Power Distribution Design

[PhaseShift]

I'm in the process of designing a service feed to a remote location in order to provide power for various equipment. At the remote location I am told that I need (2) 110V 30A receptacles for tools as well as (1) 240V 20A receptacle for a battery charger. Adding this up tells me that I have 12kVA worth of load.

At the same location there is plans for a future load of 5kVA at 120V.

Because of the distance I am going to run a 480V service from an MCC out to this location where I will put a cabinet. I'm trying to decide weather it would be better to have the 480V feed two seperate 480 to 120/240V transformers for the two sets of loads or have one larger transformer and have a panel or something. If I go with the first option I will have a 12kVA transformer for the first set of loads and a 5kVA transformer for the second set of loads. If I go with the second option I will have (1) 17kVA transformer to serve all of the loads. Keep in mind the transformers can not get too large because I have to fit them in an outdoor cabinet.

The 5kVA load is something that is not yet planned and may never happen but I want to have the service avaliable for it. I want to factor this in the decision making for the economical aspect of the design.

Which of the two design choices do you guys think works the best?

 

[charlie b]

I don't recall 5, 12, or 17 KVA as being standard transformer sizes. Do you know anything about the tools or the future 5 KVA load? Perhaps the tools don't actually draw 30 amps each, or they are intermittently used so that you won't have 60 amps being drawn by the two tools for and extended time. My inclination, if cutting costs to the minimum was a design criterion, would be to go with a single 15 KVA transformer. The next size up is 30 KVA. If you are concerned about having to upgrade later to the larger transformer, you might consider sizing the primary conductors for the 30 KVA. I believe that means using #8, instead of #10, not accounting for any voltage drop considerations.

 

[PhaseShift]

I don't recall 5, 12, or 17 KVA as being standard transformer sizes. Do you know anything about the tools or the future 5 KVA load? Perhaps the tools don't actually draw 30 amps each, or they are intermittently used so that you won't have 60 amps being drawn by the two tools for and extended time. My inclination, if cutting costs to the minimum was a design criterion, would be to go with a single 15 KVA transformer. The next size up is 30 KVA. If you are concerned about having to upgrade later to the larger transformer, you might consider sizing the primary conductors for the 30 KVA. I believe that means using #8, instead of #10, not accounting for any voltage drop considerations.

I dont know anything about the tools other than the fact that I was told I needed (2) 30A circuits. Unfortunately I dont know if these tools will be used simultaneously or intermittenly. As far as the future load, all I am told is that it would be (2) 20A circuits at 120V feeding two different electrical cabinets.

I am thinking about installing a 15kVA transformer as you mentioned and then using this transformer to feed an 100A 120/240 panel.

From this panel I can then have (2) 30A circuits @ 120V, (1) 20A circuit at 240V, and (2) 20A circuits at 120V. I was thinking about also throwing in another convenience receptacle on one of these circuits somwhere for other convenience tools or lighting. I'm sure amoungst all these items there will ba a diversity factor.

What do you think of the above layout?

 

[augie47]

also, most of the transformers you would normally use for this install would alredy be suitable for outdoor installations or would have a Nema 3R outdoor kit available at a small cost. You normally would not need an "outdoor cabinet."

 

[PhaseShift]

also, most of the transformers you would normally use for this install would alredy be suitable for outdoor installations or would have a Nema 3R outdoor kit available at a small cost. You normally would not need an "outdoor cabinet."

I was thinking about using an outdoor cabinet to put the 120/240 distribution panel in and maybe mount the receptalces on the outside of the panel, since the panel is in the middle of no where. Would a 15kVA transformer fit inside a large panel or would I need need to have an outdoor transformer outside the cabinet.

I was also thinking about using a fused disconnect mounted on top of the cabinet and using it as the primary protection of the breaker.

