Looking for input

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hardworkingstiff

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Wilmington, NC
When working up voltage drop calculations at marinas, what load would be proper to use in the calculations.

Example: One type W feeder from the service equipment to a power center that has (4) 50-amp receptacles (feeding 2 slips/boats). NEC does not allow a derating for four receptacles, so the cable will need to be rated for 200-amps, but what load would be expected? Do we do a voltage drop calculation based on a 200-amp load? Do we do one at 160-amps (80%)?

Thanks all.
 
Re: Looking for input

Wouldn't that depend how those boats use the slip, or if you expect full capacity? I recall some slips in Seattle had people living on board full time.

I've got a few ideas here, but at first glance, it seems max OCD load is used for v-drop calcs in NEC FPN guidlines. Unless defined as technical-power 2001 NEC 647.4(D), or local codes enforce something similar, should we use FPN guides, expected loads, or go with ANSI C84.1-1995 guidlines?

1) NEC FPN's NOT ENFORCED
If your shooting for 3-5% v-drop from 1999 & 2001 NEC FPN's 210-19 & 215-2, the same language is used there, "..and where the maximum total voltage drop on both feeders and branch circuits.."

To me "maximum" is limited by OCD, not calculated loads that are likely modified by harmonics, or maybe some UPS equivelent for rectified/400Hz-marine loads.

NEC & ANSI codes citing voltage limits may also imply a v-drop boundary.

2) VOLTAGE DEFINITION
"The actual voltage at which a circuit operates can vary from the nominal within a range that permits satisfactory operation of equipment."

For voltage-variation limits for equipment, see ANSI C84.1-1995, Voltage Ratings for Electric Power Systems and Equipment (60 Hz). (NEC 100 "Voltage, Nominal")

4) TEST INSTRUMENT SUGGESTIONS

A) Fluke 43B - Imbalances in Inductive Motors

if any equipment depends on multi-phase motors, industry practice may keep each phase within ~1% of average voltage, and current unbalance within 10% of average current.

This argument is based on deviations causing high unbalanced currents in stator windings, resulting in overheating and reduced motor life.
For (E)or(I) MaxDeviation= ABS(largest of Each Phase - Avg) and [ABS(MaxDeviation)/Average]*100= Percentage.

B) ShureTest 61-165

For residential voltages, Ideal's 61-165 manual states, "under load, voltage should not drop below 108vac for reliable equipment operation." Except one referece to switch-mode power supplies, this ShureTest manual doesn't elaborate why 108vac is significant.

[ January 01, 2006, 08:03 AM: Message edited by: ramsy ]
 
Re: Looking for input

Wow Roger you like to cover the bases. :cool:

hardworkingstiff I have a much simpler answer, one that I think you already know.

It is entirely your choice as to how much VD you will design in the system. :cool:

Keeping in mind I am not an engineer, IMO using 80% of the circuits capacity for VD calculations would provide satisfactory results.

I have no real proof of that however I think we can safely assume that any boats electrical system using that 50 amp supply will not be designed to draw more than 40 amps continuously from a 50 amp shore cord.

I understand the boats electrical is not based on the NEC but I bet the boat builders know that the shore power will only be rated 80% continuously.

Further it seems unlikely that all 4 boats would ever draw more than 40 amps at the same time. I suspect that your 200 amp feeder will rarely if ever see more than 160 amps.

Perhaps run the calculations each way (100% and 80%, provide pricing for both and let the customer decide what they want to pay for.
 
Re: Looking for input

Bob wrote "Wow Roger you like to cover the bases. ..Perhaps run the calculations each way (100% and 80%, provide pricing for both and let the customer decide what they want to pay for."

I knew more experience would bring something else to the table. :D

[ January 01, 2006, 09:00 PM: Message edited by: ramsy ]
 
Re: Looking for input

Perhaps run the calculations each way (100% and 80%, provide pricing for both and let the customer decide what they want to pay for.
Click to expand...
First of all the 80% factor doesn't mean much. The OC can handle full load for 3 hours and by then you can damage equipment if the VD is too high.
Second the owner doesn't have a clue what you are talking about so he will choose the cheap route.
Are you willing to bet your wallet the cheap route will work?
You need to get more info on the load. Perhaps someone has data to share or a site that can provide more info to base your caculations.
 
Re: Looking for input

BTW,

I've spoken with three different boat manufacturers about voltage. They all pretty much say that a boat can handle voltage down to 200-volts before it cuts out. Now that seems pretty low to me (16.67% drop). They also said the boat captains are accustomed to selectively using power if voltage drops too far. I came away with the impression that an 80% usage would be a reasonable usage assumption (I hate using that word, assume).

With the distances for the feeders being so long, it gets pricey fast to stay within the 3% NEC recommendation if you are calculating for a 100% load, especially if the load is not 100%. I've stated before that I've come to a compromise in my recommendation of sizing the feeders to maintain 220-volts with the power company having a 2% drop and the usage being 80%. That keeps the 100% usage voltage drop (with the power company maintaining 240-volts) at less than 9%. The difference is, a #4 type W cable can be run 305' using the compromised standard. To meet the 3% NEC recommendation for feeders you would need a 2/0 cable. That works out to about a $2,500 price difference to the project (for just the one pedestal). If you have twenty pedestals, that's $50,000. These are the lightly loaded pedestals (100-amp). It gets very expensive when you are 400-500' out with a 200-amp rated pedestal.

While the less than 9% is in excess of what the NEC recommends, it seems practical to me. I welcome comments as to whether I'm getting too far out on a limb with this thought process!
 
Re: Looking for input

Originally posted by ramsy:
hardworkingstiff might check the "American Boat and Yacht Council (ABCY) standards."
Click to expand...
Thanks, I did as you suggested and here is what I found.

"Q. Our marina is thinking about re-wiring our docks. How do we do it so we are ABYC compliant?

A. ABYC standards stop addressing systems at the shore power plug, so we don't have anything in our book that targets this issue. However, NFPA 303 Fire Protection Standards for Marinas and Boatyards, covers this exact topic in accordance with the National Electric Code. Contact information and document ordering information can be found at the FPA website at www.NFPA.org."

Hummmm, I wonder why they stopped addressing this.
 
Re: Looking for input

In my experience with the marinas is that holidays are the worst case. You could have a water source heat pump that has a LRA of 300 amps or more. You could figure the first boat's heat pump running while the second boat's starts. Try to find out about the size of the slips or get the owner to tell you the size of the boats. Otherwise, I might have to settle with 160.
 
Re: Looking for input

Originally posted by hardworkingstiff: . . . what load would be proper to use in the calculations. . . . Do we do a voltage drop calculation based on a 200-amp load?
Click to expand...
You do a VD calculation based on the calculated load. In this case, that is 200 amps. When you get your answer, then you get do decide whether or not to make any design changes, in an effort to improve the voltage.
 
Re: Looking for input

Originally posted by charlie b:
You do a VD calculation based on the calculated load. In this case, that is 200 amps.
Click to expand...
How does anyone here know what the calculated load is?

When we have a building with 1,000 - 20 amp outlets we do not count that as 20,000 amps of calculated load. :eek:
 
Re: Looking for input

Thanks all.

I've been gone all week working out of town and just got near a computer. If I go "by the book", looks like I'll be getting out of the marina wiring business.
 
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