480v disconnect

[Dsg319]

It's for a 200 amp service. If the load is over a 100 amps, which it will be, I assume 100 amp fuses won't be sufficient.

The primary 480volt side will only see have of that amperage of the secondary

 

[Dsg319]

The primary 480volt side will only see have of that amperage of the secondary

Half*******

 

[ggunn]

Im replacing an underground service where the sub pole is 1000' away. At the pole I have the 200amp meter main disco to a transformer right next to pole to jump up to 480v for voltage loss over the 1000'. Then into a disco and another step down transformer by the house before going into the panel. My question is does the disco for the step down transformer have to be fusible? Having a hard time finding fused 480v 200 amp 2 phase disco.

480V is not a standard size; for 480V we use 600V rated switches. You could use two poles of a three pole 600V switch

 

[electrofelon]

It's an 40 year old 100 amp direct bury #1 aluminum service. I'd need 750 kcmil to account for voltage loss over 1000'. This was the cheaper solution to upgrade to 200 amp.

Ok so lets say we want to design for 5% VD at 100 amps, 240 V (24KVA) That should be plenty unless they have something like an on demand water heater or an electric furnace.

I get 600KCMIL AL at 240V with 4.5% VD. At 480V, I get 1/0 with 4.3%. BUT, that is ignoring the voltage drop of the transformers. You actually should pretty much double that, maybe around 2% each, so really you are at 8%. Now lets go back to the no transformer route and plug in the 8% and see what we get. Looks like only 250 is needed. And dont forget the $300/yr in no load losses.

People always ignore the transformer VD and losses. I have a step up step down at my house (1900 feet) and trust me, it is not a great way to go, your voltage will swing a lot more than you think. I would never entertain doing that for 1000 feet.

 

[hartr082988]

Ok so lets say we want to design for 5% VD at 100 amps, 240 V (24KVA) That should be plenty unless they have something like an on demand water heater or an electric furnace.

I get 600KCMIL AL at 240V with 4.5% VD. At 480V, I get 1/0 with 4.3%. BUT, that is ignoring the voltage drop of the transformers. You actually should pretty much double that, maybe around 2% each, so really you are at 8%. Now lets go back to the no transformer route and plug in the 8% and see what we get. Looks like only 250 is needed. And dont forget the $300/yr in no load losses.

People always ignore the transformer VD and losses. I have a step up step down at my house (1900 feet) and trust me, it is not a great way to go, your voltage will swing a lot more than you think. I would never entertain doing that for 1000 feet.

We are doing 200 amps because it's a decent sized house with a jammed panel and they plan on adding a barn/shop. The cost difference is significant to do it without transformers. The existing service has 2 transformers and appears to have been satisfactory for powering their home. Do the math on 200 amp wire

 

[electrofelon]

Do the math on 200 amp wire

I have. You need to do a full and proper analysis. OF course the transformers look good when you assume they are 100% efficient and IGNORE the losses and voltage drop in them. When you use the ACTUAL entire system voltage drop you will have with transformers, and use that figure for no transformers, you will find you dont need as big a wire as you think. (2) 750's with a 500 for the neutral will cost you about 8K. How much you got in transformers and disconnects? Remember the $3000 in 10 years of losses, throw that into the mix....

 

[electrofelon]

Why not at least use a 600V transformer?

 

[hartr082988]

Why not at least use a 600V transformer?

wait time. The house is without power because the wire has a short somewhere. I could acquire 480v quicker.

 

[electrofelon]

wait time. The house is without power because the wire has a short somewhere. I could acquire 480v quicker.

I understand there are often "externalities" in the real world such as equipment availability/wait times, using an existing wire run or equipment, and perhaps most importantly what the customer wants to do. Transformer losses are potentially another one, where a customer may prefer to amortize an amount of money into transformer losses over decades versus paying a lump sum upfront for more wire for a no transformer system. These may steer you in the 480 step up step down direction.

However, I stand by my position that without these other factors, 1,000 ft is better off done without transformers. If you DONT IGNORE transformer voltage drop and losses, you will clearly come out ahead without transformers.

 

[tortuga]

The existing service has 2 transformers

Do you mean two utility transformers ? Or it already has a step up/down ? what is the size of the existing transformers?
Since your already trenching can you just run empty conduit and have the utility run primary closer to the load (s)?
Thats what I usually see done.

We are doing 200 amps

Whats the calculated load of the service in kVA? Whats the kVA of the new step up/down transformers?

The house is without power because the wire has a short somewhere.

I would locate the short with a thumper and make a temporary repair to restore power to buy some time to look into all your options.