-thinking of the idea of converting at the machines, with VFDs or otherwise.
 
How much load is being added, if you are converting from single phase the amp draw on the single phase side of conversion equipment should be about 1.732 times the draw on the three phase side - depending on what you have already might need to upgrade the single phase supply equipment to handle added load anyway.

In many instances if installing the thee phase service you would probably be better off to just upgrade service to a single three phase supply - even if you end up leaving existing items mostly as is. Supply existing panel from the three phase supply either as a second service disconnecting means or convert to a sub feed from a new service.
 
Probably the biggest factor will be how much the utility will charge you for the new service. Have they given you a cost? In my experience it can vary widely from free or a few hundred to ten thousand or more (even for a relatively small service).
 
electrofelon said:
Probably the biggest factor will be how much the utility will charge you for the new service. Have they given you a cost? In my experience it can vary widely from free or a few hundred to ten thousand or more (even for a relatively small service).
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How much energy you are expected to use can be a factor in that cost. Large service with high demand at times but low overall usage - is going to cost more then larger service that is at a moderate demand nearly constantly - they will get faster return on their equipment investment in energy sales.
 
petersonra said:
most POCOs no longer sell energy. they just deliver it. the supply of electricity is a separate entity, sometimes directly affiliated with the POCO, sometimes not.
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Electric utilities are all publicly owned here - so not used to that concept, but I agree. Those places that are not also selling the energy probably do charge more for providing services since usage amounts don't really impact their return on investment.
 
electrofelon said:
Probably the biggest factor will be how much the utility will charge you for the new service. Have they given you a cost? In my experience it can vary widely from free or a few hundred to ten thousand or more (even for a relatively small service).
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The customer really needs to be talking to the power company about some of this stuff. They are the one's that will have to set up an account and get the power turned on. Even the deposit on a three phase commercial service can be more than most would think ( probably help that they have a history with the power company, if it's good history).
 
growler said:
The customer really needs to be talking to the power company about some of this stuff. They are the one's that will have to set up an account and get the power turned on. Even the deposit on a three phase commercial service can be more than most would think ( probably help that they have a history with the power company, if it's good history).
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And if two different voltages (I guess technically OP sort of is with 208 vs 240) it may be better to separately derive one system and only be supplied by a single service. Get beyond 2000 amps (maybe less on single phase services) and that may favor multiple services cost wise.
 
Have to think he is better off with one service. Chances are existing single phase has to be upgraded to handle added loads plus cost of VFD’s to convert to three phase loads. On the other hand will existing 240 volt loads tolerant 208 supply (assuming 3 phase 208/120 wye service is available)? What will three phase service cost from power company? Demand charge may come into play. Time to plan wisely and move slowly IMHO.

Chances are 240 devices will tolerate 208, and 3 phase is superior, but it is better to stick to your wheelhouse so I reckon I might suggest upgrading existing service and add VFD’s. Can’t believe I just recommend that!
 
Majority of 240 volt loads will be fine on 208. If there is one that absolutely can't tolerate for some reason - buck/boost transformer will work there and likely not all that expensive.

Straight resistance loads will have lower watt output - but usually just means longer time heating up. Unless they were marginally designed for the application to begin with - will still work.
 
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