If I need the mentioned 30A circuits for tools, do I need to make these receptacles rated for 30A? I guess this would mean that I could not put other 20A convenicne receptacles rated for 20A on this 30A circuit. I'm trying to squeeze another convenience receptalce or two in somewhrere. Do you think there is a divirsity factor I can assume in some places to squeeze in these receptacles. What is typically assumed?

 

[Pierre C Belarge]

I like the idea of one transformer/panel. A clean and easy install.

 

[PhaseShift]

Im thinking about using the following branc circuits for this design:

One duplex receptacle with each receptacle being served from a 30A breaker. This way each receptacle will be dedicated for the (2) 30A tools if necessary.

One 240V receptacle fed from a 20A breaker to be used for the battery charger.

Two duplex receptacles fed from a 20A breaker which can be used for convenience receptacles for dropt lights, other tools etc...

I will then have two extra 20A breakers to be avaliable for the (2) future 20A loads.

This would give me a connected load load of 90A on one L1 and 70A on L2 of divided up. Using a 15kVA transformer I will have about 62A avaliable on each leg. I am hoping with the diversity factor of the loads used above that this design will work.

What do you guys think about this design?

 

[LawnGuyLandSparky]

Make it so.

 

[EEJaime]

Phaseshift,
I am not sure how you are calculating your load, but from your OP you have:

2 circuits at 120V, 30A rated [= 2 x 120 x 30 x 80%] = 5760 VA +
1 circuit at 240 V, 20A rated [= 240 x 20 x 80%] = 3840 VA = 9600 VA or 9.6 kVA.

If you add to that your future load of 5 kVA @ 120V, (hopefully you can split that into two circuits to balance your load), you will still have an ultimate load of 14.6 kVA.

I would recommend insalling a 15 kVA, 480V, 1ph to 120/240V, 1 ph, 3w Mini-Power-Zone panel that comes complete with primary breaker, transformer, secondary main and branch breakers. It is a neat and simple package that provides plenty of flexibility and sufficient space for growth. You will most likely not see all loads simultaneously. These units were a Square D piece of gear initially, but today, many manufacturers offer similar products.

Good Luck,
EEJaime

 

[PhaseShift]

OK, thanks for all responses.

Now that I have my branch circuit figured out I am trying to determine what I will use for the feeder circuit.

Since I am using a 15kVA transformer, it has a primary draw of 32A. Looking at table 450.3(B) I see the max primary protection without secondary protection if 125%. Taking 125% of 32A gives me 40A. I am going to use a 40A fused disconnect for the primary protection. Do we typically need to protect the secondary of the transformer in an application like this? Should I use secondary fuses as well?

As far as the feeder from the MCC out to the fused disconnect I know I have a load of 32A as mentioned above. Do I need to take 125% of this load for figuring cable size? Assuming I need to take 125% I come up with 40A again. Ignoring voltage drop this would require a #8 fed from a 50A breaker.

Figuring in voltage drop, I have a distance of about 1000ft. When calculating voltage drop, do I use the 32A load or the 40A load (125% rated) when figuring the load current. Assuming I use the 32A and 3% VD I get the following: (Not sure if I should use 3% or 5% for this feeder circuit)

CMILL = [(10.3)(2)(32A)(1000ft)] / 24.4
= 45777 CMILL

This would require a #3 AWG conductor to account for voltage drop.

Last but not least I need to size feeder breaker in MCC. When sizing this breaker do I need to account for the cable size as a result of votlage drop calc or just based on the load current. If based on load current then I would be able to use a 40A breaker. If I account for voltage drop de-ratted cable then I would need an 100A breaker for the #3 AWG cable. Which of the two would be an appropriate breaker size?

I appreciate all the help, this is the first time I have done such a design so I want to make sure that I am learning to apply the code right with this design decision. The design will be reviewd by other, but I want to try to work through it myself before they go correcting things.

