Boat "hoist" load calculation

D

Dale lee

Member
Location
Fort myers Fl
Occupation
Electrician
Hey guys, in the process for the second time this week of trying to determine an adjustment factor for just motors serving multiple boat lifts

the specific setup is 13 boat slips with a 100 amp feeder to a dock panel, from the panel to each slip is a combination of 20 and 30 amp branch circuits. each slip is a 2 motor boat lift, there is no pedestal.

currently i have been denied by permitting department essentially for not having a load calculation for an existing system. i was contracted to replace 2 sub panels at the dock and condense them into one panel.

no while I'm not sure why I'm required to do this I am attempting anyway, mostly to figure out the load calc question.

so heres the facts-

100A feeder
13 slips with 2- 3/4 hp motor(s) @ 240V per lift, worst case you can assume 2 or three could be used at the exact same time and and 3 min (being generous)
555.6 demand factors only apply to shore power receptacles
430.24 says 125% flc of highest rated motor plus sum of 100% flc for remaining motors
430.26 say ahj can grant permission for feeder to have less ampacity than specified in 430.24 if all motors aren't operating at 1 time
430.248- 3/4 hp motor @ 230V = 6.9A each
125% largest fla= 8.625, sum of (25) remaining motors @ 6.9= 172.5+ 8.625= 181.125

I've been told that other guys have used the motor as a outlet and then used 555.6 but that doesnt make sense

any help would be appreciated as i think its absurd to tell customer their panel change is gonna cost 15,000 (no its not easy to upsize the feeders)
 
I can't find any information about demand factors for multiple boat lifts, but a simple approximation suggests that the number should be very low. You should probably ask the permitting department what evidence you need to provide to support using a reasonable demand factor. Would they permit the use of utility demand data from previous years, or a calculation based on % time the lifts actually operate, or do they just need you to do the calculation for different numbers of lifts operating and then simply state (without) support the expected number of lifts operating at a given time?

Failing that, would they permit some sort of interlock that prevents too many lifts from operating at the same time?

I did a very simple 'back of the envelope' probability calculation. I imagined that every single lift has to get operated once randomly in a half hour period. I also assumed that each lift operates for a full three minutes. I assumed a completely random distribution of operation in that half hour window. This implies that each lift has a 10% probability of operating at any point in time. I'm sure anyone around the marina would attest that 10% is a very high estimate of the operating %.

Given this 10% operating percentage, you can use the same calculations used for 'how many heads in N coin tosses given an unfair coin' to calculate the chance of n lifts operating at any given time.

The chance of 0 lifts operating is 25.4%
The chance of 1 lift operating is 36.7%
The chance of 2 lifts operating is 24.5%
The chance of 3 lifts operating is 10%
The chance of 4 lifts operating is 2.8%
The chance of 5 lifts operating is 0.6%
The chance of 6 lifts operating is 0.1%

Obviously there is no guarantee that all the lifts would not operate at once, but the chance is phenomenally low. Given a realistic operating % number, I expect that it is unlikely for even 3 lifts to be operated at the same time.
 
winnie said:
I can't find any information about demand factors for multiple boat lifts, but a simple approximation suggests that the number should be very low. You should probably ask the permitting department what evidence you need to provide to support using a reasonable demand factor. Would they permit the use of utility demand data from previous years, or a calculation based on % time the lifts actually operate, or do they just need you to do the calculation for different numbers of lifts operating and then simply state (without) support the expected number of lifts operating at a given time?

Failing that, would they permit some sort of interlock that prevents too many lifts from operating at the same time?

I did a very simple 'back of the envelope' probability calculation. I imagined that every single lift has to get operated once randomly in a half hour period. I also assumed that each lift operates for a full three minutes. I assumed a completely random distribution of operation in that half hour window. This implies that each lift has a 10% probability of operating at any point in time. I'm sure anyone around the marina would attest that 10% is a very high estimate of the operating %.

Given this 10% operating percentage, you can use the same calculations used for 'how many heads in N coin tosses given an unfair coin' to calculate the chance of n lifts operating at any given time.

The chance of 0 lifts operating is 25.4%
The chance of 1 lift operating is 36.7%
The chance of 2 lifts operating is 24.5%
The chance of 3 lifts operating is 10%
The chance of 4 lifts operating is 2.8%
The chance of 5 lifts operating is 0.6%
The chance of 6 lifts operating is 0.1%

Obviously there is no guarantee that all the lifts would not operate at once, but the chance is phenomenally low. Given a realistic operating % number, I expect that it is unlikely for even 3 lifts to be operated at the same time.
Click to expand...
In reality, a boat lift might get operated twice per day.
 
retirede said:
In reality, a boat lift might get operated twice per day.
Click to expand...

I absolutely agree. But I suspect that the operating times are not random throughout the day; it seems plausible that all the lifts might get operated shortly after sunrise and again shortly before sunset. (Or more likely most of the lifts wouldn't get operated at all on any given day. Boats are expensive!)

But I was trying to be very conservative about the numbers. If the OP can show that even with wildly high estimates of lift use the chance of having more than a few lifts operating at once is tiny, then it makes their case that the current feeder is sufficient.

-Jonathan
 
This is one of those situations where you could probably just make the numbers do whatever you need them to do to get the result you want.
 
ptonsparky said:
Do both motors start at one time for each lift? I would be using 13.8 A for each. Doesn't help you at all with mine coming to 183.