 

[wireguru]

Im thinking about using the following branc circuits for this design:

One duplex receptacle with each receptacle being served from a 30A breaker. This way each receptacle will be dedicated for the (2) 30A tools if necessary.

where do you get a 30a duplex receptacle?

seems like the 30amp circuits should be L5-30 twistlock, or maybe 5-30 straight blade receptacles, depending on the client's needs. Or put in both, or sell them adapters....

 

[PhaseShift]

where do you get a 30a duplex receptacle?

seems like the 30amp circuits should be L5-30 twistlock, or maybe 5-30 straight blade receptacles, depending on the client's needs. Or put in both, or sell them adapters....

Thanks, I will look for these type of receptacles to include.

 

[PhaseShift]

The two things that are puzzling me and I cant seem to find any information on are:

1) When sizing the breaker for my feeders do I need to size the breaker per the size of the cable required for the load, or do I size the breaker per the increased cable size as a result of the VD calcs?

2) When calculating feeder load do I need to use 125% of transformer primary current since this is a continuous load?

Thanks

 

[mull982]

The two things that are puzzling me and I cant seem to find any information on are:

1) When sizing the breaker for my feeders do I need to size the breaker per the size of the cable required for the load, or do I size the breaker per the increased cable size as a result of the VD calcs?

I believe that you only need to have the OCPD to protect the cable that would be sized for the load current and not the cable sized for Voltage drop. In this case it would only need to be sized for 32A

2) When calculating feeder load do I need to use 125% of transformer primary current since this is a continuous load?

Thanks

For a transformer I believe that you do not have to take 125% of the transformer primary rated current but rather just the primary current. In this case the feeders would need to be sized for just 32A.

Please correct me someone if I am wrong

 

[Cold Fusion]

I dont know anything about the tools other than the fact that I was told I needed (2) 30A circuits. Unfortunately I dont know if these tools will be used simultaneously or intermittenly. As far as the future load, all I am told is that it would be (2) 20A circuits at 120V feeding two different electrical cabinets.

I am thinking about installing a 15kVA transformer as you mentioned and then using this transformer to feed an 100A 120/240 panel.

From this panel I can then have (2) 30A circuits @ 120V, (1) 20A circuit at 240V, and (2) 20A circuits at 120V. I was thinking about also throwing in another convenience receptacle on one of these circuits somwhere for other convenience tools or lighting. I'm sure amoungst all these items there will ba a diversity factor.

What do you think of the above layout? (Emphasis is mine)

Not much. You are setting your self up for a marginal installation. As you said, "I dont know if these tools will be used simultaneously or intermittenly." If you don't know that, then how do you know, "I'm sure amoungst all these items there will ba a diversity factor."

If you don't understand the customers needs, how can you expect to meet them?

I'd RFI the customer and ask your two questions. If the boss said, "Don't do that.", I don't know what to tell you. Maybe figure out a way to ask the boss, "Okay, do I do this cheapeast possible? Or guarenteed to work?"

cf

 

[PhaseShift]

Not much. You are setting your self up for a marginal installation. As you said, "I dont know if these tools will be used simultaneously or intermittenly." If you don't know that, then how do you know, "I'm sure amoungst all these items there will ba a diversity factor."

If you don't understand the customers needs, how can you expect to meet them?

I'd RFI the customer and ask your two questions. If the boss said, "Don't do that.", I don't know what to tell you. Maybe figure out a way to ask the boss, "Okay, do I do this cheapeast possible? Or guarenteed to work?"

cf

I hear what you are saying. But similar to a panel in your house, the circuit breaker ratings always add up to more than the feed of the panel. The idea there is that there is obviously some sort of diversity, or else all of those circuit breakers would not be avaliable.

As an example, I have seen in an MCC a 15kVA transformer serving a 225A 120/240 panel. Certainly 225A is much larger than the 62A the transformer can supply and therefore can never be fully utilized. The panel can only have 62A of load. I'm sure there is a diversity factor assumed here?

What is the rule of thumb for such installations? In this case, I am the client, and am designing this for our use. Maintance has told me what I stated above regarding the two tools.

 

[Cold Fusion]

---But similar to a panel in your house, the circuit breaker ratings always add up to more than the feed of the panel. The idea there is that there is obviously some sort of diversity, ---

It's not a house. It's a machine shop(?). There are two circuits, not 40. I have no idea what the diversity is.

---What is the rule of thumb for such installations? ---

Rules for a house? Lots, I guess. I don't do houses. For a machine shop - none I know of.

--- In this case, I am the client, and am designing this for our use. Maintance has told me what I stated above regarding the two tools.

With the information so easily available, I'm baffled why you would even consider guessing. You shouldn't have any trouble going down and looking at the machinery nameplates, asking the foreman, "Are you going to use these two machines at the same time?" If she says no, tell them you are interlocking them - bet they change their mind.

It's a machine shop where the workers do stuff to make the company money. You down size the install and save the owners a few bucks. First time they get a rush job and turn on every thing at once and trip a breaker - guess who's name is mud. I don't know this is true. Maybe cheap and an occasional tripped CB is okay. You could know pretty easily.

cf

 

[PhaseShift]

It's not a house. It's a machine shop(?). There are two circuits, not 40. I have no idea what the diversity is.


Rules for a house? Lots, I guess. I don't do houses. For a machine shop - none I know of.


With the information so easily available, I'm baffled why you would even consider guessing. You shouldn't have any trouble going down and looking at the machinery nameplates, asking the foreman, "Are you going to use these two machines at the same time?" If she says no, tell them you are interlocking them - bet they change their mind.

It's a machine shop where the workers do stuff to make the company money. You down size the install and save the owners a few bucks. First time they get a rush job and turn on every thing at once and trip a breaker - guess who's name is mud. I don't know this is true. Maybe cheap and an occasional tripped CB is okay. You could know pretty easily.

cf

CF

Thanks for the advice. I'll reconsider my branch circuit design.

Can you help me in regards to my two earlier questions regarding the feeder breaker and feeder cable sizing. I want to make sure that I clearly understand these when I designe the feeder service both now and in the future.

Thanks for the help.

 

[Cold Fusion]

...Can you help me in regards to my two earlier questions regarding the feeder breaker and feeder cable sizing. ...

1) When sizing the breaker for my feeders do I need to size the breaker per the size of the cable required for the load, or do I size the breaker per the increased cable size as a result of the VD calcs?

2) When calculating feeder load do I need to use 125% of transformer primary current since this is a continuous load?

I'm guessing you are talking about the transformer primary cb and primary conductors. If not, none of the applies.

One usually sizes the CB feeding a transformer at least 125% nameplate current so the CB doesn't trip on inrush. Look at T450.3.B. Keep in mind that 450 addresses transformer protection not conductor protection.

For sizing the primary conductors, look at 240.4 and 240.4.B. They have to be protected by the CB you picked in 450. So the conductor ampacity is going to be 125% nameplate current. You can oversize the conductors all you want

Nothing stopping you from sizing the primary CB less than 125%, but it looks pretty bad if the CB trips on inrush

cf

 

[wasasparky]

At the remote location I am told that I need (2) 110V 30A receptacles for tools as well as (1) 240V 20A receptacle for a battery charger. Adding this up tells me that I have 12kVA worth of load.

Not even close. Do not use breaker amperages to size load!

Lots of questions:
What kind of tools? (load and duty can vary greatly)
Hand/portable that are infrequebtly used?
Machine type that could operate for hours?
What's the battery charger for? (load and duty can vary greatly)
Charger for forklift? (material handling equip)
Charger for a heavily loaded UPS/DC power supply?
Where is this thing? (I hear 3R enclosure, and 1000' away)
Is it a remote outdoor thing way outside in the yard?
Is it in the far corner of the shop?
I would have to assume outside far from the building, because if indoors you would want to install a bit more capacity since you are already far from usable power distribution and don't want to do this again next time you need to add some equip...