Please note I'm from Central Nebraska and have worked only one boatlift. It was homemade .
Click to expand...
yes, absolutely. thats the actual numbers im using, was just throwing the numbers out real quick.
 
winnie said:
I can't find any information about demand factors for multiple boat lifts, but a simple approximation suggests that the number should be very low. You should probably ask the permitting department what evidence you need to provide to support using a reasonable demand factor. Would they permit the use of utility demand data from previous years, or a calculation based on % time the lifts actually operate, or do they just need you to do the calculation for different numbers of lifts operating and then simply state (without) support the expected number of lifts operating at a given time?

Failing that, would they permit some sort of interlock that prevents too many lifts from operating at the same time?

I did a very simple 'back of the envelope' probability calculation. I imagined that every single lift has to get operated once randomly in a half hour period. I also assumed that each lift operates for a full three minutes. I assumed a completely random distribution of operation in that half hour window. This implies that each lift has a 10% probability of operating at any point in time. I'm sure anyone around the marina would attest that 10% is a very high estimate of the operating %.

Given this 10% operating percentage, you can use the same calculations used for 'how many heads in N coin tosses given an unfair coin' to calculate the chance of n lifts operating at any given time.

The chance of 0 lifts operating is 25.4%
The chance of 1 lift operating is 36.7%
The chance of 2 lifts operating is 24.5%
The chance of 3 lifts operating is 10%
The chance of 4 lifts operating is 2.8%
The chance of 5 lifts operating is 0.6%
The chance of 6 lifts operating is 0.1%

Obviously there is no guarantee that all the lifts would not operate at once, but the chance is phenomenally low. Given a realistic operating % number, I expect that it is unlikely for even 3 lifts to be operated at the same time.
Click to expand...
wow, thanks for the in depth response. that is the frustrating part is there is no direct demand factors for boat lift motors in the code. and yes the permitting department expects me to do the load calculation exactly how the code permits. i spoke to the guy on the phone and told him i dont understand why they are asking for a load calculation as its been this way at least 20 years. his response was "show me a code that says its ok because it existing". right then i knew other than an actual black and white code reference, im not getting around him.

it frustrates me because i would like to be able to come to a peaceful solution for the customers sake, but the guy is about 80 and completely bull headed.
 
Most reasonable inspectors allow utility data as a reference. Show him the load calculations and show him how it is meaningless with the actual utility data. If you have to, go over his head.
 
Thank you all for the replies, much appreciated.

I'm going to do the motor load calculation per 430.24 and take the same approach of probability or in this case improbability of (x) lifts operating at exactly the same time. considering these are at a condo association where half the owners don't even own a boat, and the other half are only here (Florida) one month of the year.
 
CoolWill said:
Most reasonable inspectors allow utility data as a reference. Show him the load calculations and show him how it is meaningless with the actual utility data. If you have to, go over his head.
Click to expand...
Absolutely, I'm just trying to resolve it peacefully before going over his head considering in this situation he is actually the AHJ not the inspector.
 
IMHO I think you could write something up on the basis of 220.60 'non-coincident loads'. The code text is:

If it is unlikely that two or more noncoincident loads will be in use simultaneously, using only the largest load(s) that will be used at one time for calculating the total load of a feeder or service shall be permitted. If a motor or air-conditioning load is part of the noncoincident load and is not the largest of the noncoincident loads, 125 percent of either the motor load or air-conditioning load, whichever is larger, shall be used in the calculation.

Note that the code doesn't say 'impossible', but says 'unlikely'.

Get enough data to show that more than N lifts operating simultaneously is 'unlikely' and then do the calculation for N lifts.

IMHO the honest number is that three lifts operating simultaneously is unlikely, but it will be easier to demonstrate that a higher number is unlikely. As long as whatever number you pick is good on the existing feeder your problem is solved. So if you can gather enough data to reliably claim that operating more than 6 lifts simultaneously is unlikely, then you should be good to go.
 
winnie said:
IMHO I think you could write something up on the basis of 220.60 'non-coincident loads'. The code text is:

If it is unlikely that two or more noncoincident loads will be in use simultaneously, using only the largest load(s) that will be used at one time for calculating the total load of a feeder or service shall be permitted. If a motor or air-conditioning load is part of the noncoincident load and is not the largest of the noncoincident loads, 125 percent of either the motor load or air-conditioning load, whichever is larger, shall be used in the calculation.

Note that the code doesn't say 'impossible', but says 'unlikely'.

Get enough data to show that more than N lifts operating simultaneously is 'unlikely' and then do the calculation for N lifts.

IMHO the honest number is that three lifts operating simultaneously is unlikely, but it will be easier to demonstrate that a higher number is unlikely. As long as whatever number you pick is good on the existing feeder your problem is solved. So if you can gather enough data to reliably claim that operating more than 6 lifts simultaneously is unlikely, then you should be good to go.
Click to expand...
I was just looking at that, trying to find a way to get from 555 to 220. which I will add was my original argument but was told is "not the right application."

555.6 says general lighting and other loads SHALL be calculated with part 3 article 220

220.60 noncoincident loads is in part 3 of article 220

but i agree 100% now just need to gather the "evidence"
 
You must log in or register to reply here.
Top